BPG Animal Skin and Leather

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This chapter pertains to the materials that are used for form leather, the chemical composition of untanned hides and the chemical alteration of both the tanning and tawing processes, the degradation of tanned hides, and common conservation treatments for leather. For a thorough discussion of similar topics pertaining to untanned hides such as vellum and parchment, please refer to the page on Parchment.

See also Use of Leather in Book Conservation, Leather Research, and Leather and Skin (OSG).

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American Institute for Conservation (AIC). "BPG Animal Skin and Leather." AIC Wiki. April 18, 2024. https://www.conservation-wiki.com/wiki/BPG_Animal_Skin_and_Leather.

Sources of Leather

A main attribute of leather is its resistance to putrification even if kept wet. Other materials made from animal skins, such as parchment and raw-hide, degrade if repeatedly wet (Roy Thomson 2011c). Leather is created from animal hides. While almost any hide from a vertebrate can be tanned, a select few species of animal are commonly used for bookbinding. Calf, pig, sheep, and goat hides are the most traditional sources of bookbinding leather. These have been selected through the years for this purpose due to the workability of the final leather product for the needs of bookbinders, but also due to their ideal size to cover a complete quarto.

Other hides, such as larger and smaller mammals, reptiles, fish, and even humans, have been experimented at one time or another for bookbinding materials, though none are found in collections in great quantities, nor do they match the traditional sources in durability and utility for their purpose. More information on less commonly used animal skins and leathers can be found in Section 10.5.

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Chemical Composition of Untanned Hides

Skins are primarily composed of collagen, a protein that is extremely long in relation to its cross section, imparting strength and flexibility. Leather is composed of these collagen-based animal hides that have been chemically altered by tanning to render them imputrescible, have desired handling and working characteristics, and increase chemical and physical durability over time. To investigate the chemical composition of leather, it is best to first understand the chemical composition of skins and hides and how tanning alters these structures.

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Collagen Molecules

(Unless otherwise cited, information for this section is from Ch. 1, “The Nature and Properties of Leather” by Roy Thomson in Conservation of Leather and Related Materials)

Fig. 2.1: Basic structure of an amino acid

Collagen is a protein molecule built of sequential chains of amino acids twisted and bound to form a strong, fibrous molecular structure. The sequence that the amino acids are linked determines which protein is formed (Roy Thomson 2011c). The amino acid monomers that are the basis of the collagen protein are composed of a carboxyl and an amino group and a variable side chain off a central carbon (see Fig. 2.1).

Fig. 2.2: Condensation reaction of two amino acids to form a peptide bond

These side chains, which give each individual amino acid its unique chemical characteristics, can range from a simple hydrogen to reasonably large functional groups that can be polar or non-polar, acidic or basic, aromatic or aliphatic. Non-polar side chains involve only carbon and hydrogen atoms, however polar side chains contain oxygen and can frequently involve carbonyl and hydroxyl groups, amino and amide groups, or thiols (also called mercaptans, -SH). The different amino acids are linked together by a covalent peptide bond formed by a condensation reaction between the carbonyl group of one amino acid and the amino group of another amino acid (see Fig. 2.2) to form a polymer chain called a polypeptide. The backbone of proteins is the same but they are distinguished by the sequence of amino acids.

Polypeptide Chains, Procollagen and Tropocollagen Structure

(Unless otherwise cited, information for this section is from Ch. 1, “The Nature and Properties of Leather” by Roy Thomson in Conservation of Leather and Related Materials)

Collagen’s backbone, the polypeptide strand, is formed by a known twenty different amino acids that form a chain of about 1000 units in length. However, collagen consists mainly of three amino acids: glycine (30%), hydroxyproline (10%) and proline (10%) (Roy Thomson 2011c). In the chain, a common sequence of amino acids is glycene-X-proline or glycene-X-hydoxyproline, where X is a range of other commonly occurring amino acid residues. Hydroxyproline, an amino acid found in all collagen molecules (see Fig. 2.3), is rare in almost all other protein structures and its presence is used as an indicator for collagen. Proline has a ring shape, and it is this that causes the protein chain to twist, with three protein polymers twisted together in a triple helix to form collagen (Roy Thomson 2011c). Spatially, this sequence forms a left-handed helix.

Fig. 2.3: Hydroxyproline

The procollagen structure is formed by the twisting together of three left-handed helical polypeptides into a triple helix with a right handed twist with three amino acid groups per twist. Chemical crosslinks and hydrogen bonding between the three chains further stabilized the collagen molecule. For this, the three chains must be closely packed and staggered to allow smaller side chains (glycine) to orient into the center, and larger side chains to project outwards. From this, the terminal extension peptide groups (found at each end of the polypeptide chain) are removed by specific proteases to form non-helical telopeptide regions thus finalizing the formation of the tropocollagen structure. This final quaternary structure is stabilized by multiple hydrogen bonds between the amino and carboxyl groups of adjacent helices. Due to the necessity of a tight helical structure, all large functional groups on amino acids are oriented to the outside of the helix.

Fibril and Fiber Structures

(Unless otherwise cited, information for this section is from Ch. 2, “Collagen: the leather making protein” by B.M. Haines in Conservation of Leather and Related Materials)

Collagen is a multi hierarchical structure that is further developed from the collagen molecules, resulting in four levels of macromolecular structure: first the molecules pack together into an organized secondary helical structure called a fibril, then those fibrils further organize into larger bundles called fibril bundles, then into fascicles, and finally into fiber bundles. Fibrils are the first level of the collagen structure that is visible via scanning electron microscopy (SEM).

The collagen fibril is stabilized by the formation of two types of chemical bonds: Salt links and covalent intermolecular bonds. Salt links are electrostatic links formed between acidic and basic functional groups on the amino acid side chains (see Fig. 2.8) whose strength is maximized by aligning polar regions of the fibrils.

Covalent intermolecular bonds are formed by staggering the telopeptide regions (the terminal non-helical areas of the tropocollagen structure described above) with helical portions of adjacent molecules, thus resulting in a long fiber structure with no weak points. The repeated coiling structure within the collagen molecule gives strength to the fibril.

Mammalian Skin Structure

(Unless otherwise cited, information for this section is from Ch. 3, “The fiber structure of leather” by B.M. Haines in Conservation of Leather and Related Materials)

There are three distinct layers of the mammalian skin: the grain layer which consists of the outer surface to the root of the hair follicles with fine collagen fibers, the corium layer where the fiber bundles are larger and interweave at a higher angle relative to those at the skin's surface, and the flesh layer that separates skin from muscles and has finer fibers that run in a horizontal plane (B M Haines 2011b). The proportion of these three layers varies between different animals, and this, along with the grain surface patterns, can be used to visually identify the source of leather.

The skin structure influences the strength and working properties of a skin. The strength of leather is dependent on thickness as well as the proportion of corium tissue, and the interweave of the corium and grain layers. For example, young calfskin is thin (1mm thick) but very strong due to the fine interweaving of fine corium fiber bundles and fine grain surface. However, this compact weave also reduces the drape of the skin, making it more appropriate for bookbinding than clothing. The wool fibers in sheep skin prevent a strong interweave between the grain and corium layers, and along with fat naturally stored between these layers, leads to looseness in this region and the tendency of sheep skin to delaminate. The reduction of the corium layer (for, example, when paring leather for bookbinding) will greatly reduce the strength of the leather (B M Haines 2011b).

Leather's strength is dependent on the thickness of the leather, the proportion of the corium later, and the interweaving of the corium and grain layers. Leather's flexibility and ability to stretch and compress comes from fine spaces between the fibers and fibrils, allowing their movement. Reducing these spaces with stretching or processing can reduce flexibility and cause leather to tear when stretched. (B M Haines 2011b). A piece of leather’s strength is influenced by the species, location on the skin, orientation, and for bookbinding, how much of the corium layer is removed with paring. An example is: “a goat binding leather that before paring measures 1.4mm and tears at 7.5 kg: after paring to 0.6 mm the leather would tear at about 1.2 kg. The paring removes almost all of the corium tissue but whereas the thickness had been reduced by about a half, the strength was reduced by five sixths.” (B M Haines 2011b, 20).

Shrinkage Temperature

(Unless otherwise cited, information for this section is from Ch. 2, “Collagen: the leather making protein” by B.M. Haines in Conservation of Leather and Related Materials)

The hydrothermal shrinkage temperature (Ts) is the sudden shrinkage in the length of collagen when in heated water. It is used to determine the relative stability of leather. The backbone chains of collagen are held in an extended form with hydrogen bonding and when the energy from the heat is greater than the hydrogen bonds, these bonds break and the extended form collapses and the fiber shrinks. The collagen molecules are then held together by only covalent and salt links and take on a rubber-like consistency (B M Haines 2011a).

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Physical Alterations Through Tanning

(Unless otherwise cited, information for this section is from Ch. 1, “The nature and properties of leather” by Roy Thomson in Conservation of Leather and Related Materials)

The sought-after characteristics of leather produced through the tanning process are brought about by many physical and chemical changes to the untanned hide. Ideally, all leathers have increased hydrothermal stability (an increased Ts), improved softness and drape through the opening of the fiber structure and removal of the ground substance, are rendered imputrescible, and have fibers that do not stick together when wetted and dried. These changes are brought about through the combination of the many physical processes of tanning outlined above, but also a series of chemical alterations to the collagen structure.

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Increased Hydrothermal Stability

The Ts can be increased through the process of tanning, and is often used as one of the primary measures of the efficacy of a particular tan. The value is dependent on a number of factors, such as animal type, pre-tanning and tanning treatments, and condition of the leather. In most cases, tannage increases Ts, and hydrolytic and oxidative aging reduces Ts.

Recorded Ts using international standards: Raw mammalian skin 58-64C Limed unhaired cattle hide 53-57C Parchment 55-64C Alum-tawed skin 55-60C Formaldehyde—tanned leather 65-70C Alum-tanned skins 70-80C Vegetable tanned leather (hydrolysable) 75-80C Vegetable tanned leather (condensed) 80-85C Chrome tanned leather 100-120C (B M Haines 2011a)

Standards for the measurement of Ts are covered in section 7.3.2.1 and on the Leather Research page.

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Imputrescence

Imputrescence is achieved by removing many of the non-collagen substances found in untanned hides, but also through de-watering steps in leather production, such as salt-curing and drying (Reich 1999).

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Flexibility

In tanned skins, the fibers don’t stick when wetted and dried–unlike parchment. This allows for continued flexibility. This is partly due to fatliquoring and mechanical action such as staking which act as lubrication.

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Increased Softness and Drape

• Liming step allows for the swelling of the pelt by the imbibition of water
• The mucopolysaccharides in the ground substance are polyelectrolytes and bond water very firmly, so few other types of ions can reach the fibers of the dermal network while the ground is present.
• With liming, the hydroxyl ions in the skin break down the bonds between the mucopolysaccharides and water, thus dispersing the ground substance and exposing the collagen fibers in the dermal network to hydroxl ions (and swelling).
• Bating and/or deliming lower the pH and reduce the # of hydroxyl ions bound to the dermal network, reducing swelling and softening the pelt.

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Chemical Alterations Through Tanning

(Unless otherwise cited, information for this section is from Ch. 4, “The chemistry of tanning materials” by A.D. Covington in Conservation of Leather and Related Materials)

In order to achieve many of the physical changes to make a hide into leather, many chemical changes must also take place. The following is an overview of the chemistry of the major tanning processes, vegetable tanning, mineral tanning, brain tanning, aldehyde tanning and syntans. For the purposes of this page, vegetable tanning will be the focus as it is the most common leather used, both historically and in modern times, for book binding. However, other tanning processes and non-tan processes, such as tawing will also be covered. For a summary of the production of vellum and parchment, refer to the Paper Conservation Catalog chapter Parchment.

Tanning is defined by the process in which a putrescible material becomes resistant to microorganisms. Tanning changes the physical properties of the skin, such as color, opacity and smell. Chemically, tanning increases hydrothermal stability. Tanning also allows leather to be wetted and dried without fibers sticking together and becoming stiff or opaque (A. D. Covington 2001). Unlike tanned leather, parchment is an example of a material that is resistant to putrefaction but that can not be wet and retain its appearance and flexibility. Tanning is not reversed with wetting, unlike tawed materials.

The complex chemistry of tanning agents and collagen is still being researched. Current theories suggest that multiple mechanisms occur and the reaction is different depending on the tanning agent. However, the process is contingent on creating an environment where the collagen is confined, but this is possible in multiple ways (A. D. Covington 2001). It has been found that the greatest hydrothermal stability results from both linking steps and an additional step that locks the components of the matrix together, allowing the matrix to act like a single chemical compound (A. Covington et al. 2008). It had previously been believed that mainly chemical crosslinks introduced into the collagen by tanning agents raise the Ts (depending on the type of tanning material and process) and increase hydrothermal stability. (B M Haines 2011a; A. D. Covington 2011). Covington notes that the change in Ts from different tanning agents contradicts this theory, since there is a large range in Ts that does not correlate with the expected ability to covalently bond–another mechanism must be involved (A. D. Covington 2001). Instead, it is believed that the mechanism depends on the creation of a supramolecular matrix around the triple helix (A. D. Covington 2001). The theory of “polymer in a box” is a contributing factor. The idea that a single collagen triple helix fiber in a theoretical box has lower thermal resistance than a network of triple helix fibers, because the networking reduces entropy and increases enthalpy by reducing freedom of movement and ability to denature (A. D. Covington 2001; Miles 1999).

A component of the tanning mechanism is through the alteration of the water matrix of the supramolecular structure. The water matrix for raw collagen is similar to a solvent shell where water molecules are bound to the outside of the structure, particularly at the hydroxyproline residue sites (A. D. Covington 2011). This forms a sheath of water around the supramolecular structure, bound by hydrogen bonds. There are two previously proposed roles the formation of the supramolecular solvation plays: one based on hydrogen bonding with water bridging and another based on the inductive effect of the hydroxy group (A. D. Covington 2001). Furthermore, during the tanning process, much of this water matrix is replaced by tanning agents, thus changing the chemical composition of the matrix surrounding the structure and increasing the number of covalent bonds, and creating a more rigid structure (A. D. Covington 2001). It is the ability of tanning agents to fit into or displace the water structure and bind the matrix covalently to collagen that affects the magnitude of the change in the shrinkage temperature of the collagen (A. D. Covington 2011).

Also, counterions may play a significant role in the resulting hydrothermal stability. Chromium (III) molecules have a moderate effect on Ts, whereas Chromium (III) sulfate tanning has a much stronger effect (A. D. Covington 2001). The specific counter ion also has an effect, as seen by the higher Ts of sulphate compared to chloride in conjunction with Chromium (III) (A. Covington et al. 2008). Water content of the collagen triple helix is also a factor: research has found that reducing water content within a certain range brings the fibers closed together creating a tighter network that makes them less likely to denature (A. Covington et al. 2008). The amount of hydrothermal stability imparted with tanning is dependent on entropic and enthalpic contributions to the modified collagen structure influenced by the composition of the collagen, the tanning agent, counterions or secondary tanning agents, and water.

