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This page is currently being reorganized and edited as a generic materials page to be linked out to specialty applications. Please see BPG Animal Skin and Leather and BPG Use of Leather in Book Conservation for additional information

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Preparation of animal skin to produce leather or parchment at J Hewit & Sons (David Lanning, 2018)

Leather and skin objects include artifacts made using the hides, tissues and inner membranes of animals. These may include objects made from parchment, guts, or rawhide as well as tawed, oil tanned, brain-tanned, vegetable tanned, or mineral tanned leathers. The raw materials for skin and leather products are obtained from a wide range of animals, all of which are composed of the protein collagen. Various procedures are carried out to prevent putrefaction and impart specific working properties to the finished product. Untanned and cured products can be manipulated by processes such as cutting or sewing. Decoration and protective coatings are often applied. This sections below discuss the materials used to create 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- and skin-containing artifacts and artworks.

Chemical Composition of Untanned Hides

Collagen Molecules

Figure 2.1: Basic structure of an amino acid

(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 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).

Figure 2.2: Condensation reaction of two amino acids to form a peptide bond (SOURCE)

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)

Figure 2.3: Molecular diagram of Hydroxyproline (SOURCE)

Figure 2.4: Single helical polypeptide chain of tropocollagen (B M Haines 2011a)

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.

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 which 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). (B M Haines 2011a).

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 (B M Haines 2011a).  The repeated coiling structure within the collagen molecule gives strength to the fibril.

Chemical and Physical Alterations Through Tanning

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 an increased hydrothermal stability (an increased Ts), an 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.

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.

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.

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).

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.

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.

Tanning techniques and manufacturing

Full tan

The most popular methods for producing full-tanned leathers include mineral, aldehyde, and vegetable tanning methods. These methods can also be combined. It is sometimes difficult to differentiate between the different tanning methods, especially if the leather has been dyed or is very degraded. Vegetable tanned leather is the most common type of leather found in western cultures. There are a variety of tests that can indicate the presence of tannins that are characteristic of vegetable tanned leather, or differentiate between the types of tannins produced by different plants used in the tanning process.

Vegetable tan

(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)

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 [add charts with tannin types and characteristics Katie and Kristi have charts]

Sources of Vegetable tannins of commercial importance (Calnan 1991b, 16)

Hydrolysable/pyrogallol

Hydrolysable (also known as pyrogallol) tannins can be subclassed into two subcategories:

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

Characteristics of Hydrolysable tannins:

• Are sugar based – mostly glucose, though also contain some larger polysaccharides [see Figure 4,2]
• 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
• 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).
• Content more non-tans compared to condensed tannins (Betty M. Haines 1991).

[Image: Hydrolysable tannin molecular structure, gallotannin and ellagitannin]

Condensed/catechol tannins

Figure: The flavonoid ring system of condensed polyphenols (A. D. Covington 2011)

Example of condensed tannin: mimosa

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.

[Figure: Condensed tannin molecular structure]

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.

Figure: Model of hydrogen bonding polyphenols and peptide link in protein (A. D. Covington 2011)

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).

Figure: The structure of mimosa tannin (A. D. Covington 2011)

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)

Pre-tanning

• 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.

Figure: Historic illustration showing removal of hair from skin with two-handled knife (Roy Thomson 2011b)

Figure: Historic illustration showing removal of hair from skin with two-handled knife (Roy Thomson 2011b)

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.

Figure: Historic illustration showing the handling of hides in weak tan liquors (Roy Thomson 2011b) Note: this is from Diderot - maybe use the original source?

Figure: Historic illustration showing the handling of hides in weak tan liquors (Roy Thomson 2011b) Note: this is from Diderot - maybe use the original source?

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 which 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.

Pre-tanning

• 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. Katie has images of staking and splitting machines as well as vat and wheels for bating and washing
• 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 19^th century, due to increased demand and importation, condensed tannins from natural hardwoods (such as quebracho,  mimosa, mangrove or gambier) began to be used, to 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 being 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 chrom 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 effect (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 (for a definition of syn-tans see section 4.2.2.2.)

STRIPPING AND RETANNING

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.  

Vegetable tanning outside Europe

*This section is not complete. 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 was 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 the use of the juice of limes for deliming, and the citric acid in the limes reacting with the lime to form calcium citrate, and insoluble salt with buffering capacity (Betty M. Haines 1991).

Mineral tan

[Mineral Tanning Chemistry:]

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).

Salts are added to a wide variety of leathers today for different reasons, and many leather are stripped and retanned and have multiple tannins.

Chrome tanning

• Introduced in the turn of the 19^th 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 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, allow 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.

semi-Aluminum (III) salts in veg tan process

• Alum is often used in conjunction with vegetable tanning to enhance dyeing.

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 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.

Manufacture (mineral tan)

(NEED TO ADD!!)