Suggested reading on tanning chemistry: Covington, A. D. 2001. “Atkin Memorial Lecture Theory and Mechanism of Tanning: Present Thinking and Future Implications for Industry.” JOURNAL- SOCIETY OF LEATHER TECHNOLOGISTS AND CHEMISTS 85: 24–34.

Covington, Anthony, L Song, Ono Suparno, H. Koon, and Matthew Collins. 2008. “Link-Lock: The Mechanism of Stabilising Collagen by Chemical Reactions.” Journal of the Society of Leather Technologists and Chemists 92 (January): 1–7.

Covington, A D. 2011. “The Chemistry of Tanning Materials.” In Care and Conservation of Leather and Related Materials, 14.

Miles, Christopher A., Ghelashvili, Michael. 1999. “Polymer-in-a-Box Mechanism for the Thermal Stabilization of Collagen Molecules in Fibers.” BPJ Biophysical Journal 76 (6): 3243–52.

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Vegetable Tanning Chemistry

Many plants contain polyphenols used for tanning. The molecular mass of the polyphenols of tannins is 500-3000, lower molecular mass molecules in the tannin are called non-tans, and higher molecular mass species are gums (A. D. Covington 2011). Different tannins give different properties to the leather produced.

Other materials also impact the stability of tanned leather from the tannin source. These include non-tans, various organic salts present in leather that act as effective buffers against acidic atmospheric pollutants (Roy Thomson 2011b; McLean 1997).

Classes of vegetable tannins are: Hydolysable/pyrogallol, Condensed/catechol, and non-tans.

Hydolysable/pyrogallol

Hydolysable/pyrogallol tannins can be subclassed to

• Gallotannins (tannic acid/sumac)
  
• Ellagitannins (chestnut/oak)
  

Characteristics of Hydrolysable tannins:

• Are sugar based – mostly glucose, though also contain some larger polysaccharides
• Gallotannins are characterized by glucose esterified by gallic acid, and ellagitannins have sugar cores esterified not only with gallic acid but also with ellagic acid.
• Raise the shrinkage temperature (Ts) to 75-80 C
• The presence of a trihydroxyphenyl moiety (the phenol molecule with 3 OH groups attached) can allow for the complexation of metal ions, resulting in a semi-metal tannage, which raises the Ts as high as 120C.
• Hydrolysable tannins break down by hydrolysis, then depositing esterifying acids within the fiber structure – called a “bloom”
• Very reactive tannins due to the high number of hydroxy groups on the hydrolysable tannin, and very reactive/astringent as tannins.
• They are pale colored, and light fast (do not darken readily on exposure to light).
• Sumac is an example of a traditional hydrolysable tannin used historically in the Mediterranean region. It produced lighter colored leathers and is known to have a high salt content which may be why it is known to be a durable leather. (Calnan 1991a).
• Contain more non-tans compared to condensed tannins (Betty M. Haines 1991).

Condensed/catechol tannins

Characteristics of Condensed tannins:

• Found in Mimosa/quebracho
• Based on flavanoid rings
• As seen in Figure 4.3, the flavonoid ring system of condensed polyphenols has an aromatic A compound, which is reactive to forming carbon-carbon bonds, creating flavonoid polymers. The B ring is not as reactive, and often contains the dihydroxyphenol moiety (giving the alternate name of catechol tannin).
• The overall structure of a condensed tannin can be hydroxylated in different ways, but they do NOT typically form semi-metal tans (though in rare cases where this does occur it produces a very high Ts)
• Typically raise Ts to 80-85 C
• Condensed tannins do NOT break down by hydrolysis, but do deposit a precipitate, an aggregate of polyphenol molecules called “reds” or phlobaphenes (which are reddish-colored, water-insoluble phenolic substances ).
• Condensed tannins redden markedly with light exposure. This is because of their linked ring structurestrucutre which undergoes oxidative crosslinking.

[insert figure 6: Flavanoid ring]

Non-tans

Vegetable tan extracts contain both tans and non-tans. Non-tans are sugars and salts, mainly potassium salts or organic acids (Calnan 1991b, 70). Hydrolysable tans contain more non-tans (ibid). Non-tans provide protection against acid hydrolysis. Washing and processing during leather manufacture can remove non-tans.

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Vegetable tannins and their reaction with collagen

• React with the collagen molecule via hydrogen bonding at the collagen peptide links
• Additionally the polyphenols can fix to the amino and carboxylic acid groups on the side chains.
• Condensed tannins also can form covalent bonds between the collagen molecule and the aromatic carbon groups in the tannins via “quinoid” structures which are more stable than H+ bonding and accounts for the increased Ts for condensed tannins (A. D. Covington 2011). [see Figure 7]

[insert Figure 7: Quinoid Structure]

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Mineral and Other Tans

Historic

Many inorganic elements could, in theory, be used for tanning, but in practice, they are limited due to effectiveness, availability, toxicity, and cost. The most practical inorganic compounds are: titanium (III), or (IV), zirconium (IV), Chromium (III), and iron (III).

Chrome tanning

• Introduced in the turn of the 19th century, and in 2006, 90% of leather production was chrome tanned (A. D. Covington 2011)
• Is relatively cheap, readily available.
• Raises the Ts to over 100C.
• Chromium III interacts with collagen through the ionized carboxy groups (those with aspartic and glutamic side chains)
• Chromium (III) forms basic salts in the pH range 2-5, and the ionized carboxy groups are most reactive at pH 2-6. The overlap of the pH values allows the reaction to occur.
• To maximize tan, initiated at pH 2.5-3 w/ 33% basic chromium III sulphate. This allows for better penetration by the chromium species
• pH is then gradually raised to 3.5-4, which increases the number of free reaction sites on the collagen while it increases the size of the chromium species
• The temperature and pH are elevated during tanning to allow chrome fixation.
• The higher the chrome content in the leather, the higher the Ts, although the industrial requirement is to achieve the highest Ts with the least amount of chrome possible. The tanner does this by controlling the reaction rate and penetration.
• It has been thought that the high Ts from chrome tanning is due to cross-linking at the carboxylate side chains, but more recently it has been found that crosslinking is not necessary for chrome tanning.
• Masking of chromium (III) by use of ligands (an ion or molecule attached to a metal atom by coordinate bonding) of monodentate or bidentate ligand salts, allows for a reduction of reactivity by bonding with reactive sites, and accelerates the reaction rate without increasing the size of the chrome species, which would decrease penetration.
• Because only a small amount of chrome tanning agent is necessary for tanning, re-tanning materials can be added to chrome-tanned leather. This allows for the leather to be used for a wide range of products.

Aluminum (III) taws

• There is a long history of using potash alum in leathermaking, dating back to the Egyptians.
• Alum is often used in conjunction with vegetable tanning to enhance dyeing.
• Alum interacts weakly with collagen, and hardly raises the Ts.
• Because the alum salts can be washed out, the process is called ‘tawing’ rather than tanning.
• Aluminum (III) reacts with the collagen carboxyls, but does not form stable covalent complexes with the carboxyl groups. The interaction is largely electrovalent, making hydrolysis easier.
• The reaction is improved by modifying aluminum sulphate with masking salts and basifying the tannage to pH 4. This reduces the reversibility of the process, and raises the Ts up to 90C.
• Aluminum (III) salts have a limited use as tanning agents. They produce a firm leather, which can dry translucent because the fiber structure can stick.

Titanium (IV) salts

• Titanium (IV) salts are superior to Al(III) for tanning.
• The reaction is more electrostatic than covalent due to a weak coordinating power with the collagen carboxyl groups.
• Titanium (IV) is traditionally used to re-tan vegetable tanned leather for hatbanding.
• To achieve a high Ts (90C) large quantities of titanium (IV) are required, therefore it is not ideal.
• Colorless and makes white leather.

Zirconium (IV) salts

• It has superior tanning properties to Al(III) and Ti(IV), but is nowhere near as effective as Cr(III).
• The process is somewhat similar to tanning with plant polyphenols, in that the reaction is hydrogen bonding with the carboxy, amino, or hydroxy groups on the collagen chain.

Oil and Brain Tanning Chemistry

Oil tans - chamois

• Chamois are tanned with unsaturated oil, preferably cod liver oil.
• Tanning oils need to contain fatty acids, either free or a glyceride derivative, which are polyunsaturated.
• The level of unsaturation is specific—if there is too little unsaturation the oil will not oxidize readily and only act as a lubricant; if there is too much unsaturation the oil will crosslink and harden with oxidation.
• The natural oils (linseed, fish and castor are most common) are first sulphonated to form derivatives that are ionizable. These ionized derivatives (with -OSO2OH functional groups) can then react with water and form a dispersion. The level of sulphonation of the oils is directly related to the depth of penetration into the dermal fiber network, so more highly sulphonated oils penetrate more deeply into the leather.
• Dewooled split sheep skins are used.
• Oil tanning does not significantly raise the Ts, so is considered a leathering process rather than a true tanning process.
• Is very hydrophilic, and can take up at least 600% water to its dry mass.
• Heat-shrunk oil tanned leather can regain 80% of its dimension if put in cold water (the Ewald effect).

Synthetic versions of oil tanning are made with sulphonyl chloride.

Brain tans – buckskin

• Animal brains are partly cooked in water, mashed into a paste, and worked into the pelt. The phospholipids in the brain act as a lubricant, resulting in Buckskin, a soft, open-structure leather.
• To allow the leather to be re-wetted, it must be smoked over wood to make the tannage permanent, otherwise the leather will harden on drying due to fibers re-sticking.

Modern

Aldehyde tanning

• Formaldehyde tanning: Reaction occurs primarily at the amino groups of the collagen chain. This N-hydroxymethyl group is highly reactive and can crosslink to the amino group of another collagen chain. The crosslinking is inefficient because the formaldehyde species are not monomeric. The Ts is typically raised to 80-85C.
• Glutaraldehyde tanning: The terminal hydroxy groups of the polymer react with the amino groups of collagen. The polymer can also form hydrogen bonds with the peptide links in the collagen. This gives the leather a spongy, hydrophilic character. This tanning process gives an undesirable yellow-orange color. Attempts to modify the color have not been fully successful.
• Oxazolidine tanning: The compounds are alicyclic derivatives of an amino alcohol and formaldehyde. The compound reacts with one or more amino sites on the collagen chain.

Syntans

Definition from Bookbinding and the Conservation of Books: A Dictionary of Descriptive Terminology (Roberts 1994):

“A contraction of "synthetic tannins," which are chemicals that combine with, or affect, the protein constituents of hides and skins and produce a product that is flexible, porous, and has the desirable qualities of leather. The most widely known syntans are made by treating aromatic substances, e.g., cresols, phenols, naphthalenes, etc., with formaldehyde and sulfuric acid. There are many variations in the ingredients of syntans, relative quantities used, and methods of manufacture. Syntans produce white or buff-colored leather, depending on the ingredients, which darken upon exposure to light, and generally behave much like vegetable-tanned leathers. Although syntans do exist which can be used alone to produce leather (so-called exchange or replacement syntans), many syntans lack the filling power of vegetable tannins and produce an undesirably thin, "papery" leather. They are also more expensive than the natural tannins. Syntans do have desirable properties, however, and are widely used in both chrome and vegetable tannages. When used in conjunction with other tanning agents, where they are known as "auxiliary syntans," they perform the following functions: 1) the presence of 5% syntan helps dissolve solid vegetable tannin extracts and reduces any tendency to form REDS (condensed tannins) or BLOOM (pyrogallol tannins); 2) a pretannage with 5 to 10% syntan improves the shade, i.e., makes it paler, and the levelness of color of a subsequent vegetable tannage; 3) a pretannage with a syntan or admixture with a vegetable tannage improves penetration of tannin into the skin; 4) when syntan is used with a vegetable tannin the leather develops a more uniform but paler color upon being dyed, but the syntan generally prevents the development of deep, full shades: 5) some types of syntan may be adjusted with an alkali to a pH of 6.0 to become "neutral syntans," often called synthetic mordants (but should be called "synthetic leveling agents") which have value in dyeing leather: and 6) some syntans retard mold growth and/ or remove iron stains. (248 , 291 , 306 , 363 )”

• Auxiliary syntans: Are often based on naphthalene and are synthesized by the ‘Nerdol’ method (the base material is sulphonated to a high degree and then polymerized). Because there are sulphonate groups, the compounds interact strongly with collagen amino side chains at pH<6. This blocks the reaction sites of vegetable tannins and promotes penetration, as well as solubilizing aggregated phlobaphenes of condensed tannins and reduces the reaction on the surface of the hide. (A. D. Covington 2011)
• Combination or retanning syntans: Are usually based on simple phenolic compounds, and are synthesized by the ‘Novolac’ method (the base material is polymerized and the product may be partially sulphonated). The product is more complex than auxiliary syntans: they have a higher molecular mass and can be crosslinked in two dimensions. This allows them to increase Ts more, and have more of a filling effect. They are small polymers with weak tanning ability and work best as retanning agents, and are largely used with chrome tanning. (A. D. Covington 2011)
• Replacement syntans: Syntans with increased tanning power that can be used in the place of vegetable tannins and can be used on their own for tanning. They are distinguished from retanning sytans only by their increased effectiveness. The result is similar to vegetable tanning, raising the Ts to 80-85C. (A. D. Covington 2011)

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Leather Manufacture

(Unless otherwise cited, information for this section is from Ch. 7, “The manufacture of leather” by Roy Thomson in Conservation of Leather and Related Materials)

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Vegetable Tanning

Historic Process in Europe

The process of historic vegetable tanning in Europe is perhaps the best documented and related the most closely to tannages used most commonly on historic bindings, including encyclopedia entries such as LaLande (Lalande 1764; LaLande 1764) and Diderot (Diderot 2010; 1772; Diderot and d’Alembert 1771). In England, under the Tudors and the Stuarts, leather manufacture was strictly regulated–such as skins only remaining in lime for as long as necessary to remove the hair, only lime and pigeon dropping could be used, and tanning liquors had to be prepared from oak bark. Many of these regulations were only removed in 1808. (R. S. Thomson 1991)

Pretanning

• Skins would arrive in different conditions. If dried and cured with salts, the skins were soaked (often in a river) to rehydrate them.
• Putrefaction brought about the decay of non-collagen materials in the raw pelt. This was accelerated by the addition of various materials. Sweating was performed with controlled microbial attack through partial purification. Acid raising or drenching was performed using acidic solutions made from bacterial fermentation of sources such as bran (beer). Liming was an alkaline treatment, as well as the use of wood ash.
• Hair removal was eased by thorough putrefaction. To remove the hair a dull double-handled knife was used to scrape the hair side of the skin once the follicles were loosened.
• Fleshing was performed on the flesh side of the skin, which was again enhanced by putrefaction. This was done by using a sharp, double-handled knife to scrape away small bits of clinging flesh from the pelt, often on a beam.
• Scudding was the careful scraping of grain surface (hair side) of the pelt to remove debris left on the skin once the hair was removed.
• Trimming was done on the sides of the pelt, by removing any small unwanted tags or bits of skin.
• Rounding is a term used for the sorting of the hides, and trimming down if necessary, to parse them into different piles for various qualities of leathers and different tanning treatments.
• Deliming was done if the skin had originally been treated with lime (or “limed”) as part of the putrefaction.
• Bating was the practice of softening the pelt by immersing it in baths of dog or pigeon dung, which partially digests the remaining protein structure in the hide and thus opens up the flesh for softer drape. Used for leather for clothing mainly.
• Drenching (also called Raising) is similar to bating, which involves the fermentation of the pelt in soured grain matter with Lactobacillus sp. bacteria. Organic acids and enzymes produced dissolved the non-fibrous protein and removed excess lime (if present). Through the use of their enzymes, these bacteria digest the souring grain to produce lactic and acetic acid and off-gas carbon dioxide. This forms bubbles of CO2 within the fiber network of the pelt while other enzymes digest the mucopolysaccharides in the ground, helping to break it down and increase the absorption of tanning agents into the fiber network.
• Scraping again follows if any bating and drenching steps were taken to remove any loosened proteins or ground materials.

Tanning

• Oak bark was historically used in the United Kingdom and is both condensed and hydrolysable–about ⅓ hydrolysable and ⅔ condensed. Sumac, a purley hydrolysable tannin, was used in the Mediterranean region in early times. (Calnan 1991a).
• The hides are first submerged in a very weak tanning liquor along with constant agitation to allow full permeation of the tanning liquor throughout the hide. This was an important step to prepare the skin for the more astringent concentrated tanning liquor.
• The wet hides are then laid in pits and layered with pulverized vegetable tanning materials and then with more hides, and then more tanning materials until the pit is filled. The pile is then topped off with either clean water or water mixed with extracted tanning materials and allowed to steep for at least one year.
• After necessary tanning time has passed, the hides are pulled from the pit and thoroughly rinsed and smoothed.
• The hides are lastly dried very slowly in the dark.

Finishing

• This work was performed by a currier for large skins such as cattle hides. For smaller skins like sheep, goat, deer, and dog, were processed by the fellmonger, the whittawyer or the glover.
• Scour: The leather was scrubbed clean with stiff brushes
• Smooth: The leather was smoothed using stone or metal-bladed slickers.
• Pare: This was done with the currier’s shaving knife, which had a rectangular, double-edged blade fitted with two handles. The damp leather was placed over a currier’s beam , and the blade passed over it at almost a right angle. In this way thin shavings were removed from the flesh side of the skin, until the desired thickness was achieved.
• Stretch: The skin was worked on a bench with stone, slickers and brushes. This flattened and stretched it, and removed any loose tanning materials.
• Fatliquoring: was performed on partially dried skins. A warm mixture of tallow and fish oils is allowed to impregnate the skin. The skins are stacked to allow even penetration, and then hung in a warm room to dry, and then the surface grease is removed. Sometimes the surface layer of the flesh side was also removed. The process uses fats to impart suppleness to thinner leathers. Because oils and fats are hydrophobic and do not disperse in water, the natural oils (linseed, fish and castor are most common) are first sulphonated to form derivatives that are ionizable. These ionized derivatives (with -OSO2OH functional groups) can then react with water and form a dispersion. The level of sulphonation of the oils is directly related to the depth of penetration into the dermal fiber network, so more highly sulphonated oils penetrate more deeply into the leather.
• Currying is a process similar to fatliquoring, but practiced only on thicker leathers (not on bookbinding leathers). The process uses cod oil, paraffin oil and meat tallows, and is applied by hand. When leather is curried under tension, a high tensile strength, low stretch leather is produced.
• Boarding involves rolling the hide repeatedly with hand rollers to soften and smooth the finish. This also brings out the natural grain of the skin, and can achieve different aesthetics.
• Staking (or perching) is another softening step that involves the rubbing of the flesh side of the hide on a large convex blunt blade.
• Dying is done only as a near-final step. Historically, only natural dyestuffs were used.
• Some leathers (maybe not for bookbinding) had weak animal glue, waxes, milk, and blood, wiped on the grain layer, and the leather was polished.

19th-20th Century Industrial Changes to Vegetable Tanning in Europe

As the demand for leather increased, so too did the need for speed in the tanning process. Causes for increased demand included industrialization and population growth, as well as wars such as the Napoleonic Wars, that demanded many leather products. Although the basic process of vegetable tanning changed little, many mechanized functions and more aggressive chemicals were introduced to speed the tanning process. The use of local agricultural byproducts to prepare and tan leather were also impacted by changes in contemporary changes in agriculture and industrialization. Breeding practices changed, to produce larger animals faster, resulting in thicker poorer quality skins. For sheep skins, the introduction of Merino strains for improved wool led to poorer quality leather. Also, resources and quality materials became scarce in Europe, materials were imported from other countries and tied to European Colonization, including skin tanned before exportation with both condensed or hydrolysable tannins, and the reprocessing in Europe often had detrimental effects. Efforts to make local materials more efficient were also explored. (Roy Thomson 2011b; R. S. Thomson 1991; Roy. Thomson 2001)

Below are the many of the steps that were changed by mechanization or the introduction of chemical treatments.

Pretanning

• Putrefaction was carried out almost exclusively with alkaline baths. Rather than using pure lime, an orpiment-lime mixture was used (sulfide), which allowed skins to be unhaired in 36 hours rather than the previous 3-4 weeks. This was later adapted to using cheap waste lime liquors from gas works (removing hydrogen sulfide from coal gas) resulting in a mixture of lime and sodium sulfide.
• Splitting was practiced to extend the square footage of tanned leather from each hide. Each hide could be split into a “grain split” and a “flesh split” hide, or paper-thin skiver. Splitting was done with band knives. The splitting machine was patented by William Powers in 1768 (R. S. Thomson 1991)
• Deliming involved the immersion of hides in a weak lime stock (pH 10-13) of weak ammonium sulfate, ammonium chloride, sodium bisulfate or boric acid to remove excess lime.
• New machines with spiral bladed cylinder knives began to be used for unhairing, fleshing, scudding, setting, scouring, striking, staking and shaving.
• Imported skins often arrived already tanned in their country of origin. European tanners removed this tanning by stripping with alkalis. However, this not only removed the tannins but also the non-tans. East Indian tanned skins were often tanned with condensed tannins, whereas Nigerian skins were often tanned with sumac (Betty M. Haines 1991).

Tanning

• In the 19th century, due to increased demand and importation, condensed tannins from natural hardwoods (such as quebracho, mimosa, mangrove or gambier) began to be used, with detrimental effects.
• Other new imported hydrolysable tannins were used which gave the best quality light leathers, such as divi divi, myrabolams, algarobilla, tara and bulbool.
• Explored new ways to extract tannins, rather than the traditional but slow layering in pits with skins and leaching with cold water.
• Use of mechanized tanning drum.
• Once it was determined that the leather produced in the 19th century was not adequately resistant to deterioration, different agencies began researching the cause and solutions. One experiment was to require vegetable tanned bookbinding leather be semi-alum tanned containing no less than 2.8% Al2O3, or chrome re-tanned which has been treated with aluminum salts–but these leathers could not be satisfactorily worked by bookbinders because they had too much mineral character, and did not wet well, take up paste well, did not block well, and quickly dulled paring knives (R. S. Thomson 1991). Potassium lactate or citrate was also added by tanners around the 1930s to act as buffers (Betty M. Haines 1991).

Finishing

• The increased use of sulfuric acid produced skins that were pale buff rather than reddish-brown, affecting dying practices. One reason for the increased use of sulfuric acid was staining from iron from the new machines used to prepare the skins. Sulfuric acid was also used as part of dye fixation. Another reason was to strip imported already tanned leather for re-tanning and dyeing (Roy. Thomson 2001)
• New synthetic dyestuffs had negative effects. Previously used dyestuffs had been applied with alum beneficially, new dyestuffs sometimes used mordants such as potassium bichromate or iron sulfate, which are damaging. Likewise, these dyes required more acidic environments for fixing and clearing, and sulphuric acid was used with negative effects (Roy Thomson 2011b).
• Until the introduction of bright red synthetic dyes, red leathers (particularly Russia leathers) were dyed with cochineal mordanted with tin chloride which involved using concentrated hydrochloric acid and lead to leather that did not age well (R. S. Thomson 1991).
• Immersion dyeing was noted to remove non-tans from hydrolysable tanned leather with detrimental effects (Betty M. Haines 1991).

Modern Vegetable Tanning

Modern tanning has become increasingly mechanized and reliant upon chemicals.

The 2021 AIC conference panel “Leather selection and Use” has interviews with leather manufacturers making archival bookbinding leather and is a useful resource.

Pretanning

• Skins usually arrive cured
• Washing and soaking in dilute alkalis
• Liming in calcium hydroxide, often with sharpening agents
• Hair removal now uses mechanized cylinder rollers and knife cylinders to remove hair after chemical putrefaction.
• Fleshing also uses cylinder rollers and knife cylinders that are sharper than those used in hair removal.
• Deliming uses acids and buffer salts
• Bating is accomplished by immersion in baths of proteolytic enzymes.
• Pickling reduces the pH of the skin to 2.5 using sulphuric or hydrochloric acid with the addition of salts. Skins are preserved and transported in this state, although sometimes skins are lightly tanned instead.
• Drenching now uses acidic baths of lactic, acetic, or formic acid to lower the pH and break down protein structures.

Tanning

• Syn-tans are regularly used in conjunction with vegetable tannins to speed up the process for light leathers

Finishing

• Some or all of the traditional post-tanning steps take place, although increasingly mechanized.
• Fatliquoring with an emulsion of various animal, marine, vegetable or mineral oils, either emulsified with anionic sulfated oils (made with sulphuric acid), or using other wetting agents. Because fatliquoring requires a small amount of oil to be distributed in a large amount of skins, the oil needs to be kept the in emulsion (Calnan 1991b). Historically this was done with egg yolks and vegetable lecithin, but today oils are treated with sulphuric acid to introduce hydrophilic sulphate groups into the fat molecule, or with bisulphite to introduce sulphonate groups into the oils (ibid).
• Finishing with pigments coating systems in some cases (maybe less so for bookbinding). Earlier casein and nitrocellulose finishes were used but now ‘resin’ finishes based on acrylic of polyurethane polymer systems are used.

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Vegetable tanning outside Europe

This section is not complete and can be broken down into specific regions. Please submit any additional information to the compiler to be added.

• Skins imported into Europe in the 19th were often pretanned, either with hydrolysable or condensed tannins.
• Research by the US Department of Agriculture and the British Leather Manufacturers Research Association in the 19th century found that native Nigerian goatskins were very resistant to deterioration unless they were stripped when processed in Europe–this was found to be due to protective non-tanning materials used in the tanning liquors in Africa (R. S. Thomson 1991).
• Sumac/ extract from Acacia arabica pods was used widely in North Africa and the Mediterranean (Betty M. Haines 1991)
• Examples of Nigerian leather (tanned with sumac)from the 1930s were found to be very sound, and washing, which normally removed non-tans and made leather more susceptible, was not found to have noticeable adverse effects. This was found to be due to the use of the juice of limes for deliming--the citric acid in the limes reacting with the lime to form calcium citrate, and insoluble salt with buffering capacity (Betty M. Haines 1991).

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Alum tawing

References for this section include:

Barlee, Roger. 2001. “Aluminium Tannages.” The Biannual Newsletter from J. Hewit &Sons Ltd No.11.

Vest, Marie. 1999. “White Tawed Leather : Aspects of Conservation.” IADA Preprints 1999., 67–72.

Alum is a double salt of aluminum and potassium phosphate, and occurs naturally in many warm climates, hence its early use as a tanning agent. The basic process consists of immersing raw pelts in a solution of alum, and this process was well established in Egypt 1600 BC. The production of alum leathers spread throughout the Mediterranean through both Arab traders and the Roman Empire. The production of alum leathers became very widespread during the Middle Ages, and was used for the production of bookingbinding leathers, gloving leather, ladies shoe leather and fur skins. Whilst there has been a dramatic reduction in the quantities of alum leather produced since the advent of chrome tanning, alum leathers are still used for bookbinding, high quality gloves, furs and cricket balls.

The manufacture of alum leathers is called tawing, and is quite distinct from the tanning process described above. Whilst the aluminum in the alum does combine with the leather fibers, as can be seen by the increased shrinkage temperature of the leather, the leather is sensitive to being washed in water. Unlike “normal” leathers that can withstand washing, when alum leathers are thoroughly immersed in water, the tanning salts are washed out, and sulphuric acid is produced. When dried the resulting material is hard and inflexible having characteristics similar to those of a dried raw pet. Whilst the problems associated with the washing out of the alum are indeed serious, alum leathers handled correctly are among the most stable leathers ever produced. Many fine examples of Medieval alum tawed leathers are still available in libraries and museums around the world.

Pretanning

In Medieval times, skins were unhaired and then given a bran drench. The fermentation that resulted produced acetic acid that removed the lime from the unhairing, after which the skins would be scudded to remove any remaining hair and pigment from the skin.

Tanning

Whilst the original alum leathers were produced using only alum, the process was modified fairly early in its history to include salt, egg yolk, and flour. These ingredients give the leather a fuller substance, and also a softer handle. The skins would then be placed in a tub containing alum, salt, flour, and egg yolk, and would be agitated by hand or using wooden poles over a period of a few days. After being left over a wooden horse, the skins would be hung up to dry, producing a very hard and inflexible material. This crusted leather would be allowed to age for a few weeks to allow the alum to stabilize. The leather was then conditioned using damp sawdust, and then hand staked. The stake was a wooden support, at the upper end of which was a blunt steel knife.

The conditioned skin would be laid over the stake, and the staker, holding both sides of skin, would forcibly draw the skin over the knife in all directions. This action would stretch the leather fully, and in the process remove the stiffness producing a very soft pliant piece of leather. The process has changed very little during the centuries apart from the usual mechanization that has occurred widely within the trade. Nowadays the skins are placed in a wooden drum to increase the agitation during the tawing process, and the hand staking is now carried out by machine.

Finishing

The one area however where knowledge has been lost is in the dyeing of alum leathers. Alum tawed skins are peculiar in the way that they dye, and special methods and dyestuffs were used. The leathers were dyed using vegetable dyes and mordanting agents, modern synthetic dyes being of little use. The leather was first washed in an alkaline solution (usually stale urine or ammonia), and then repeatedly brushed or dipped in a dye-wood or vegetable dye liquor. Following this the skins would be given a mordant wash using a metallic salt in order to either enhance the color or to bring out a special tone. The use of the mordanting also had the effect of making the colors generated more permanent. Typical woods or vegetable dyes were: Oak Bark, Logwood, Sumac, Fustic, Elderberry Juice, Cochineal and Persian Berries. Mordanting agents included: Copper Salts, (blue), Iron Salts (black/dark shades), Tin Salts (red).