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)

Semi-Tan

There are a variety of ways to produce semi-tanned leathers, but brain tanning, smoke tanning, and alum tawing are the most common methods. Well preserved or new brain tanned leathers tend to have ìa soft suede-like nap,î and are plum and flexible. When deteriorated, the leather becomes stiff, deformed, and turns from a light buff color to a grayish-brown color (CCI Notes 8/4, 1). Smoke tanned leathers exhibit many of the same tactile qualities as brain tanned leathers, but also frequently have a characteristic smoky smell and amber brown color (CCI Notes 8/4, 1). Alum tawed leather is flexible and light colored. It also tends to have a fine nap that makes it soft (CCI Notes 8/2, 1). Because of its flexibility and soft finish, semi-tanned leather is typically used in clothing, including jackets, moccasins, leggings, and gloves. This kind of leather is also used for accessories such as pouches (CCI Notes 8/4, 1).

Alum tawing

• 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.

Manufacture

(DP)

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 the 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.

Oil-tanned skin

Brain-tanned, Buckskin, smoke tanned, indigenous practices

• 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.

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 it’s 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.

Non-tan

Parchment and rawhide are two non-tanned skins that may be encountered on a regular basis. Please see the books and paper chapter on parchment in this wiki for more information about parchment. Rawhide is comprised of de-fleshed and depilated animal skin that has been dried out. This type of leather has not been treated and is very tough and rigid. This makes it ideal for use in drum heads, parfleches, and shields (CCI Notes 8/4, 1).

Finishing Treatments

Historically, coatings have been applied to seal, waterproof, or decorate leather. On horse-drawn vehicles, coated leathers have been used to create hoods, dashboards, and fenders as well as decorative trimming. Coated leathers are also found on apparel.

Japanned or Patent Leather

The process of rendering leather waterproof through application of multiple layers of a baked-on colored glossy coating was first patented in England by Charles Frederick Mollersten in 1805 (Anon 1806, 229). In the United States, Newark, NJ inventor Seth Boyden began producing a similarly made waterproofed leather c. 1819, referred to as patent leather (Bishop, Freedley, and Young 1864). Distribution of patent leather became more wide spread after 1822. This same material is also referred to as japanned leather in literature (Proctor 1922, 473). Though what results is a glossy, often black colored smooth surface similar to what one sees on lacquered or japanned decorative arts, the manner in which this coating was produced was different.

Splits of vegetable tanned cow hide were used in the 19th century as a substrate for coated leathers to be used on vehicles (FitzGerald 2007, 30). Various historic accounts supply recipes for the coating material thus variations on artifacts are likely to exist, but typically contained linseed oil as a major component with whale oil, horse grease, Prussian blue, lamp black, gum benzoin, gamboge, and litharge as possible additives (Seeley and Sutherland 1991). According to period literature, prior to coating, the hides were degreased to enhance the bond with the coating. Some makers applied a coarse grade of japan directly against the leather as a sort of primer before applying subsequent coats. Each layer was baked in a stove at temperatures between 60-93°C (140-200°F) and polished before the next layer was applied (Proctor 1922, 477.) After a number of layers of this coating was built up, the surface was finished with a top coat, referred to as a varnish. Depending on the time period and the location in which the japanned leather was produced, the varnish layer may have consisted of resins such as copal, amber or shellac; combined with gums; linseed oil; and/or asphaltum with red lead or litharge used as driers (Seeley and Sutherland 1991) or nitrocellulose (Proctor 1922, 481).

During their period of use, japanned leather on horse drawn vehicles may have been polished using a slurry of egg white, sugar and lamp black mixed with spirit of wine as a maintenance procedure (FitzGerald 2007, 314). Another period recommendation was to use a mixture of wax, olive oil, lard, oil of turpentine, and oil of lavender to polish coated leathers (Seeley and Sutherland 1991).

While 19th and early 20th century trade manuals describe the manner in which japanned and patent leather was produced, little technical analysis has been done to determine whether the descriptions are accurate (Ravenel 2011).

Enameled Leather

Coated leathers that are referred to as enameled have the coating applied to the grain side of the leather, often used for hoods on vehicles. FitzGerald points out that the grain split may be used for coated leathers other than enameled, but in these cases the coating is not applied to the grain but the split side (FitzGerald 2007, 31). While japanned leather became available commercially after 1822, enameled leather was not produced until after 1836 (Proctor 1922, 476).

Other Coatings on Leather

• Paint
• Nitrocellulose

As early as 1897, nitrocellulose was being used as a coating material on leather (Proctor 1922, 483).

• India Rubber

Seeley and Sutherland describe black pigmented finishes containing india rubber used as early as 1858 (Seeley and Sutherland 1991, 24).

Other Decorative Finishing Techniques

Gilding

Paint

Tooling

Identifying Leather

Many aspects can alter the appearance of animal skin leather such as tanning, leather treatments, and deterioration from use or burial. Despite this manipulation, identification of the type of skin the conservator is preserving may still be possible. The grain, hair follicle pattern, collagen fiber configuration, use of the leather, and infrared spectroscopy can offer clues to what animal the leather came from.

Grain and hair follicles

Perhaps the easiest method of identifying leather is by looking at it under magnification. With a microscope or even a loupe, the conservator can sometimes discern how the grain and hair follicle pattern is arranged. These two characteristics are sometimes good indicators of the animal that the leather came from. [1]

Leather from cattle skin has hair follicles that are mostly “equal in size and arranged in regular rows” (Haines 2006, 17). Calfskin follicles are smaller and closer together, and adult cattle skin has larger follicles. No matter the age of the animal from which the leather comes, the follicle pattern will be the same. Leather from goat skin has alternating rows of large and fine hair follicles that form clusters.The follicles in sheep skin leather are all the same size and are relatively small. Like goat skin, the follicles of sheep skin are arranged in clusters (Haines 2006, 19).

Collagen configuration

Under greater magnification, cross sections of the leather in questioncan reveal the configuration of the collagen layers, which can be diagnostic of the species from which the leather came.

In leather made from cattle, the nature of the skin changes depending on the age of the cattle. Mature cow skins are thicker, usually between 4 and 6 mm (Haines 2006, 12). The grain layer, which constitutes the outer portion of the skin, contains hairs that are equally spaced (Haines 2006, 13). The corium layer consists of thick fibers, which resemble a tangled mass beneath the grain layer.Figure 5. Cross section of mature cattle skin leather. (Haines 2006, 13).Skin and leather from calves is thinner than adult skin, but becomes thicker with age. While calf skin and leather has the same structure as adult skin, the corium layer is much thinner and has finer bundles of fibers (Haines 2006, 14).

Goat skins range in thickness between 1 and 3mm, and the grain layer takes up a greater portion of the skin (about a third of its thickness) than the grain layer in cow skin. Corium fiber bundles are fairly fine and the difference between the grain and corium layers is rather indistinct (Haines 2006, 14).

Sheep skin leather sometimes includes the wool, and this may be a useful clue to what species of mammal the skin is from. The type of wool the sheep produces determines what the skin and leather cross sections will look like. Sheep that grow light wool tend to have thinner skins (.8mm average) with a finer corium layer. Sheep that produce heavy, copious wool have thicker skins (2 to 3mm) with a coarser corium layer. The corium layer of sheep skin tends to be relatively loosely woven, and there is a layer of fat between the grain layer and corium that makes the two layers loose, and makes the leather drape well (Haines 2006, 14-15).

Deer skin is relatively thick (2 to 3mm), and has a thin grain layer with a loosely woven corium.

Pig skin has a coarse grain with indistinguishable grain and corium layers. The hair shafts pass all the way through the skin, rather than stopping in the grain layer. The fibers of the skin are compact and form a distinct basket weave pattern (Haines 2006, 15).

Use

When trying to identify leather, the conservator may sometimes be able to take into consideration how the leather was used. Thick leathers tend to be unacceptable for clothing or shoe uppers. Because of this, adult cow skin is often used for sole leather, harnesses and horse tack, and machinery belts. Adult cattle skin can be split to make it thinner and more malleable. When split, the leather can be used for shoe uppers, upholstery, or case leather (Haines 2006, 13). Calfskin, on the other hand, does not have to be split as it is already fairly thin leather. The compactness of the fiber weave means that the leather, although thin, is still unacceptable for clothing. Calfskin leather is strong and makes for good footwear, handbags, and bookbinding.

Likewise, the compact weave of pig skin leather makes the substance ideal for bookbinding and case leather (Haines 2006, 15).

Sheep skin leather and leather from young goats are loosely woven and are ideal for making gloves and other clothing articles that require the leather to have a soft handle and good drape (Haines 2006, 14-15).

Spectroscopy

Infrared spectroscopy is yet another way of determining the species from which leather came. While this may be a useful technique, it does require that the surface layer of the leather be sloughed off to remove any scum that has collected over the years and any finishing agents that may skew the results. Although only a small section would have to be prepared in this way, the conservator would definitely have to weigh the costs and benefits of this method.

Although all mammalian skins are all primarily composed of collagen, the chemical signature of the types of collagen in different animal skins varies between species. The infrared spectrometer is able to pick up these differences, and report them in the form of a spectrum that can tell the conservator or scientist what the constituent parts of the leather are. In order to be able to discern what species the leather comes from, the scientist or conservator might have to make a database of known leather samples to compare to the unknown samples (Shao 2005, 49-50).

Tanning Techniques

When conservators must treat leather artifacts, it is important to be able to determine how the leather was tanned. The leatherís tanning and finishing may give the conservator some idea of how the object will continue to deteriorate, or how it may react to certain treatments.

Non-tanned leathers

Parchment and rawhide are two non-tanned leathers that may be encountered on a regular basis. Please see the books and paper chapter on parchment in this wiki for more information about parchment. Rawhide is comprised of de-fleshed and depilated animal skin that has been dried out. This type of leather has not been treated and is very tough and rigid. This makes it ideal for use in drum heads, parfleches, and shields (CCI Notes 8/4, 1).