Following the dyeing, in the case of dipped skins, the leathers were generally “re-egged” in order to replace the egg yolk lost during the dyeing. The skins would then be dried to the crust state again, prior to being conditioned and staked as before.

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Leather Deterioration

The complex structure of leather leads to challenges in discovering the exact decay pattern of a particular skin, which depends on several factors including the animal species, the leather processing, as well as environmental conditions. Deterioration mechanisms and pathways are becoming more understood through the use of natural and artificial aging studies, primarily on vegetable tanned leather. Though there are great strides being made into the investigation of leather deterioration, there is much that remains unknown.

Degradation, either natural or chemically enhanced, can lower the Ts by reducing the number of existing hydrogen bonds. Natural degradation functions by intermittently breaking the molecular backbone via oxidation and reduction reactions, thus shortening the polypeptide chain. Additionally, oxidation/reduction reactions also chemically alter the composition of the amino acid side chains, which can affect the number of hydrogen bonds (B M Haines 2011a).

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Breakdown of the collagen structure

(Unless otherwise cited, information for this section is from Ch. 5, “The mechanisms of deterioration in leather” by Mary-Lou Florian in Conservation of Leather and Related Materials)

Collagen is held together by a complex organization with hydrogen and covalent bonds, and the breaking of these bonds causes denaturation and gelatinization.

The deterioration of vegetable tanned leather has been the focus of many studies, such as those by the British Leather Confederation formerly the British Leather Manufacturers’ Research Association (BLMRA) in Northampton, UK; the Canadian Conservation Institute in Ottawa, Canada; the Leather Conservation Centre in Northampton, UK; and by the co-operative work of the STEP Leather Project Group in the European Community. (For more on these studies see the Leather Research Page).

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Biological deterioration

Although leather is defined by being resistant to rotting and bacterial degradation, in extreme conditions leather can be damaged by the action of bacteria. This includes if the leather has been damaged in other ways making it more susceptible, such as alum tawed skin that has been partially stripped of aluminum salts from wetting (R. Thomson 2006).

Mold can damage leather, but it is thought that mold does not attack the collagen/tannin complex, but rather other materials in leather such as vegetable tannins and non-tans, fatty lubricating materials, and humectants (R. Thomson 2006). Hydrolysable tanned skins are more prone to mold attach than condensed tanned skins because of the high levels of water soluble non-tanning material (R. Thomson 2006). The conditions that lead to mold growth, warm and moist environments, also promote hydrolytic chemical degradation of the skin; which is further accelerated by reduction of pH from mold growth (R. Thomson 2006).

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Physical deterioration

Leather is hygroscopic and readily shrinks and swells with changes in RH leading to physical damage, especially when partially restrained. Age hardening is the darkening in color and loss of flexibility along with cracking, due fluctuations in RH (R. Thomson 2006). High RH partially dissolves water soluble components which then settle on the surface when the RH drops. This leads to restriction in movement of the grain layers, and reduces flexibility by increasing adhesion between fibrils and fibers (R. Thomson 2006)

Thermal degradation

Thermal degradation of leather results when the heat supplied is such that the increased movement of the molecules leads to bonds breaking between the three protein chains of the collagen triple helix. Repeated heat fluctuations lead to leather losing its ability to absorb water from the air, causing the leather to become hard and brittle, and internal chemical compounds become concentrated. (Florian 2011)

Degradation caused by water

Water is present in leather in two forms: multilayer water (free water) present between the network of interwoven collagen bundles; and molecularly bound water bound to the protein molecules. The amount of free water present can contribute to the acid hydrolysis and oxidation. The removal of bound water, such as with freeze drying, alters the intra- and intermolecular structure, causing irreversible stiffness. Loss of water with aging will also increase the concentration of acids in leather and lower the pH. When leather has less free water it will be stiffer, but this is reversible with increased RH if the leather is stable. Similar to heat, repeated fluctuations in moisture content can lead to permanent damage, known as hysteresis. Over time the leather is not able to re-absorb the same amount of water, and becomes stiff. This may be due to the realigning of polymers. (Florian 2011).

In treating deteriorated leather, it is often noted that application of water can cause darkening and hardening of leather. Polar solvents, including water (and to a lesser extent, alcohols), will dissolve vegetable tans and bring them to the surface which becomes darkened and embrittled (Calnan 1991b).

In leather that was exposed to high humidity or cycling conditions, tan breakdown and other products accumulate between the leather fibers causing them to stick and making the leather darker and stiffer, particularly in hydrolysable tanned leather (Calnan 1991b).

Experimental evidence shows that hydrolysable tanned leather will have a greater decrease in Ts than condensed tanned leather when artificially aged in warm moist conditions (Calnan 1991b). The same experiment showed that this was likely due to loss of bound tan resulting from hydrolytic changes in the tan rather than hydrolytic degradation of the collagen, and that there was not an appreciable loss in strength with the reduction of Ts (Calnan 1991b).

[insert figure: SEM images showing that artificial aging in warm moist conditions of hydrolysable tanned leather leads to the sticking of the collagen fibers. (Calnan 1991b, 49)]

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Chemical deterioration

(Unless otherwise cited, information for this section is from Ch. 5, “The mechanisms of deterioration in leather” by Mary-Lou Florian in Conservation of Leather and Related Materials)

The two main mechanisms by which collagen degrades in leather are acid hydrolysis and oxidation, which affect not only the protein backbone of the collagen molecule but also the tannins and lubricants used in the leather-making process. Hydrolysis can be recognized with the breaking of the polymer chain, whereas oxidation with the increase in amino acid end groups (see fig. X) (Haines 1991). These reactions are influenced by the processing of the leather but also the environment.

[insert figure: Diagram comparing hydrolysis and oxidation mechanisms of collagen (Haines 1991, 72).]

Acid Hydrolysis

Hydrolysis is breaking of bonds in a molecule by hydronium ions, H3O+. Acid hydrolysis is when an acid (ex. Sulphuric acid) dissociates in water and forms hydronium that cleaves bonds in a molecule The cleaving of bonds by acidic agents (hydronium ions, H3O+) is termed acid hydrolysis. In collagen this breaking of bonds occurs between the amino acids along the protein backbone at the peptide linkages (C--N). This creates smaller polymers (peptides) and releases free amino acids.

Leathers contain a significant amount of acidic moieties, though sulfuric acid is thought to be the most prevalent player in acid hydrolysis. Sulfuric acid was thought to come primarily from atmospheric pollution, though there may be excess sulfur compounds, including sulfuric acid, from leather processing. However, research in 1905 suggested that sulfuric acid used in early stages of leather processing, such as deliming and pickling, were displaced with other steps, and that it was the use of sulfuric acid in later stages, such as dyeing, that was more damaging (Betty M. Haines 1991). Sulfur dioxide is prevalent in polluted air, and is absorbed into the leather. The sulfur dioxide is readily oxidized to sulfur trioxide with sunlight, which combines with moisture to form sulfuric acid.

Even though sulfuric acid is thought to be the primary acidic influence, other acids exist in leathers and contribute to hydrolytic breakdown. These include acids in tannins and amino acids present in leather. Although acid hydrolysis is a major cause of leather deterioration, leather is inherently acidic when stable.

Acid hydrolysis leads to the breaking of bonds selectively in the polymeric structure of collagen, leading to it losing its structural integrity and becoming gelatin. Environmental factors, such as high heat and high relative humidity will increase the rate of reaction for acid hydrolysis. The rate of acid hydrolysis is also dependent on the water content of the leather. Acid hydrolysis in the protein chain results in the production of amine groups.

[insert figure: Hydrolytic breakdown of Collagen]

Oxidation

Oxidation and reduction involve a transfer of electrons and are frequently called redox reactions. Oxidation is the loss of electrons, even if oxygen is not present, and can be initiated by several factors, including, light, heat, free radicals, and oxygen. Oxidation by free radicals frequently results in a chain reaction. Results of redox reactions include both the breaking of bonds and polymerization. Metal ions and increases in relative humidity will serve as catalysts for oxidative deterioration. Situations that produce free radicals and thus oxidation of leather include radiation from UV; air pollutants such as ozone and sulfur dioxide; peroxides forming from oxidizing agents; products from auto-oxidation of lipids, amino acids, and tannins (especially condensed tannins) and their breakdown products.

Oxidation of collagen occurs only at specific amino acid residues and tripeptide segments. Oxidative degradation of the protein polypeptide chain results in peptides, or in amide groups, which eventually break down into ammonia. This ammonia can react with sulfuric acid and form ammonium sulfate thus sequestering the sulfuric acid and impeding deterioration by acid hydrolysis.

Oxidation of leather can result in loss of chemical strength, embrittlement, crazing, cracking, lowering of pH, and color change. These changes are caused by chain scissions, changes in the amorphous/crystalline ratio, crosslinking, density change, and the production of new functional groups and acidic products.

[insert figure: Image of Oxidative breakdown of Collagen]

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Degradation properties of different tans

(Unless otherwise cited, information for this section is from Ch. 5, “The mechanisms of deterioration in leather” by Mary-Lou Florian in Conservation of Leather and Related Materials)

Vegetable tannins

As stated before, the vast majority of bookbinding leather is vegetable tanned. The two main types of vegetable tannin are condensed and hydrolysable. Most historic leathers will be tanned with some combination of tannins. Both condensed and hydrolysable tannins are weakly acidic phenolic compounds, which have antioxidant ability depending on their structure, with hydrolysable tannins having a greater antioxidant ability. The antioxidant capability of tannins is reduced upon exposure to strong oxidizing agents (such as UV).

As tannins themselves degrade, the acidity of their environment is increased, which can increase acid hydrolysis reactions of other components of the leather. The crosslinks between tannin and collagen are broken, and the leather de-tans resulting in a decrease in shrinkage temperature. Analysis using high power liquid chromatography (HPLC) of deterioration mechanisms of tannins in aged leather looks at the amount of tannin and monomers of phenolic acids present, measured by optical density (OD/100mg). The absolute or relative amounts of monomers present can be used to measure the level of deterioration of vegetable tanned leather. Aged leathers made with hydrolysable tannins show new monomers and an increase in gallic and ellagic acid monomers. It was also found that in aged leather tanned with hydrolysable tannins there was a loss of tannins compared to new leather, potentially due to their breakdown into organic acids and sugars. This was not found definitively for leather tanned with condensed tannins, and therefore the total amount of tannin extracted cannot be used to determine deterioration of leather. Aged leathers made with condensed tannins showed a shift towards more hydrophobic compounds, and an increase in ellagic acid as well. An increase in gallic acid monomers is also an indication of deterioration for leather tanned with either process. Overall, condensed tanned leathers have more monomers in the extracted tannin than hydrolysable tanned leather, indicating the greater deterioration of condensed tanned leather.

Hydrolysable (pyrogallols)

Hydrolysable tannins are those that when hydrolyzed break down to sugars and phenolic compounds, primarily gallic and ellagic acid. Especially with exposure to water, these smaller compounds can cause sticking between leather fibers, stiffening and darkening the leather. They are known historically to be more resistant to deterioration. Hydrolysable tannins have more naturally occurring salts and non-tans, which are thought to have a shielding effect against acid hydrolysis. Hydrolysable tannins also have a high degree of antioxidant ability, which protects them from oxidation. In artificial aging of oak bark, mimosa, and sumac tanned skins, sumac (hydrolysable) tanned leather was found to lose flexibility in warm conditions, and a significant embrittlement of grain surface in warm moist conditions (Calnan 1991a). In warm moist conditions the Ts was also reduced most significantly for the sumac tanned leather compared to the other tannages (Calnan 1991a). HPLC (High Performance Liquid Chromatography) found that extracted sumac tannage from artificially aged leather had the most significant change of the tannins tested for high and fluctuating RH, with a marked reduction in high molecular weight fractions and and increase in smaller molecular weight fractions, indicating hydrolysis (Calnan 1991a). The sumac tanned leathers were found to have more resticking of the fibers after aging in high or fluctuating RH due to accumulation of small tan break down products from the hydrolysis of the tannin (Calnan 1991a). In testing it was found that sumac tanned leather artificially aged in warm moist conditions had a slightly larger decrease in Ts compared to the other leathers tested, but based on other tests it was suggested that this change mainly resulted from loss of bound tan rather than hydrolysis of the collagen molecule (Calnan 1991a). Other testing found that the rate of hydrolysis was influenced by pH, with an increased rate of hydrolysis under pH 3.0 (Calnan 1991a).

Condensed (catechol)

Condensed tannins primarily degrade into insoluble compounds that are colored from yellow-brown to red. Condensed tannins do not break down by hydrolysis, but are more likely to polymerize into larger tan fractions (Calnan 1991b). Condensed tannins deposit a precipitate–an aggregate of polyphenol molecules called “reds or phlobaphenes, which are reddish-colored water-insoluble phenolic substances. Condensed tannins redden markedly with light exposure. This is because of their linked ring structurestrucutre which undergoes oxidative crosslinking.

Condensed tannins absorb twice as much sulfur dioxide as hydrolysable tannins. These tans have a reputation for higher degrees of deterioration than hydrolysable tannins. Condensed tans have a lower antioxidant activity and are more susceptible to oxidative degradation.

In artificial aging of oak bark, mimosa, and sumac tanned skins, warm moist conditions did not cause change in Ts of mimosa tanned skin (Calnan 1991a). HPLC (High Performance Liquid Chromatography) found that extracted mimosa tannage from artificially aged leather with high and fluctuating RH, showed a reduction in high molecular weight fractions but not a corresponding increase in smaller MW fractions, indicating polymerization and condensation had occurred (ibid).

(See section on Red Rot)

Chromium

The bond between chromium tanning salts and collagen is much stronger than in vegetable tanned leathers. This results in a higher shrinkage temperature and leather that is more resistant to deterioration. Chrome-tanned leathers do not absorb large amounts of sulfur dioxide and do not show signs of appreciable degradation in accelerated aging trials. The working properties of chrome-tanned skins are not ideal for bookbinding.

Alum-tawed

Alum-tawed skins, while not truly “leather,” are used fairly heavily in bookbinding. These skins have a high concentration of alum and salts that are thought to be protective against deterioration. Since the alum salts are not bonded to the collagen, and can be washed out with water, effectively “detanning” the skin. Alum-tawed skins do not take up significant concentrations of sulfur dioxide and have proven to be very resistant to degradation.

Combination Tans

Working on combination tannages to combat deterioration, thus far they look like….

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Degradation of leather processing components

Fats, oils and waxes used in leather manufacture result in the deterioration of collagen by producing high energy radicals that catalyze oxidation.