Semi-tanned leathers

There are a variety of ways to produce semi-tanned leathers, but brain tanning, smoke tanning, and alum tawing are the most common methods. Well preserved or new brain tanned leathers tend to have ìa soft suede-like nap,î and are plum and flexible. When deteriorated, the leather becomes stiff, deformed, and turns from a light buff color to a grayish-brown color (CCI Notes 8/4, 1). Smoke tanned leathers exhibit many of the same tactile qualities as brain tanned leathers, but also frequently have a characteristic smoky smell and amber brown color (CCI Notes 8/4, 1). Alum tawed leather is flexible and light colored. It also tends to have a fine nap that makes it soft (CCI Notes 8/2, 1). Because of its flexibility and soft finish, semi-tanned leather is typically used in clothing, including jackets, moccasins, leggings, and gloves. This kind of leather is also used for accessories such as pouches (CCI Notes 8/4, 1).

Full-tanned leathers

The most popular methods for producing full-tanned leathers include mineral, aldehyde, and vegetable tanning methods. These methods can also be combined. It is sometimes difficult to differentiate between the different tanning methods, especially if the leather has been dyed or is very degraded. Vegetable tanned leather is the most common type of leather found in western cultures. There are a variety of tests that can indicate the presence of tannins that are characteristic of vegetable tanned leather, or differentiate between the types of tannins produced by different plants used in the tanning process.

The ferric test involves applying a ferric reagent to the leather which can indicate the presence of tannins in leather. If tannins are present, the area where the ferric reagent was applied will turn grey or black (Falcao and Araujo 2010, 152). Although the ferric test is a useful indicator of tannins, it is limited because it cannot detect what kinds of tannins are present in the leather and it can permanently mark the surface. There are two different kinds of tannins, condensed and hydrolysable, that are found in plants. The presence of one or the other kind of tannin may indicate what specie or group of plants was used to tan the leather. Determining the type of plants used might be of diagnostic use for artifacts whose origins are unknown.

Vanillin and acid butanol can be used to detect the presence of condensed tannins in leather. Plants that contain condensed tannins include quebracho, oak, and mimosa (Falcao and Araujo 2010, 151). The vanillin test consists of adding drops of vanillin reagent to the leather sample, and then adding two drops of HCl after the vanillin is absorbed. A red color in the test zone indicates that condensed tannins are present in the leather.

The acid butanol test must be carried out on individual leather fibers. Leather fibers are added to acid butanol reagent. Then, iron reagent is added, and the mixture is heated. If a red-orange to red-crimson product appears in the solution, condensed tannins are present (Falcao and Araujo 2010, 152-153). Nitrous acid can be used to detect the presence of ellagitannins, which are a type of hydrolysable tannin. Myrabolans, chestnut, Valona, oak, and divi-divi are plants that contain ellagitannins (Falcao and Araujo 2010, 151).

In the nitrous acid test, leather fibers are added to HCl and heated. Then, aqueous sodium nitrate is added and the mix continues to be heated. After about 20 minutes, the formation of a progressive blue color in the solution indicates the presence of ellagitannins (Falcao and Araujo 2010, 153).

Rhodanine can be used to test for gallotannins, which are another type of hydrolysable tannin. Plants containing gallotannins include tara, Aleppo, sumac, and Chinese (Falcao and Araujo 2010, 151). In this test, leather fibers are tested with rhodanine reagent, followed by aqueous potassium hydroxide. If a pink formation appears on the tested fibers, gallic acid is present (Falcao and Araujo 2010, 153).If the leather tests negative for tannins, the leather may have been tanned using other methods. Mineral (e.g. chrome) tanning replaced vegetable tanning by the end of the 19th century, and was used widely throughout Europe (Falcao and Araujo 2010, 149).

Chrome tanned leather can sometimes be identified by ìa blue-green line on a cut edgeî that indicates where the mineral salts used for the tanning penetrated into the leather (CCI Notes 8/2, 1).

Mineral tanned leather can also be identified by burning a small piece of the leather until the residual carbon is burned off. The presence of a green ash indicates that chrome (or occasionally aluminum or zirconium) is present in the leather (Identifying Chrome Tanned Leather).

Aldehyde tanned leathers are those that were tanned using formaldehyde or other aldehyde compounds. After tanning, these leathers are white in color and are supple (Aldehyde Tannage).

Technology

Processing

Parchment

• [Composition/Chemistry/Characteristics]

• [Manufacture]

Gut

Rawhide

Leather and tanning

• Archaeological Leather

Archaeological leather must be stabilized; this is necessary because it may be missing a percentage of its original tannins. Tannins, as the name implies, is part of the tanning process; it is a polyphenolic compound that binds to and precipitates proteins. Conservators must fully understand their leather in order to treat it, without understanding the tanning process or environment in which it was found, a conservator has less of a chance in being able to aid an artifact. Their goals, put simplistically are to: dry waterlogged leather and treat desiccated leather so it does not continue to lose its original integrity; this may include “humidifying” it in order for it to acquire a stable water balance.