Autoxidation of lipids from the production of leather and treating the surface of leather with dressings is another issue for leather. These fats and oils may contain unsaturated fatty acids or oils which from autoxidation can become gummy or hard. Some metals found in leather from processing contribute to oxidation as catalysts. These include magnesium, iron, cobalt, and copper. On the other hand, zinc and aluminum act as antioxidants and slow oxidation. Sulfur compounds can be used at different stages of the tanning process to make conditions that promote tannage, such as maintaining an acidic pH, solubility of components, or controlling the viscosity. Inorganic water soluble sulfur compounds will be washed out, but organic sulphonated materials (e.g. sulphonated condensed tannins and syntans) may remain. The degradation of these compounds releases sulfur, which can lead to hydrolysis.

Free fatty acids that are solid at room temperature tend to recrystallized on the leather surface, resulting in spue. These fatty acids can be from leather manufacture or later treatment. Oxidation of drying oils often results in spue. Besides the visible change, the oxidation of fatty acids that results in spue can also make the material sticky, making the surface of the leather tacky. This can attract dirt and dust and exacerbate issues. (Calnan 1991b)

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Macroscopic effects of degradation

Red Rot

[See Red Rot definition].

Red rot is often used to indicate degraded powdery leather, but this can be more generally referred to as acid hydrolysis. The term red rot is often used to refer to powdery reddened leather from the 19th century that is associated with condensed tanned leather. Condensed tannins deposit a precipitate, an aggregate of polyphenol molecules called “reds” or phlobaphenes, which are reddish-colored. Hydrolysable tannins are not known to darken or change color with aging (other than denaturalization and darkening of the leather with moisture). (OS 2022)

Condensed tannins absorb more sulfuric acid from the atmosphere than hydrolysable tannins, leading to increased acid hydrolysis of the leather, leading to shortened fibers that make leather powdery. However, condensed tannins themselves do not break down with acid hydrolysis.

Condensed tannins were more frequently used in the 19th century (see section 5.1.2), and manufacturer practices also included the increased use of sulfuric acid or sulfur-containing products. This removed non-tans from the leather, making the leather more prone to degradation by acid hydrolysis. Before 1850 organic acids were used during the hair removal process which was less damaging than inorganic acids (sulfuric acid) because the organic acids did not displace the residual calcium salts in the skin which acted as a buffer, whereas sulphuric acid had a sufficiently low pH to displace the calcium (Dirksen 1997; Betty M. Haines 1991).

Spue/Bloom

waxy vs. salt Include in here the thought that salts protect from deterioration...

[insert figure: IMAGE OF SPUE/BLOOM]

Blackening of Leather

Excess tans as well as non-tans migrate to the surface of the leather upon introduction of water or other polar solvents causing a darkening of the leather surfaces.

Increase in stiffness of leather

In leather that was exposed to high humidity or cycling conditions, tan breakdown and other products accumulate between the leather fibers causing them to stick and making the leather darker and stiffer, particularly in hydrolysable tanned leather (Calnan 1991b).

[See section Physical Deterioration]

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Conservation treatments for leather bindings

This information is intended to be used by conservators, museum professionals, and members of the public for educational purposes only. It is not designed to substitute for the consultation of a trained conservator.

To find a conservator, please visit AIC's Find a Conservator page.

In 2000 and 2011, surveys were conducted of leather conservation practices in common use in conservation labs (St. John 2000, and Teper and Straw 2011). These practices included the use of:

• repair materials (new leather, Japanese paper, western papers, book cloth, and other cloth)
• techniques (tissue hinges, linen hinges, board tacketing, board slotting, and sewing support extensions)
• adhesives used (paste, methylcellulose, PVA, Lascaux 498 and 360HV, gelatin, and hide glue)
• toning (aniline dye, acrylic, watercolors, etc.)
• surface treatments (SC6000, Renaissance Wax, Klucel G, red rot cocktail, leather dressing)

In 2018, a session at the annual AIC meeting discussed the use of leather in bookbinding. The proceedings were published (Hebert Kaye 2018) and also made into a wiki page: Use of Leather in Book Conservation. A 2020 AIC session, “Leather Selection and use” discussed treatment techniques.

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Preventive conservation

The most successful method of preserving leather and skin products is a good preventive conservation program. This program needs to include systematic collection care, handling and storage practices, as well as regular inspection and condition evaluation. For longer life of skin and leather objects is necessary:

• When possible, identify the general category of the skin product correctly (i.e. Vegtable tanned, sheep skin).
• Understand the product’s characteristics, including its deterioration features (i.e. 19th century leather exhibiting red rot and friability).
• Upgrade the general environmental measures, including controlling climatic conditions, minimizing light exposure, providing physical support, protecting from mishandling, soil accumulation and pest infestation.
• It is also important to inspect, evaluate, monitor and document an object’s condition, periodically, recording the urgency for conservation treatment.

Preferred environmental conditions align with general conservation practices for organic material: 55-65% RH, avoiding large fluctuations in temperature and RH, and cooler temperatures reduce chemical reactions (Calnan 1991b).

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Handling

Much of the damage caused to leather and skin products is due to improper handling. Therefore, there are a few essential rules for the safe handling of these objects:

• Be prepared for handling by having a clean area ready to receive the object.
• Consider the weight of the entire object before lifting, be aware that aged and deteriorated fibers cannot tolerate much physical stress.
• A good option is to use a rigid support or keep items in a box/container when moving them. If direct handling is necessary use both hands and support the object from underneath.

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Cleaning

Cleaning is an irreversible action so it is important to carry it out knowing as much as possible about the nature of the soil and the sensitivity of the object under treatment. In general, cleaning an object is necessary only to remove airborne soil accumulation. It is not possible to remove some surface soil by simple cleaning methods and other soils are not removable at all. Highly deteriorated objects cannot be cleaned by routine procedures so degraded surfaces should be protected in order to avoid cleaning. The principal cleaning techniques are:

• Vacuuming is the safest cleaning method if carefully executed. It uses a fine plastic screening and a vacuum cleaner with adjustable suction or a rheostat and a small standard nozzle attachment. An important caution is to protect the leather when screening between the leather and the nozzle.
• Dusting is the most frequently used technique. It can be combined with vacuuming. It can be carried out using soft brushes. Each time a material is brushed, surface material may be removed. Brushing can increase the danger of knocking off delicate pieces.

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Consolidation, Repairing, Stabilization, Structural Treatments

Prior to treatment, testing should be performed to determine the stability of the leather, and/or how it responds to certain materials. This can include MHT for TS to determine whether the use of moisture at room temperature is appropriate, or spot testing water or alcohols to see if they cause darkening of the leather.

General stabilization for leather can include mixtures of conservation-grade acrylic adhesives (e.g. Lascaux 498 HV®, Lascaux 360 HV®). One of the advantages of acrylic adhesives is their flexibility along with strength. Compared to naturally derived adhesives such as starch paste for instance, those acrylic adhesives have more resistance to biological organisms, and also have a lower water content. However, they are not reversible with water like wheat starch paste after drying, and contain additives such as plasticizers and

Leather repair techniques can also allow strips of non woven polyester fabric (e.g. Reemay®) or Japanese Tissue or other conservation grade paper, which can be toned for aesthetic integration with acrylic paints, watercolor or with conservation-grade acrylic adhesives. Texture fills made to match the grain pattern of the leather can be made using acrylic mediums and a support material (see BPG Use of Leather in Book Conservation/ Reidell). Losses can also be repaired by filling with new archival leather, matched according to the grain, thickness and color, using conservation-grade adhesives to insert fills.

Degraded and powdery leather fibers (see Red Rot Treatments) can be treated with Klucel G® (Hydroxipropylcellulose) in a low concentration (2% or less) in DI water or ethyl alcohol (IMS®), while some products ready made, like Cellugel® (Hydroxipropylcellulose and Isopropanol). Applied with a soft brush it can also be used to stabilize the surface and prevent further damage.

Structural treatments such as reshaping or removal of deteriorated previous structural repairs can be carried out by Humidification chamber. This can be used to reshape leather items, gradually increasing relative humidity within the chamber. Leather items can be reshaped using ultrasonic humidifier, especially when local treatment are required. While reshaping treatments are carried out, leather need to be supported in order to hold fragile areas in place also during the drying process. Nylon net can be used as temporary support as well as rigid foam boards can help during handling processes.

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Retouching

Where loss compensation is required aesthetic integration can be achieved using watercolor or acrylic paints that can be used to color matching of missing areas. QoR paints have been used to taxidermy treatments and are suitable to leather with an Aquazol binder soluble in alcohols.

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Pests and Fungi

Reducing RH to below 65% is shown to inhibit mold growth, along with increased air flow, raising temperatures, and removing the moisture source. Fungicides are used in leather manufacture, but not readily applicable to treatment and many have serious health and safety hazards. In some cases cold and heat can be used, but many fungi are resistant to even extreme conditions. (R. Thomson 2006)

For pests and insects, reducing the temperature to -25°C is often an effective treatment, although the eggs of some insects may survive (R. Thomson 2006). Anoxic treatments are also applicable.

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Historic and contemporary coatings and consolidants

[Please submit information on additional coatings and consolidants to page compiler to be added]

Lubricants applied with non-polar solvent will penetrate the leather but may migrate back to the surface when the solvent evaporates. Polar solvents may dissolve the tannins and bring them to the surface, causing darkening and embrittlement. The tight fiber network on aged and hardened leathers will likely prevent penetration of lubricants or consolidants. (Calnan 1991b)

See STEP and ENVIRONMENT publications for information on testing and analysis of different coatings and consolidants used in conservation. (Larsen et al. 1994) (Larsen and Environment Leather Project. 1995)

• Leather Dressing: consists of Lanolin and Neatsfoot oil and contains no wax. It was developed by the New York Public Library and tested by the US Department of agriculture (“Leather Dressing”, Talas website). Calhan recommends diluting neatsfoot oil with mineral spirits, Stoddards’s solvent or petroleum ether before use, because otherwise neatsfoot oil can increase the fat content too much, by up to 10% (Fredericks 1997). As rendered animal fat from cattle, neatsfoot oil is a non-drying oil with a major fatty acid component (for cow/sheep source of oil) of 30% palmitic, 30% steric, and 35% oleic (Horie 1987)(Horie 1987, 151). Lanolin is the grease derived from sheep fleeces, and is not a triglyceride like neatsfoot oil, but a lipid, or a mixture of esters of fatty acids and long-chain alcohols (ibid).

• Leather Saver: composition is proprietary, but is recommended for furniture and sold on Talas for bookbinding treatment. The label states that the oil bonds with leather fibers, and uses only the “finest oils available,” containing no neatsfoot oil (a neutral oil which the label says leaves books oily), and also has no sulfur. As the type of oil is not listed it is not known if the oil is drying or non-drying, and therefore its aging characteristics are also not known.

• SC6000: The composition of SC6000 was reformulated in 1996 to SC7400 to meet British Health and Safety Standards but is still supplied under the name SC600 (Brewer 2006, 33). SC6000 is a combination of natural and artificial waxes blended with a soft acrylic resin that was first formulated for the shoe industry in the 1970s and began to be used for book conservation in the 1980s (ibid, 34). Its Material Safety Data Sheets states that SC6000 contains 30-40% isopropyl alcohol and less that 1% aromatic hydrocarbon, there is 1% ammonia, which is used for emulsification, and diacetone alcohol (less than 5%) as a solvent present; the boiling point is 80 degrees Celsius (MSDS SC6000, 2016, from link on Talas). Its new formula is said to be more alkaline (Fredericks 1997, 6). One of the main uses of SC6000 is to block damage from atmospheric pollutants. The goal of blocking penetration of acidic atmospheric pollutants with SC6000 was evaluated by Haines (2002), and she notes that the current (2002) formulation of SC600 provided a less effective barrier as compared to the SC600 used in the research done in 1980 by the British Leather Makers Research Association (BLMRA) for the British Library. This was determined by the color change of vegetable tanned leathers exposed to pollutants, although the new formulation of SC6000 was still found to act as a barrier in comparison to other coatings (Haines 2002).

• Klucel G and Cellugel: Klucel G is a hydroxypropyl cellulose (HPC) polymer. Klucel is a product of Aqualon (formerly from Hercules) LLC and comes in six viscosities, H, M, G, J, L and E, with H having the highest molecular weight and viscosity, and E having the lowest molecular weight and viscosity. HPC is soluble in water and polar solvents (Horie 1987, 128). Cellugel is a ready-made mixture of hydroxypropyl cellulose and solvent. Research by Paula Steere found that the HPC is Klucel G with the solvent isopropanol, in a 3% weight-to-volume concentration of HPC to isopropanol (Steere 2018, 2). Concerns with the application of Cellugel and Klucel G are that they are large molecules and do not penetrate leather well, and when used with ethanol it can darken leather, but this might be mitigated by using it with isopropanol (Fredericks 1997: 6) The use of a polar solvent can also cause the leather to become embrittled (along with the darkening) because vegetable tannins are soluble in polar solvents and will migrate to the surface (Haines 2002, introduction) (Calnan 1991b). It is worth noting that alcohols are less polar than water, and therefore may be preferred.

• British Museum Leather Dressing: lanolin, beeswax, and cedarwood oil mixed with hexane to give a pasty consistency

• Pliantine: lanolin, beeswax, and cedarwood oil mixed with trichlorethane (Genkleen).

• Talas Dressing: lanolin, neatsfoot oil, and cedarwood oil mixed with hexane

• Cire 212 (BnF):

• Cire 213 (BnF):(http://editions.bnf.fr/sites/default/files/FDS%20Cire%20213_2015.pdf)

The BnF site says: “Produite et commercialisée par la Bibliothèque nationale de France, la cire 213 est spécialement recommandée pour l’entretien des reliures en cuir. C’est une émulsion à base d’huile de pied de bœuf qui a la propriété de lubrifier le cuir en même temps qu’elle le réhydrate. La cire apporte les éléments indispensables qui permettront à un cuir possédant encore de bonnes propriétés physico-chimiques de garder une certaine souplesse.”

An online retailer (“Cire 213” n.d.) described it as: “oil-based emulsion, colourless, derived from beef tallow, pH neutral or slightly alkaline so that the leather cannot be attacked, contains fungicide and insecticide as essential protection against biological pests”

The SDS sheet says it is a beeswax emulsion with glycerol trioleate, lists 15-20% Hydrocarbures, C8-C9, isoalcanes; and 0.1-0.5% biphényl-2-ol. The pH is listed at 8-8.5. It is very toxic to aquatic animals and long term effects are not known. Sites selling the product say is not meant to be used on very degraded leather, but as a preventive measure and to rehydrate cracked leather. Environment testing from 1996 found Cire 213 protected leather from pollutants, showing better tensile strength of treated sample against control (Larsen and Environment Leather Project. 1995). (Bibliothèque nationale de France and Direction des services et des réseaux - Département de la conservation n.d.)