Leather can be made in a variety of ways. A conservator needs to understand the tanning process before treating the archaeological leather because depending on the leather, different stabilizing techniques should be used (Smith 2003). There are a variety of archaeological leathers but the most common tanning processes found are: smoking, oil process, and vegetable tanning. Vegetable tanning, in conjunction with smoke tanning were most likely the first methods of tanning used (Florian 2007; May 2006). After the pre-tanning process, which includes cleaning, de-hairing, de-liming, bating and pickeling, the skins are placed in a tannin bath for 3-4 months (Florian 2007; May 2006). Vegetable tannins are organic compounds that can be found in most trees and shrubs; the tannins are water-soluble and contain polyphenolic substances. The hides are placed in multiple baths to create the desired grain color and to make sure they are fully tanned (May 2006).

Polyphenolic substances are polymers of phenol monomers. Vegetable tanning is effective because phenols are astringent, which means they are able to bind with the hydrogen bonds of collagen (Florian 2007). Collagen makes up most of the skin structure in leather; collagen is the fibres and fibre bundles that retain and are stabilized during the pre-tanning and tanning process (May 2006). Phenols are “water-soluble, weakly acidic and easily oxidized under alkaline conditions; they form complexes with metals (iron and aluminum in particular); they act as antioxidants; they are inhibitors in auto-oxidation of lipid compounds; and they are toxic to micro-organisms” (Florian 2007). Many of these properties are initially beneficial in the tanning process; they help stabilize the animal hide. However, over time, these vegetable tannins may contribute to leather deterioration (Florian 2007).

The smoking process is less complex for the tanner to complete. In simplest forms, the clean skin is exposed to smoke from burning wood. The skins become preserved because of an aldehyde mixture (formalehyde) given off from the partial dry distillation of the burning wood along with the other resinous material in the wood (Florian 2007). This process was popular among American Indians, not only because of the ease of making the leathers but also because of the positive qualities that came with smoke-tanned leather. The hide could conceivable be waterproofed because the polyphenolic tannage in wood could combine with formaldehyde; this combination formed an insoluble phenol formaldehyde resin (Florian 2007). The formaldehyde may continue to help the hide by precipitating the naturally occurring globular proteins; this may create a biocide because of the cross-linkage with amines (Florian 2007).

For the oil process, oils were simply trampled into the skins; traditionally the tawyer worked the mixture in by using his bare feet (May 2006). This process is effective because oily unsaturated fats oxidize once exposed to air; “the tanning action is due to hydrolytic and oxidation products reacting with the collagen” (Florian 2007). Once fully oiled, the skins were hung in warm, spacious stoves for the oxidation process to occur; the oiling and heating process occurs about four time before being completely washed off in alkaline liquors (May 2006). The alkaline is added to remove excess oil; the hide is then worked till it is softened. Oil-tanned leather does not tend to be affected by red rot; it is tough, washable and durable (Florian 2007).

Once the tanning process is complete, leathers may be subject to “fat-liquoring, staining, dying, graining or embossing, plating, boarding, enamelling or abrading” (Florian 2007). The conservator must be aware of the leather’s life history after the tanning process in order to account for other factors that may affect the treatment of leather. Without stabilization, leather can continue to worsen its condition post excavation; frequent changes in relative humidity can negatively affect the archaeological material. The tanning process is such that it does not allow conservators to simply add more tannins to the leather in order to properly conserve the artifact (Volken 2001). This is why is it so important to understand the tanning process of archaeological leather found; it adds insight in how to treat it.

• Alum Tawing

• Oil Tanning

• Brain tanning

• Vegetable Tanning

• What is Vegetable Tanning?

• Process used for Historic Vegetable Tanning

• Mineral Tanning

Finishing Treatments

Historically, coatings have been applied to seal, waterproof, or decorate leather. On horse-drawn vehicles, coated leathers have been used to create hoods, dashboards, and fenders as well as decorative trimming. Coated leathers are also found on apparel.

Japanned or Patent Leather
The process of rendering leather waterproof through application of multiple layers of a baked-on colored glossy coating was first patented in England by Charles Frederick Mollersten in 1805 (Anon 1806, 229). In the United States, Newark, NJ inventor Seth Boyden began producing a similarly made waterproofed leather c. 1819, referred to as patent leather (Bishop, Freedley, and Young 1864). Distribution of patent leather became more wide spread after 1822. This same material is also referred to as japanned leather in literature (Proctor 1922, 473). Though what results is a glossy, often black colored smooth surface similar to what one sees on lacquered or japanned decorative arts, the manner in which this coating was produced was different.

Splits of vegetable tanned cow hide were used in the 19th century as a substrate for coated leathers to be used on vehicles (FitzGerald 2007, 30). Various historic accounts supply recipes for the coating material thus variations on artifacts are likely to exist, but typically contained linseed oil as a major component with whale oil, horse grease, Prussian blue, lamp black, gum benzoin, gamboge, and litharge as possible additives (Seeley and Sutherland 1991). According to period literature, prior to coating, the hides were degreased to enhance the bond with the coating. Some makers applied a coarse grade of japan directly against the leather as a sort of primer before applying subsequent coats. Each layer was baked in a stove at temperatures between 60-93°C (140-200°F) and polished before the next layer was applied (Proctor 1922, 477.) After a number of layers of this coating was built up, the surface was finished with a top coat, referred to as a varnish. Depending on the time period and the location in which the japanned leather was produced, the varnish layer may have consisted of resins such as copal, amber or shellac; combined with gums; linseed oil; and/or asphaltum with red lead or litharge used as driers (Seeley and Sutherland 1991) or nitrocellulose (Proctor 1922, 481).