Between 10 and 18% of neat foot oil, Paraffin 5%, Beeswax 5%, Ortho-phenylphenol (fungicide) 2%, Silicone oil 1%, Potassium oleate 1%, Emulsifier

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Examples of leather treatments

C2CC The Care of Leather and Fur

NPS Museum Handbook, Part I, Appendix S, Curatorial Care of Objects made from Leather and Skin Products, 1996. (“Part I, Appendix S, Curatorial Care of Objects Made from Leather and Skin Products” 1996)

[please submit additional resources to page compiler to be added]

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Leather Identification and Analysis

(Unless otherwise cited, information for this section is from Ch. 6, “Testing leathers and related materials” by Roy Thomson in Conservation of Leather and Related Materials)

Leather testing and analysis for conservation and research purposes falls broadly into three categories (as defined by Thomson, Ch.6 Testing leathers and related materials) (Roy Thomson 2011a)

• The determination of the type of raw materials. This includes species identification through visual examination and instrumental analysis
• The determination of tanning process
• Assessment of type and degree of deterioration.

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Species Identification

Magnification

Visual examination under magnification is often sufficient to identify the species of animal used for leather. Mammalian skin, used almost exclusively on book bindings, has three distance layers: the grain layer on the outer surface with the root of the hair follicles, the corium layer where much of the strength of the skin is, and the flesh layer that separates the skin from the muscle (B M Haines 2011b). The proportion of these layers, fiber size, fiber angle and fiber weave density differ for different species. The most accessible and often used approach for identifying species in examining the follicle pattern in the grain layer. The placement of follicles and follicle size can identify the species.

[insert table/figure: Visual identification of species from follicle pattern. (B M Haines 2011b, 17–19)]

Additional resources: (Duffy 2013)

Proteomics

Mass spectrometry analysis

Species identification of leather by proteomics relies on the detection of diagnostic markers from collagen. Differences in the amino sequences of the collagen chains of different organisms leads to different peptidic profiles that can be matched to species. The pool of peptides is obtained by solubilizing the proteins from the skin (for example with a solution of ammonium bicarbonate) and using a proteolytic enzyme to cut the proteins into smaller fragments (usually trypsin that cleaves proteins at the C-terminal side of arginine (R) and lysine (K) residues); the resulting peptides are made of up to 20-30 residues for collagen. Peptide mass fingerprinting for species identification relies on matching the peptidic profile of the unknown sample with known reference profiles.

Peptide mass fingerprinting uses mass spectrometry, in which molecules are ionized and identified by means of their mass-to-charge ratios (m/z). The resulting mass spectra are plots of the relative abundance of ions as a function of their m/z values. In MS mode, peptides are identified by their mass only; in MS/MS mode, a peptide is selected, isolated and fragmented in a collision cell, and the resulting fragments are acquired by a mass analyzer to form a spectrum that will be read as the amino-acid sequence of the peptide. The mass spectra generated are compared with protein sequences in databases.

The most common methods of ionization are matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI). In MALDI, gas-phase ions are formed when molecules mixed with a matrix are irradiated with a laser; the matrix absorbs the laser energy and instigates ionization of molecules as singly-charged ions. MALDI is often coupled with a time-of-flight mass spectrometer (TOF-MS) in which the ions are accelerated through a fixed electric field and their time of flight to reach the detector determine their mass-to-charge ratio (heavier ions travel slower). With this mode of analysis, samples are directly deposited on the MALDI plate and mixed with the matrix (typically 1 μL of analyte with 1 μL of matrix such as α-cyano-4-hydroxycinnamic acid). Most abundant or most easily ionized peptides are detected in a single mass spectrum, and MALDI-TOF-MS is therefore the favorite method used for taxonomic identification of specimens. Figure 7.1 illustrates how the process works by matching an unknown profile with references profiles of species.

[insert figure: schematic MALDI-TOF profiles of common animals with discriminatory peaks (Credit: Caroline Solazzo)]

Peptides can also be individually fragmented and identified in tandem MS mode (MALDI-TOF/TOF-MS) by their amino acid sequence. In ESI, the analyte is mixed with a solvent and injected through a tip, high voltage is applied to the sample which is dispersed in a spray and ions are created through desolvation (evaporation of the solvent). ESI is used for LC-MS/MS: the peptide solution (obtained from either in-solution digestion or in-gel digestion) is injected into a High Performance Liquid Chromatography (HPLC) or Ultra Performance Liquid Chromatography (UPLC) system for separation. For proteomics, the LC system is equipped with a reverse-phase column as the stationary phase and the mobile phase is a gradient of water miscible organic solvent (acetonitrile) with acidified water (e.g. 0.1% formic acid). Peptides are separated on the column by their hydrophobicity and transferred to the mass spectrometer through the ESI source where they are analyzed and fragmented resulting in a list of hundreds or thousands mass spectra that will be imported in search programs to compare with existing databases of proteins. Mass spectrometers used with ESI are, for example, quadrupole-TOF instruments or Orbitraps. ESI-LC-MS/MS allows characterization of more complex samples than MALDI-TOF and as such is preferred for studying degradation in ancient samples or for separation of complex protein samples.

ZooMS for species identification

ZooMS (zooarchaeology by mass spectrometry) is typically applied to the peptide mass fingerprinting method with MALDI-TOF-MS. It was developed for collagen in bones but has since been applied to skin tissues with success. In (Ebsen et al. 2019), over a hundred medieval shoe parts from Denmark were characterized by the grain pattern analysis and ZooMS to compare the efficiency of the two techniques. Of the 105 shoe parts tested, 37 had grain patterns preserved, and only 17 could be identified to species. ZooMS could confirm the identification in 13 of them, while the last four were identified with higher taxonomic resolution by the grain pattern (Brandt 2018). On the other hand, 71% of the 105 shoe parts (or 72% of 115 samples in a supplementary study (Brand et al. 2020) were identifiable to a species by ZooMS, making the rate of efficiency of this technique much higher. Brandt and Mannering (2021) extended their research to skin objects from the Viking period with poor preservation of grain patterns and were able to identify the nine objects to a species or family, thus characterizing the objects as belonging to equid, bovid or cervid families. The advantages of the ZooMS technique are summarized in (Ebsen et al. 2019):

• it requires only a very small amount of sample material;
• samples can be prepared in less sterile lab facilities than aDNA samples, and;
• the process of sample preparation and data analysis takes less time than the preparation of samples for aDNA and proteomics and because of this, amongst others, the method is much cheaper

Indeed, dozens of samples can be spotted on a MALDI plate and each spot takes only a few seconds to be analyzed, so the process is very efficient. With a good set of references and spectra of good quality (which is not always the case for highly degraded samples), a sample can be identified to a genus or species in a few minutes.

Species identification can also be done with LC-MS/MS, although it is a lengthier process. Proteomics search engines such as Mascot (from Matrix Science), PEAKS Studio (Bioinformatics Solutions Inc.) or Proteome DiscovererTM are used to import files of MS/MS spectra and searching them against public databases. Each peptide’s sequence is compared to sequences in a database to find the best alignment and is given a score according to the program’s algorithms. The software will then provide a list of all the proteins in which the peptides are found, assigning a score to classify proteins. In this way, a sample is generally matched to more than one species as peptides can be found in multiple organisms due to the conservative nature of proteins like collagen: proteins from related species will be matched with closed scores (Figure 7.2).

[insert figure: Example of protein identification of modern sheep leather with PEAKS. From left to right: protein accession number in NCBI, PEAKS protein score, protein percentage coverage, number of peptides identified, number of unique peptides per protein (within the database searched), types of post-translational modifications identified, Mr of the protein and protein description with [species] (Credit: Caroline Solazzo, unpublished data)]

Species markers in collagen proteins are used to validate the identification: Table X1 shows some markers that can be found in leather from the most common species (sheep, goat, cow, pig and horse), these markers often differ by only one amino acid. Figure 7.3 shows the MS/MS spectrum of peptide TGQPGAVGPAGIR in sheep leather.

[insert table: Examples of collagen markers to differentiate sheep, goat, cow, pig and horse. More markers can be found in (Izuchi et al. 2016) and (Cheung et al. 2021).]

[insert figure: MS/MS spectrum of TGQPGAVGPAGIR in sheep leather, Peaks score -10lgP=30.06, m/z=590.4285, z=2, and corresponding ion table (Credit: Caroline Solazzo, unpublished data)]

Using this technique, the identification of eleven skin garments from peat bogs in Denmark were published in 2014 (Brandt et al, 2014), and again compared to morphological identification, this time mainly of the hair on the skin by means of microscopy. The samples were identified by mass spectrometry as sheep, goat or cattle, only validating the microscopic analysis in six out of 12 samples. Other applications where proteomics analysis of leather is being developed is for gilded leather and metal threads in textiles. Leather metal threads is a variety of metal threads backed by an organic substrate (membrane, skin, or paper) on which metal is applied in the form of leaf or powder, on one side or both sides of the thin skin substrate. Proteomics has been used to identify the species in gilded leather and thread decorations in early 20th c. Chinese children hats (Cheung et al. 2021), as well as in medieval metal threads from Mongolia (Solazzo et al. 2020) and Europe (Scibè et al. 2020; Solazzo et al. 2019)

Proteomics analysis of tanned leather

Usually, the processing and tanning of leather results in most minor proteins being removed and the proteins remaining are collagen type I, alpha 1 and alpha 2 and collagen type III alpha 1. The number of peptides and protein’s percentage coverage for all three chains are shown here in a pilot study of bovine leather tanned with four different methods (unpublished data, Caroline Solazzo, @ MCI, Smithsonian Institution). Maximum coverage is obtained for the oil tanning and alum tawing methods while lower percentage coverages are seen in chrome tanning and vegetable tanning potentially highlighting that the more efficient a tanning method is, the less peptides we are able to extract.

[insert figure: MS/MS spectrum of TGQPGAVGPAGIR in sheep leather, Peaks score -10lgP=30.06, m/z=590.4285, z=2, and corresponding ion table (Credit: Caroline Solazzo, unpublished data)]

The basic principle behind tanning is to induce cross-linking between the collagen chains, thus preventing the degradation of the proteins by micro-organisms. However, other types of interactions between the tannin agents and the proteins have a role in stabilizing the collagen fiber structure (Covington, 2006). Alum tawing is not considered a true tanning technique; with alum tawing, little chemical bonding is introduced and the aluminium salts can be washed out, resulting in a skin with little water resistance (Reed, 1972, pp. 62-64). The oil-tanning method similarly produces a weak cross-linked leather; this method works by introducing large amounts of oil and fats into the fiber network of the skin then heating to oxidize the oil molecules, thus converting them to aldehydes (cross-linking the proteins) and polymers (filling the dermal network) (Reed, 1972, pp. 68-71). The unevenness and low degree of aldehyde tannage (Reed, 1972, pp. 68-71) has led this method to be called a leathering process (physical stabilizing effect) rather than a tanning process (chemical interactions with the triple helix) (Covington, 2006).

In chrome tanning, the collagen structure is stabilized with chromium (III) ions, not so much by crosslinking but by “locking” the molecules together (Covington, 2006). With the vegetable tanning method, solubilization of proteins in traditional proteomics extraction buffer is much harder; the tannins interact with collagen via hydrogen bonding at the peptide bonds and at the carboxylic acids and amino groups on side chains, and other covalent reactions (Covington, 2006). In her PhD thesis, Van Doorn (2012) also indicates low sequence coverage in vegetable tanned leather (39%) compared to chrome tanned leather (60%) possibly due to tannins extract bound to lysine residues and obstructing the enzymatic cleavage reaction.

RESOURCES

Brandt LØ, Schmidt AL, Mannering U, Sarret M, Kelstrup CD, et al. (2014) Species Identification of Archaeological Skin Objects from Danish Bogs: Comparison between Mass Spectrometry-Based Peptide Sequencing and Microscopy-Based Methods. PLOS ONE 9(9): e106875. https://doi.org/10.1371/journal.pone.0106875

Brandt, LØ 2018, High-definition urban fashion: Proteins reveal preferred resources for medieval leather shoes. in R Raja & SM Sindbæk (eds), Urban network evolutions: Towards a high-definition archaeology. Aarhus Universitetsforlag, Aarhus, pp. 241-247.

Brandt LØ, Mannering U. Taxonomic identification of Danish Viking Age shoes and skin objects by ZooMS (Zooarchaeology by mass spectrometry). J Proteomics. 2021 Jan 16;231:104038. doi: 10.1016/j.jprot.2020.104038. Epub 2020 Nov 3.

Brandt, L. Ø., Ebsen, J. A. and Haase, K. (2020) “Leather Shoes in Early Danish Cities: Choices of Animal Resources and Specialization of Crafts in Viking and Medieval Denmark,” European Journal of Archaeology. Cambridge University Press, 23(3), pp. 428–450. doi: 10.1017/eaa.2020.2.

Cheung Angela, Solazzo Caroline & Tsui Wai-shan (2021) Unveil the Gold – Revealing Metal Threads and Decorative Materials of Early Twentieth Century Traditional Chinese Children's Hats, Studies in Conservation, 66:6, 357-374, DOI: 10.1080/00393630.2020.1845922

Covington, A.D., 2006. The Chemistry of tanning materials, in Conservation of Leather and Related Materials, Eds. Kite M. and Thomson R., Butterworth-Heinemann, pp. 22-35

Ebsen, J.A., Haase, K., Larsen, R., Sommer, D.V.P., Brandt, L.Ø., 2019. Identifying archaeological leather – discussing the potential of grain pattern analysis and zooarchaeology by mass spectrometry (ZooMS) through a case study involving medieval shoe parts from Denmark, Journal of Cultural Heritage 39, 21-31. https://doi.org/10.1016/j.culher.2019.04.008

Izuchi Y, Takashima T, Hatano N. Rapid and Accurate Identification of Animal Species in Natural Leather Goods by Liquid Chromatography/Mass Spectrometry. Mass Spectrom (Tokyo). 2016;5(1):A0046. doi: 10.5702/masss pectrometry.A0046. Epub 2016 Jun 6.

Reed R, 1972. Ancient Skins, Parchments and Leather, Ed. SEMINAR PRESS LTD.

Scibè C., Solazzo C., Tosini I., Lam T., Vicenzi E., González López M.J., 2020 “Gilt leather threads in 11th-15th century textiles”, in Proceedings of the 11th Interim Meeting of The Leather & Related Materials Working Group, 6-7 June 2019 Paris (France), pp. 162-169

Solazzo Caroline, Jamsranjav Bayarsaikhan, Lee Boyoung, Scibè Cristina, Pearson Kristen “Understanding medieval textile production and provenance in the Darkhad Valley through biomolecular analyses”, Proceedings of the Mongolian Studies Conference, Washington DC February 2020

Solazzo C., Scibè C. and Eng-Wilmot K., 2019. “Proteomics characterization of "organic" metal threads - First results and future directions”, In: McGath, Molly, Research and Technical Studies Specialty Group Postprints, from the 47th Annual Meeting of the American Institute for Conservation in New England, May 13 – 18, 2019, Vol 7. Washington, DC: The Research and Technical Studies Group (RATS) and the American Institute for Conservation (AIC) pp. 78-82.