During their period of use, japanned leather on horse drawn vehicles may have been polished using a slurry of egg white, sugar and lamp black mixed with spirit of wine as a maintenance procedure (FitzGerald 2007, 314). Another period recommendation was to use a mixture of wax, olive oil, lard, oil of turpentine, and oil of lavender to polish coated leathers (Seeley and Sutherland 1991).

While 19th and early 20th century trade manuals describe the manner in which japanned and patent leather was produced, little technical analysis has been done to determine whether the descriptions are accurate (Ravenel 2011).

Enameled Leather
Coated leathers that are referred to as enameled have the coating applied to the grain side of the leather, often used for hoods on vehicles. FitzGerald points out that the grain split may be used for coated leathers other than enameled, but in these cases the coating is not applied to the grain but the split side (FitzGerald 2007, 31). While japanned leather became available commercially after 1822, enameled leather was not produced until after 1836 (Proctor 1922, 476).

Other Coatings on Leather

• Paint
• Nitrocellulose

As early as 1897, nitrocellulose was being used as a coating material on leather (Proctor 1922, 483).

• India Rubber

Seeley and Sutherland describe black pigmented finishes containing india rubber used as early as 1858 (Seeley and Sutherland 1991, 24).

===Identification===
If appropriate: visual examination, microscopy, spot tests, instrumental techniques, etc. In order to reduce overlap, generic identification issues could refer to the appropriate RATS wiki section such as Materials Testing, Analytical Techniques, or Technical Studies.

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).

Effects of Degradation

• Red Rot (see veg tan section)

• 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 6.1.2, Physical Deterioration

• 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).

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).

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) 44

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) 45

Degradation caused by water

[Deterioration of Waterlogged Archaeological Leather (link to archeological page)]

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). *NB it was not clear from the literature whether the solubilized tannins were free in the leather are bonded, nor the effect the removal of these tannins has on the tannage of the leather. Calnan notes that evidence suggests that the physical changes in leather are likely due to the adhesion of fibers caused by the movement of water soluble materials, rather than the degradation itself. These points left me to consider the use of polar solvents when treating vegetable tanned leather even when leather does not have a critically low Ts (OS).

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).

Leather is a product created from the skin of animals, which is processed in order to make it “non-putrescible even under warm moist conditions” (Thomson 2006). Made from the corium, the thickest layer of the skin, leather derives its strength from the collagen fibers that compose it. When viewing leather that is in good condition, the smooth “grain” side shows a pattern of hair follicles, while the rough “flesh” side shows the ends of collagen fibers. Water makes the skin pliable during an animal’s life, This flexibility is maintained in leather by replacing the water with another substance, such as oil, during the tanning process (Cronyn 1990). This process also helps prevent leather from rotting, even if it becomes wet (Thomson 2006). Some leather products only undergo a partial tanning process, resulting in semi-tanned leather. Other products, like rawhide, and parchment, are processed without tanning (Cronyn 1990). But even in the best of situations, leather does not survive burial well. Waterlogged leather is even less likely to survive a long burial. Waterlogged environments create a unique set of issues for leather, and its condition when excavated is influenced by both pre- and post-depositional factors (Cameron et al 2006). The tannins used to tan some types of leather can react with iron in the environment, darkening the leather as iron tannates are formed. As the pH rises, the color of the leather will darken (Cronyn 1990). Leather can also darken due to saturation by other components of the burial environment (Cameron et al 2006). As a result, the original color of the leather can be obscured, especially if it was light colored. In spite of these color changes, leather may appear to be in good shape, even after a long period of burial. This being said, the oils and tannins used to tan the leather may be leached out, making it less flexible as it dries. Penetration by water can also cause hydrolysis of collagen fibers. This process makes the leather weaker, increasing the likelihood that it will crumble if mishandled. Fragmentation is a greater problem as hydrolysis and detanning progresses. Splitting is another type of damage often encountered with waterlogged leather. The leather splits into layers due to discontinuity between the collagen fibers of the grain and flesh layers of leather. When tanning agents fail to penetrate the leather completely, splitting is more likely to occur. As a result, many waterlogged leather artifacts are found in many pieces, if they are found at all (Cronyn 1990). Some types of leather products, such as rawhide or semi-tanned leather, rarely survive damp conditions. This is due to the hydrolysis and breakdown of collagen fibers in the leather. However, there are some conditions that will help preserve leather. In wet environments, the presence of copper and heart oak slows decay (Cronyn 1990). This is due to the toxic nature of copper; where copper is present, bacteria cannot survive, preventing them from breaking down the leather. Certain woods, such as heart oak, contain tannins, so the penetration of these tannins from the environment can help maintain the leather. In marine environments, the lack of bacteria may help preserve leather (Cronyn 1990). Generally, only vegetable-tanned leathers survive wet anaerobic deposits, though the reason for this preservation is not completely understood (Cameron et al 2006). The treatment of waterlogged leather is necessary for several reasons. When leather dries, it shrinks. This shrinkage increases the brittleness of the leather, also increasing the likelihood that the leather will fragment. Additionally, salt and silt can penetrate the leather, which will cause abrasion whenever the leather is flexed, wet or dry. Wet leather is also prone to fungal growth, as the components of leather provide a good food source for other organisms. Finally, wet leather, when untreated, will continue to react with the water it contains and continue undergoing hydrolysis (Cronyn 1990). Although the preservation of leather in waterlogged environments is limited, even fragmented leather can provide valuable information to archaeologists. In order to be able to decide on the proper treatment for waterlogged leather, it is important for the conservator to understand the general properties of leather and how its components are affected by the introduction of water. Although waterlogged environments can cause irreversible damage to leather artifacts, certain conditions exist that can also help preserve leather.

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)

Chemical deterioration

(Unless otherwise cited, information for this section is from Ch. 5, “The Mechanisms of deterioration in leather” by Mary-Lou E. 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 (see section 6.2 for deterioration of different tannages) but also the environment (see section 8.1 for preventive measures).

Leather is usually a chemically stable material; it tends to physically deteriorate before being severely chemically altered. Chemical deterioration in leather occurs when the environment in which it is stored becomes unstable or polluted. The chemical deterioration of leather is due to a combination of oxidation and hydrolysis (May 2006). "Oxidation of the amino acid involves altering the side chains which can result in breakdown products and finally ammonia" (Florian 2007). This process can begin with: oxygen, pollutant atmospheric gases, heat, light or free radical action (Florian 2007) (May 2006). "Hydrolysis involves breaking the amino acid and carboxyl bonds and can cause either separation of the chain or the release of the amino acids from the chain" (Florian 2007). Once hydrolysis begins, the rate may exponentially increase. Hydrolysis and oxidation can interact and drastically deteriorate leather. Leathers are not all equal; leathers processed with condensed tannins do not age as well as leathers tanned with hydrolysable tannins (May 2006). Rapid decay can occur when the environment alters significantly, especially when there are influxes of pollution. Sulfur dioxide in the air, for example, may oxidize and hydrate to form sulfuric acid within a leather structure (May 2006). Sulphuric acid is formed from sulphur dioxide air pollution. Sulfuric acid in leather is known as red rot. Sulfuric acid may either originate from pollution or had been added during the tanning processes (May 2006). Acid hydrolyses may be caused by the absorption of sulphur dioxide by the tannin in leather; alkaline soaps have the potential to concentrate and cause alkaline hydrolysis (Florian 2007). Free radicals may arise from the autoxidation of lipids; free radicals may begin to start the oxidation process and arise if the tannin polyphenols break down (May 2006). Deterioration in cleaner environments is caused by oxidative change (May 2006). Leathers in polluted areas absorb sulfur dioxide twice as rapidly if they were produced with condensed tanning materials as opposed to processed with hydrolysable tannins (May 2006). Acidic environments in highly polluted atmosphere can suppress oxidative reactions (May 2006). Oxidation and hydrolytic reactions occur in chemical degraded leather, this can occur in both polluted and unpolluted environments. The exact degradation is dependent to the conditions in which the leather is subject to as well as the process in which the leather was tanned. Leather exposure to adverse conditions may cause or increase the rate of oxidation and hydrolysis as well as initiate new reactions, which would in turn, further the deterioration of the leather; some of these factors are: extremes in water vapor, heat, chemicals from the tanning process, light and air pollutants (Florian 2007).

Figure 6.2: 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 a breaking of bonds selectively in the polymeric structure of collagen, leading to it losing its structural integrity and becoming gelatin ( see Figure 6.2). 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.

[Hydrolytic breakdown of Collagen]

[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 of 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.

[Image of Oxidative breakdown of Collagen**]

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.

Degradation properties of different tans

Full tanned

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).

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 were 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, where as sulphuric acid had a sufficiently low pH to displace the calcium (Dirksen 1997; Betty M. Haines 1991).

Mineral Tans

(Broad sentence on mineral tan degradation–Maybe even just that Chrome is the most common and the focus here.

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.

Semi Tanned

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.

Non-tanned

Gut skin

Rawhide

Parchment

Degradation of leather manufacture components/finishes and additives

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)

Spue (spew) /Bloom

[IMAGE OF SPUE/BLOOM]

Patent Leather Coatings[edit | edit source]

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  Red-brown patent leather fender on a c. 1900 Jourbert & White (Glens Falls, NY) Buckboard Surrey showing one kind of surface coating deterioration seen on patent leather panels in transportation collections. Dr. and Mrs. W. Seward Webb collection, Shelburne Museum, 1947-18.4. Photo by Laura Brill.

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  Patent leather panel on a c. 1886 Brewster & Co. Victoria sleigh(Brewster #746). The bottom portion of the panel is covered with carpet, removed for the purpose of this photograph, which has protected the coating from degradation. Dr. and Mrs. W. Seward Webb collection, Shelburne Museum,1947-18.27. Photo by Laura Brill.