Van Doorn NL 2012. “The applications and limitations of a minimally destructive approach to archaeological proteomics”. Doctor of Philosophy thesis, Department of Archaeology, University of York, UK

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Tannage process analysis

Visual

For undyed skins, color can be used to ID tannage process. Hydrolysalble tanned skins are often lighter in color than condensed tanned skins, although dyeing can obscure this. In some cases the surface of the leather is dyed but the corrium and flesh remain lighter in color, helping to identify the tanning process.

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Chemical testing

Ash test

Chrome tanned leather will give a green ash Alum tawed materials will give a white ash Vegetable, oil and untanned materials will combust completely

Spot tests

Descriptions of these tests are left intentionally vague. Please see Ch. 6 in Conservation of Leather and Related Materials, or Odegaard Material Characterization Tests for Objects of Art and Archaeology for full description of testing procedures, and consult MSDS information for relevant chemicals prior to testing

• Vegetable tanning is identified using an aqueous solution of ferric chloride. A positive result for vegetable tannins is indicated by a blue-black or green-black color.
• Aluminum tanning is identified using aqueous ammonium hydroxide solution, sodium alizarin sulphonate solution in ethyl alcohol, and acetic acid solution. A positive test results in a red color that does not turn yellow with acidification.
• Condensed tannins are identified using vanillin and ethanol, and hydrochloric acid. A positive test results in a deep red color.
• Hydrolysable tannins are identified using sulphuric acid, rhodomine and ethanol, and potassium hydroxide. A positive test results in red color, indicating gallic acid from acid hydrolysis of hydrolysable tannins.

Instrumental

High power liquid chromatography

Using high power liquid chromatography (HPLC) extracts from leather can be tested for levels of tannin and monomers of the phenolic acids, and measured by optical density (OD/100mg). Aged hydrolysable tanned leathers show an increase in gallic and ellagic acid monomers. Aged condensed tanned leathers show a shift towards more hydrophobic compounds, as well as an increase in ellagic acid. (Larsen et al. 1994; Larsen and Environment Leather Project. 1995)

FTIR

See: (Falcao 2014; M., R., and M. 2017)

XRF

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Degradation analysis

(Larsen et al. 1994; Larsen and Environment Leather Project. 1995; Carsote and Badea 2019; Vyskočilová et al. 2019; Dirksen 1997)

One difficulty in assessing the level of degradation of leather objects is that the sampling location can have a significant impact on the results, and damage may not be consistent.

Researchers characterize the degree of degradation in a variety of ways, and some of the more common are mentioned here.

Degradation of larger polymers can be analyzed by looking at extractable monomers. The amount of N-terminal amino acid residue (from the breakdown of the polypeptide chain at peptide linkages between amino acids) can be a measure of deterioration with the increase in N-terminals indicating deterioration. Both hydrolysis and oxidation lead to the production of peptides.

The ratio of basic to acidic amino acids can be used as an indicator for degree of degradation, expressed as the B/A ratio (the sum in mol% of basic amino acids to the sum of the mol% of acidic amino acids). The B/A for new leather is around 0.69, and 0.50 for artificially aged leather. Common deterioration reactions of amino acids result in deamination (loss of amine group) and decarboxylation (loss of carboxyl group), or the conversion of one amino group to another (trananimation). These reactions result in the formation of products that suggest specific types of deterioration. The presence of sulphates has been used as an indicator for hydrolytic deterioration, and ammonia as an indicator for oxidative deterioration.

The deterioration of tannins can be categorized in this way, and as mentioned above, amino acid profiles are used to look at very specific degradation of free and chain terminal amino acids.

Hydrolysis by sulfuric acid leads to the formation of sulfates. Analysis of sulfate contents has been used as an indicator of deterioration by acid hydrolysis (which does not take into account hydrolysis by other acids).

Shrinkage temperature is used as a measure of degradation of leather. It is the temperature at which leather shrinks under specified conditions. Shrinkage temperature can roughly be correlated to collagen denaturation, the breakdown of the higher structural levels of collagen. As the structurally supportive hydrogen bonds and tannin crosslinks break down, the collagen structure is disrupted and the shrinkage temperature decreases. There is a direct relationship between shrinkage temperature and degree of degradation.

Visual

Color

[Red-orange color indicates red-rot, typical of condensed tanned leather.]

Fiber cohesion analysis

Information from ENVIRONMENT leather project (Larsen and Environment Leather Project. 1995)

Scraping procedure:

The fibre sample is taken from the corium part (flesh side) of the leather.

1) The fibres are scraped off with the point of a scalpel turned upside down (photo 1 ).

2) The freed fibres (photo 2) are assessed on the basis of the reference photos and the written definition of the 5 states.

1 = Fibres very coherent 2 = Fibres coherent + slightly powdery 3 = Fibres moderate coherence + moderate powdery 4 =Fibres slight coherence + powdery 5 = Fibres no coherence + very powdery

Fibres given the rank 1 are in a good physical state and 5 a terminal state of physical disintegration.

Physical properties

[Is leather stiff, abrade easily...]

Observations on contact with water and alcohol

A low-tech solution to determining the stability of leather, especially in relation to treatment, is to take a sample of fibers, and under magnification add a small amount of water or alcohol to observe the result. If the fibers denature readily with water, water-based adhesives should be avoided. This test can also be performed at different temperatures (room or liquid), because Ts is also affected by heat, and can be raised in cold conditions.

Chemical tests

Shrinkage temperature analysis

For MHT and DSC information see: (Carsote and Badea 2019; Larsen and Environment Leather Project. 1995; Larsen et al. 1994)

Micro Hot Table (MHT)

Fiber samples are wetted and slowly heated using specialized equipment to measure the temperature the collagen fibers denature. This is determined by observing the shrinking and movement of fibers under magnification. Three intervals are recorded:

• Interval A· Distinct shrinkage activity is observed in individual fibres.
• Interval B· Shrinkage activity in one fibre (occasionally more) is immediately followed by shrinkage activity in another fibre.
• Interval C· At least two fibres show shrinkage activity simultaneously and continuously. The start temperature of the interval is interpreted as the shrinkage temperature, Ts, and the length of the interval, fl. Ts=Te-Ts, corresponds to the shrinkage interval. Te is the end temperature of the interval.

For precise measurements, specialized equipment is necessary, such as the Linkam LTS120 stage equipped with a temperature controller and Linksys32 temperature control software which enables full PC programming of temperature.

Differential Scanning Colorimetry (DSC)

Measures the temperature when a phase transition occurs, which studies have found is close to the Ts for leather.

pH Testing

Because pH is measured based on dissociation in solution, pH of leather is measured using an aqueous extract. Standard Test IUC 11 requires 5g of leather to 100mL of water, but when this sample size is not possible for historic items, the ratio must be maintained. It has been found empirically that leather with a pH above 3.2 is unlikely to have quantities of strong acids significant enough to result in red rot, and leathers with a pH 2.8 or less are likely to have red-rot.

Sulphate content

pH alone cannot be used alone to determine level of degradation and concentration of sulphate is also necessary. It is possible for degraded leathers with a low Ts to have a normal pH, but high sulphate content. This is because the formation of ammonium sulphate can raise the pH, and that the leather could have previously had a low pH due to sulphuric acid content that had since been neutralized, and all sulphate present can be assumed to have existed as free sulphuric acid at some point. (Florian 2011)

Other than sulphuric acid, many sulphur compounds were used in leather production and may not always indicate a risk to the leather.

Fat content analysis

See (Larsen and Environment Leather Project. 1995)

Moisture content analysis

This is done by weighing the sample at a certain temperature and RH, and the drying the sample, with the material lost presumed to be water. New leathers are expected to contain 14% water, and aged deteriorated leathers 10%. (Larsen and Environment Leather Project. 1995)

HPLC / Amino Acid content analysis

Amino acids within the protein can deteriorate through three main pathways, de-amination (loss of an amine group), de-carboxylation (loss of a carboxylic acid group), and transamination (mutation of one amino acid to another). Researchers can use what they know of amino acid profiles of new and aged leathers to determine degradation pathways.

​​“The ratio of basic to acidic amino acids can be used as an indicator for degree of degradation, expressed as the B/A ratio (the sum in mol% of basic amino acids to the sum of the mol% of acidic amino acids). The B/A for new leather is around 0.69, and 0.50 for artificially aged leather. Common deterioration reactions of amino acids result in deamination (loss of amine group) and decarboxylation (loss of carboxyl group), or the conversion of one amino group to another (trananimation). These reactions result in the formation of products that suggest specific types of deterioration. The presence of sulphates has been used as an indicator for hydrolytic deterioration, and ammonia as an indicator for oxidative deterioration. (Florian 2011)

About 0.2 mg (0.1 mg of chrome tanned and untanned skins) of corium sample is hydrolysed, in an evacuated and sealed glass ampoule, for 24 hours at 110 ° C in a solution consisting of 300 μ1 redistilled HCI, 15 μ1 2 % 3,3'-dithiodipropionic acid (DIDPA) in 0.2 M NaOH and 15 μl phenol in water. After hydrolysis the amino acids are separated by ion exchange HPLC on a 12.5 x 0.46 steel column packed with MCI CK 10 U resin using a gradient system and post column derivatisation with OPA (o-phthalaldehyde). Further details on the method and equipment can be found in the references I, 2 and 3. The amino acids in the standard mixture etc. are listed in table 1 with the standard abbreviations.” (Larsen and Environment Leather Project., 1995)

Degradation of larger polymers can be analyzed by looking at extractable monomers. The amount of N-terminal amino acid residue (from the breakdown of the polypeptide chain at peptide linkages between amino acids) and be a measure of deterioration with the increase in N-terminals indicating deterioration. Both hydrolysis and oxidation lead to the production of peptides. (Florian 2011)

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Use of Leather in Bookbinding

Leather has been used to cover all or parts of a book for as long as books have been bound. The most common use of leather for bookbinding is as a spine covering, but full leather bindings are also very popular, as are leather tips or corners. Leather can also be found in doublures, hinges, sewing supports, endbands, and various other pieces of the book anatomy due to its flexibility, durability, ease of decoration, and availability.

Historically, most leathers used for any purpose were vegetable tanned, with small portions being tawed, mineral, smoke or brain tanned. Books were bound in the leather available to them, but most historical exemplars were bound in alum tawed pigskin or vegetable tanned calf or goat. In the current day, almost 90% of all modern leather is chrome tanned, but book binders persist in a preference for vegetable tanned leathers for their workability, flexibility and strength.

The proceeding definitions of paring and leather decoration used in bookbinding are taken from Matt T. Roberts and Don Etherington's Bookbinding and the Conservation of Books, published 1982, and available digitally at http://cool.conservation-us.org/don/don.html.

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Paring

The process of thinning leather by cutting away the flesh side, or shaving the edges, i.e., beveling the edges that are to be turned in. A paring machine is generally used for the thinning process (or a SPOKESHAVE if no paring machine is available), while a paring knife is used for shaving or beveling.

Very little if anything is known of the method or methods used by binders to reduce thickness in the early days of covering books with leather, but it is entirely possible that from about the latter part of the 16th century they purchased leather from the manufacturer in the required thickness and then simply pared the edges.

During the 19th century there were no paring machines in use in binderies, nor were there any spokeshaves. There is no evidence of any paring of leather other than edges during the first half of the 19th century; consequently it must be assumed that the leather was purchased already pared, or was purchased and then sent out to be pared as required.

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Decoration

Tooling

Finishing has assumed a very important role in the craft of bookbinding since the earliest times. Almost all early finishing, at least in Europe, was in blind until the latter half of the 15th century, when gold tooling was introduced into Italy. In modern finishing. all but the simplest designs are measured out and drawn or tooled on thin paper. This is then positioned on the cover and heated tools are pressed through. The paper is then removed, and the blind impressions are again blinded-in. This sharpens and deepens the impressions, and, if gold is to be used, provides a smooth flat surface for the metal. In addition to making it possible to execute extremely difficult patterns without making errors on the leather itself, the use of a paper pattern eliminates the necessity of making basic guide lines in blind upon which the design is then built, and which almost invariably show beyond the tooling. It is uncertain when paper patterns were introduced, but they probably were not used much before 1830. Not all leathers can be tooled successfully. Aside from the great difficulty encountered in tooling chrome-tanned leathers, only those vegetable-tanned (or tawed) leathers with surfaces firm enough to hold a line, such as goatskin, calfskin, pigskin, etc., are suitable. With the exception of sheepskin, leathers that are loose and stretchy do not retain impressions very well.

1. Gold Tooling
   The art or process of lettering and/ or decorating the spine and covers of a book with GOLD LEAF (or, at times, other metals, e.g., platinum) impressed into the covering material, usually leather, by means of a heated letter, lettering pallet, or finishing tool. In the traditional method of gold tooling, the lettering or design is first blinded in, generally first through paper, and then again directly on the leather. The second working of the tool polishes the base of the impression and assists in creating a particular brilliance in the tooling. An adhesive (glair) is applied to the leather (either all over or directly into the blind impressions); strips of gold leaf are laid over the impressions and held in place temporarily with a thin film of vaseline or grease; and the gold is then pressed permanently into place with the heated tool When done properly, the affinity of the gold for leather is such that it will practically never come off; nor will it tarnish. Gold tooling must be ranked as one of the most important innovations in the history of bookbinding. Its origin are somewhat obscure, but it was probably introduced into Europe by way of Italy, and spread throughout the rest of Europe and England, eventually ar riving in America. There is some evidence that the technique may have been practiced in Morocco in the 13th century, but this is not conclusive. It has also been proposed that gold tooling was introduced into Italy by way of Persia (now Iran), where bookbinding and gilding flourished in the early decades of the 15th century. Very early gold tooling is difficult to evaluate because it is uncertain whether the gold was actually impressed into the leather with a (hot) tool, or was painted into blind impressions. The evidence offered by some bindings, i.e., the absence of impressions deep enough to indicate tooling, as well as what appear to be brush marks in the gold, would seem to indicate painting. Because of the elapsed time, however, which has led to the inevitable deterioration of the materials, it is difficult to differentiate between the two techniques. In any event, books were actually being tooled in gold in Venice no later than 1470, and possibly several years earlier. Gold-tooled leather bindings were not common in England before about 1530, and not in the United States until about 1669. The universal adoption of gold tooling was by no means immediate, and, in fact, blind tooling was still the predominant form of decoration until about 1580, or even 1600.
   
2. Blind Tooling
   A method of decorating a book in which impressions are made in the covering material, usually leather or tawed skin, by means of heated tools, pallets, rolls, fillets, or combinations of one or more of these. As the name implies, blind tooling does not entail the use of leaf metal, foil, or any other coloring material, with the possible exception of carbon, which is sometimes used to darken the impressions. The effect of blind tooling rests largely on the depth and uniformity of the impressions (which makes it unsuitable for use with hard covering materials) and the ability of the heated tool to produce a darkened color (see above)—factors which make leather, especially in the lighter shades, an ideal medium for this method of decoration. The critical aspects of the technique are the temperature of the tool and the degree of dampness of the leather. In general, the damper the leather the cooler the tool should be, and vice versa. In tooling leather blind, the surface is given a quick initial strike to "set" the leather in the impression. The tool is then impressed again and rocked slightly, which polishes and darkens the impression. When blind lines run across the spine of the book, polishing is accomplished by sliding a pallet along the lines; on the covers, where a fillet is used for long lines, it is fixed so that instead of rolling, it slides along the impression. Blind tooling has been used as a means of decorating books since the early days of bookbinding, and can be traced back to Coptic bindings of the 7th or 8th centuries, and even earlier. There is reason to believe that the technique was brought to Europe from the Mediterranean area about the same time as other Coptic techniques being used, possibly by imported craftsmen; however, little is known of blind-tooled bindings until the 12th century and early part of the 13th. In one form or another, the technique has been used continuously up to the present day, but during the 16th to 18th centuries, its use was more or less limited to inferior calf- and sheepskin bindings. Near the end of the 18th and during the early years of the 19th centuries blind tooling was often used on fine bindings in conjunction with gold. Also called "antique tooling."
   

Tree Calf

A form of cover decoration consisting of a smooth, light-colored calfskin treated with chemicals in such a manner as to represent a tree trunk with branches. In the usual manner, a dual design appears on upper and lower covers. The leather is first paste-washed, and the book is then hung between two rods which keep the covers flat. The book is tilted so that it inclines upward towards its head. In order to bend the boards outwards, i.e., warp slightly to a concave shape, so that the solutions will run properly, the insides of the hoards are not filled in until the decoration is done. A small amount of water is applied to the center of both covers to form the trunk, then more water is thrown on the covers so that it runs down to the trunk and to a central point at the lower edge of the hoards. Copperas (a green hydrated ferrous sulfate) is then sprinkled in fine drops on the covers, followed by potassium carbonate (salts of tartar). which causes the chemical reaction that etches the leather to form a permanent pattern in shades of gray, ranging from faint to very dark. Calfskin was used for this decoration in preference to sheepskin (although 19th century examples of sheepskin tree do exist) because in addition to being a much superior leather, it takes a better polish, which suits this style of decoration admirably. The spine of the book is protected during the marbling so that it will not be touched by the water or chemicals. The entire process calls for considerable experience and dexterity of execution, because if the result is to be effective the copperas and potassium carbonate must be applied in the correct amounts, as well as in the proper manner, while the initial water is still running down the covers; otherwise the effect will be little more than sprinkling. Late in the 19th century attempts were made to produce the tree calf effect with the use of an engraved block, which was used to print a design on the covers in black, but the results were ineffective because the block did not provide the shading which the genuine method achieved. The popularity of tree calf began to decline before the First World War, and by the late 1920s this once very popular form of decoration had virtually passed from existence. The first known tree calf decoration dates from about 1775.

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Polishing

The process of smoothing and adding gloss to the covers of a book by mechanical means. The process involves working the leather at first with a slightly warm tool, followed by repeated workings with polishers heated to higher temperatures. Small circular motions are used to prevent wide areas of darkened streaks from showing. The technique of polishing leather covers dates back at least to the second half of the 14th century.

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Leikin, Sergey, Donald C. Rau, and Vozken Adrian Parsegian. 1994. "Direct Measurement of Forces Between Self-Assembled Proteins: Temperature-dependent Exponential Forces Betweeen Collagen Triple Helices." Proceedings of the National Academy of Science 91: 276-280.

M., Mansour, Hassan R., and Salem M. 2017. “CHARACTERIZATION OF HISTORICAL BOOKBINDING LEATHER BY FTIR,SEM -EDX AND INVESTIGATION OF FUNGAL SPECIES ISOLATED FROM THE LEATHER.” Egyptian Journal of Archaeological and Restoration Studies 7 (1): 1–10. https://doi.org/10.21608/ejars.2017.6823.

McLean, William. 1997. “J Hewit & Sons Ltd - Skin Deep - Volume 4 - The Manufacture of Leather - Part 4.” 1997. https://www.hewit.com/skin_deep/?volume=4&article=2.

McCrady, Ellen. 1981. "Brief Survey of the Investigations on the Deterioration of Leather." The Abbey Newsletter 5(2). Accessed January 16, 2020.

McCrady, Ellen. 1980. "A New Consolidation Technique." The Abbey Newsletter 4(1).

McCrady, Ellen. 1981. "Dressing Leaflets Compared." The Abbey Newsletter 5(2). Accessed January 16, 2020.

McCrady, Ellen. 1981. "Research on the Dressing & Preservation of Leather." The Abbey Newsletter 5(2): 22-25. Accessed January 16, 2020.

McCrady, Ellen, and Toby Raphael. 1993. "Leather Dressing: To Dress or Not to Dress." Conserve O Gram 9(1).

McCrady, Ellen. 1990. "How Leather Dressing May Have Originated." Abbey Newsletter 14(1). Accessed January 6, 2020.

McLaughlin, George D., and Edwin R. Theis. 1945. "The Chemistry of Leather Manufacture." Journal of Chemical Education 22(11). New York : Reinhold Publishing Corporation.

Metzger, Chela, Deborah Howe, and Gillian Boal. 2003. "Use of Adhesives on Leather Discussion." AIC Book and Paper Group Annual 22: 99-104.

Middleton, Bernard C. 1972. The Restoration of Leather Bindings. Chicago : American Library Association.

Miles, Christopher A., Ghelashvili, Michael,. 1999. “Polymer-in-a-Box Mechanism for the Thermal Stabilization of Collagen Molecules in Fibers.” BPJ Biophysical Journal 76 (6): 3243–52.

Nandi, P. K., M. E. Grant, D. R. Robinson. 1985. "Destabilization of Collagen Structure by Amides and Detergents in Solution." ""International Journal of Peptide and Protein Research 25(2): 206-212.

Nesi, A., Rossignoli, G., Salvioli, N., Sperimentazioni tecniche di restauro e conservazione dei cuoi del museo Stefano Bardini, in: Lo Stato dell’arte, Atti del IV congresso Nazionale IGIIC, Siena 28-30 Settembre 2006, Nardini, Firenze 2007, pp. 239-245.

O'Flaherty, Fred, William T. Roddy, and Robert M. Lollar. 1956. The Chemistry and Technology of Leather. New York : Reinhold.

Owen, Grace and Sarah Reidell. 2011. "TIP: Cast Composites: a System for Texturing Repair Materials in Book Conservation ." Topics in Photographic Preservation 14: 250 – 262. Also presented in the “Library Collections Conservation Discussion Group 2010: Conservation in the 21st Century: Revisiting Past Practices and Their Evolution in Institutional Settings (PDF).” Book and Paper Group Annual 29.

“Part I, Appendix S, Curatorial Care of Objects Made from Leather and Skin Products.” 1996. In NPS Museum Handbook.

Petherbridge, Guy. 1987. Conservation of Library and Archive Materials and the Graphic Arts. Society of Archivists Institute of Conservation, Butterworths : London. Chapter III "Vellum and Parchment" 181-217 and Chapter IV "Books and Bindings" 218-266.

Perminova, Olga I. 1994. "The Preservation of Leather Bindings of Cartographic Atlases." IFLA Journal 20(3): 306-311.

Plenderleith, Harold James. 1946. The Preservation of Leather Bookbindings London. London: British Museum.

Raphael, Toby, and Ellen McCrady. 1984. "Leather Dressing - A Misguided Tradition?" ICOM Committee for Conservation, 7th Triennial Meeting Copenhagen, 10-14 September 1984, Preprints. 84186-84188.

Reed, Ronald. 1972. Ancient Skins, Parchments and Leathers. London, New York : Seminar Press Ltd.

Reich, G. 1999. “Communications - 1998 Atkin Memorial Lecture - The Structural Changes of Collagen During The Leather Making Processes.” Journal of the Society of Leather Technologists and Chemists. 83 (2): 63.

Roberts, Matt and Don Etherington. 1982. "Bookbinding and the Conservation of Books: A Dictionary of Descriptive Terminology." Washington D.C. : Library of Congress. Accessed January 21, 2020.

Rogers, Jerome Stanley and C. William Beebe. 1956. Leather Bookbindings: How to Preserve Them. Washington D. C. : Government Printing Office.

Rulfs, Charles L. 1979. "The pH of Things as They Are Part III: Surface pH of Leathers." The Abbey Newsletter 3(1).

Rulfs, Charles L. 1979. "The pH of Things as They Are Part IV: Chaos." The Abbey Newsletter 3(2).

Scibè, Christina, Solazzo, Caroline, Tosini, Isetta, Lam, Thomas, Vicenzi, Edward, and Lopez, Maria Jose Gonzalez. 2020. “Gilt Leather Threads in 11th-15th Century Textiles.” Proceedings of the 11th Interim Meeting of the ICOM-CC Leather and Related Materials Working Group, 162–69.

Sclawy, Adrian Conrad. 1981. "Method for Preparing Leather for Slurrying." The Abbey Newsletter 5(2). Accessed January 21, 2020.

Silverman, Randy, Anthony Cains, Glen Ruzicka, Paula Zyats, Sarah Reidell, and Olivia Primanis, Alan Puglia, Priscilla Anderson, Don Etherington, Bill Minter, David Brock, and Friederike Zimmern. 2006. "Conservation of Leather Bookbindings: a Mosaic of Contemporary Techniques." In Conservation of Leather and Related Materials. 225-243. Oxford: Butterworth-Heinemann. 225-243.

Discusses damage assessment, consolidation (Klucel G, "red rot cocktail", and Lascaux 498HV), facing adhesives, board reattachment techniques (joint tacketing, tissue or cloth hinge and joint mends, solvent-set repair tissue, split hinge board attachment, and board slotting), and adhesives for old and new leather.

Smith, Richard Daniel. 1964. The Preservation of Leather Bookbindings from Sulfuric Acid Deterioration. University of Denver : Master's Paper.

Solazzo, Caroline, Jamsranjav, Bayarsaikhan, Lee, Boyoung, Scibè, Cristina, and Pearson, Kristen. 2020. “Understanding Medieval Textile Production and Provenance in the Darkhad Valley through Biomolecular Analyses.” In . Washington DC.

Solazzo, Caroline, Scibè, Christina, and Eng-Wilmot, K. 2019. “Proteomics Characterization of ‘Organic’ Metal Threads - First Results and Future Directions.” In , 7:78–82. The Research and Technical Studies Group (RATS) and the American Institute for Conservation.

St. John, Kristen. 1997. "An Annotated Bibliography on Leather Dressing." The New Library Scene 16(2): 14-29.

St. John , Kristin. 2000. "Survey of Current Methods and Materials Used for the Conservation of Leather Bookbindings." Book and Paper Group Annual 19. Accessed January 21, 2020.

Report of a survey on leather conservation practice, focusing on special collections library treatments. Discusses frequency of use of types of repair materials, techniques, adhesives, toning, and surface treatments. The author concludes that the conservators surveyed employed a wide variety of techniques, increasingly choose minor mends over rebacks and rebindings, and have adopted newer materials like Japanese tissue, Klucel G, and waxes.

Storch, Paul. 1987. "Curatorial Care and Handling of Skin Materials." Museum Handbook Part 1. National Park Service.

Tapia, Guillermo. 1960. "Preservation of Leather Bindings." American Book Collector 15(9): 11-12

Thomson, R. 2006. “Procter Memorial Lecture 2005: The Deterioration of Leather.” JOURNAL- SOCIETY OF LEATHER TECHNOLOGISTS AND CHEMISTS 90 (4): 137–45.

Thomson, R. S. 1991. “The English Leather Industry, 1790-1990 : The Case of Bevingtons of Bermondsey.” J. Soc Leather Technologists & Chemists 75: 85. Thomson, Roy. 2001. Bookbinding Leather : Yesterday, Today and Perhaps Tomorrow. Wolstenholme Memorial Lecture 2000. [Northampton]: [B Journal of the Society of Leather Technologists and Chemists].

Thomson, Roy. 2011a. “Testing Leathers and Related Materials,” In Care and Conservation of Leather and Related Materials, 8.

———. 2011b. “The Manufacture of Leather,” 16.

———. 2011c. “The Nature and Properties of Leather,” 3. Vyskočilová, Gabriela, Matthäa Ebersbach, Radka Kopecká, Lubomír Prokeš, and Jiří Příhoda. 2019. “Model Study of the Leather Degradation by Oxidation and Hydrolysis.” Heritage Science 7 (1): 26. https://doi.org/10.1186/s40494-019-0269-7.

Thompson, J.C., On restoring sacred objects, in: “Leather conservation news: the newsletter, journal of treatment and materials research in the specialty of leather conservation”, vol. 14, no. 2, The leather conservation centre, Northampton 1998, pp. 1-5;

Teper, Jennifer Hain and Melissa Straw. 2011. “A Survey of Current Leather Conservation Practices (PDF).” Book and Paper Group Annual 30: 131-151.

Report of a survey on leather conservation practice that builds on Kristin St. John's 2000 survey. Discusses frequency of use of:

• repair materials (new leather, Japanese paper, western papers, book cloth, and other cloth)
• techniques (tissue hinges, linen hinges, board tacketing, board slotting, and sewing support extensions)
• adhesives used (paste, methylcellulose, PVA, Lascaux 498 and 360HV, gelatin, and hide glue)
• toning (aniline dye, acrylic, watercolors, etc.)
• surface treatments (SC6000, Renaissance Wax, Klucel G, red rot cocktail, leather dressing)

Vitagliano, Luigi, George Nemethy, Adriana Zagari, and Harold A. Scheraga. 1993. "Stabilization of the Triple-Helical Structure of Natural Collagen by Side-Chain Interactions." Biochemistry 32(29): 7354-7359.

Wallace, Everett Leland. 1955. "Leather Research and Technology at the National Bureau of Standards: A Review and Bibliography." U. S. Dept. of Commerce, National Bureau of Standards. Circular no. 560. Washington DC : US Government Printing Office. Accessed January 21, 2020.

Wallace, Everett L., Charles Critchfield, and John Beek Jr. 1935. "Influence of Sulphonated Cod-liver Oil on Deterioration of Vegetable-tanned Leathers by Sulfuric Acid." Journal of Research of the National Bureau of Standards 15: 73-77.

Waterer, John William. 1972. A Guide to the Conservation and Restoration of Objects Made Wholly or in Part of Leather. New York : Drake Publishers, Inc.

Wessel, Carl J. 1970. "Environmental Factors Affecting the Permanence of Library Materials." The Library Quarterly: Information, Community, Policy 40(1): 39-84. Accessed January 20, 2020.

Wingate, R. B. 1977. "Kitchen Chemistry and the Care of Books." AB Bookman's Weekly 59: 3506-3507.

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History of This Page

Prior to the creation of the AIC Conservation Wiki, this page was created as "Section 4 - Chapter 1 - Skin/Leather" of the Book Conservation Catalog by Jennifer Hain Teper, with assistance from Cara Bovet and Melissa Straw. For more see: History of the BPG Wiki.