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  The same patent leather panel as that shown to the left, on a c. 1886 Brewster & Co. Victoria sleigh (Brewster #746), as normally displayed. Photo by Laura Brill.

Nineteenth-century patent leather coatings soften in the presence of water or heat, despite claims of their makers of the material's resistance to water and hot materials. In his carriage-trimmer's manual originally published in 1881, FitzGerald included instructions for separating patent leather panels that have been stored with their coated sides together in warm conditions (FitzGerald 2007, 313). As the coating ages, it may drip, sag, crack and/or form raised scabby islands on the surface of the leather support. Various panels may exhibit different kinds of deterioration on the same vehicle, depending on the nature of the panel's exposure to light and the environment. Note that the coating remains smooth and glossy where the patent leather panel had been protected by a rug on the patent leather panel on Shelburne Museum's Brewster & Co. Victoria sleigh. On the same panel the area just above where it is more exposed, but still somewhat protected by the wool fall under the seat, the coating has formed a fine traction crackle with small raised islands and a pebbly texture. Where the coating was fully exposed to the environment, a wide traction crackle has formed with large scabby islands.

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  Sample site F from a c. 1886 Brewster & Co. Victoria sleigh's fender. Dr. and Mrs. W. Seward Webb collection Shelburne Museum, 1947-18.27. Photo by Laura Brill. The site was sampled with the hope of revealing the stratigraphy of the coating in the raised islands as well as the the glossy valleys.

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  Sample F as viewed under visible light, 100x. The sample included the leather substrate, which can be seen at the bottom of the section. The coating layers consist of a grey colored ground layer against the leather, a lighter brown layer that has flowed around the darker layer above it to create the outer wall of the raised coating island. Sample preparation and photo by Laura Brill.

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  Sample site N from a patent leather panel on a c. 1886 Brewster & Co. Victoria sleigh (Brewster #746) Dr. and Mrs. W. Seward Webb collection, Shelburne Museum,1947-18.27. Photo by Laura Brill. This protected area was sampled with the hope of revealing the stratigraphy of a minimally deteriorated coating.

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  Sample N as viewed under visible light,100x. The sample included the leather substrate which can be seen at the bottom of the section. The coating layers consist of a black colored ground layer against the leather, a coarse grey colored layer above the ground and a thick black translucent layer on top of the grey. There may be a thin varnish layer above the thick black layer. Sample preparation and photo by Laura Brill.

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  Sample N as viewed under ultraviolet light,100x. The thick black layer appears to be heterogeneous. Sample preparation and photo by Laura Brill.

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 Observations

Color

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

Darkening/black

Fiber cohesion analysis (René Larsen)

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?

Does it abrade easily?

Observation of contact with water and alcohols

A low-tech solution to determining the stability of leather, especially in relationship 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. (SOURCE)

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)

Preventive Conservation

For material/object type specific issues regarding recommendations for storage and display, handling, inhibitive conservation measures, supports, mounts, labeling, transport, condition surveys, monitoring, etc. In order to reduce overlap, general preventive care issues should refer to or be discussed in the appropriate Preventive Care section of the main AIC wiki.

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. Vegetable 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).

Documentation

For general recommendations, please refer to the Objects wiki article on Conservation Practices or for general conservation work practices, please refer to the main AIC wiki section on Work Practices.

There are several resources for identifying leather based on follicle pattern and grain cross sections that include diagnostic images (Florian 2007, Kite and Thomson 2006). When examining an object that includes a leather component, it can be useful to include the answers to the following questions:

• Was the leather tanned? Can you determine the tanning or treatment method?
• Is a follicle pattern visible? Can you identify the animal with that follicle pattern?
• What sections of the skin are present (grain layer, corium layer, etc.). Can you see the collagen configuration?
• How thick is the leather or skin?
• Does the leather have a finishing treatment?
• Are there any signs of previous repair, either during the original use of the object, or once it entered museum collections?

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.

Safety

Heavy metals

Mold (see cleaning section)

Pests/frass (see cleaning section)

Environmental

Relative humidity and Temperature

Mold

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)

Pests

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

Interventive treatments for Leather and Semi-tanned Hide/Fur

As with any other type of object, each leather or hide object must be considered unique in its conservation needs. Care must be taken to identify any culturally significant deposits so that they not be inadvertently removed along with post-collection alterations. What follows are brief descriptions of common solutions to the most frequently encountered problems, and should not be taken as prescriptions for treatment (more detailed discussion of specific treatments can be found in the references at the bottom of the page). It is recommended that any treatment be undertaken by a trained conservation professional.

Cleaning
Cleaning is done mechanically if at all possible. The preferred method is vacuuming under low suction while gently brushing dirt and debris loose with a soft-bristled brush. Often, the end of the vacuum hose is reduced in size and covered with fine netting to prevent loss of material and it never actually makes contact with the basket. Foamed latex rubber sponges are also helpful in removing some soiling when gently applied to soiled surfaces.

Degraded leather dressings applied in the past can pose tough cleaning challenges; solvent cleaning can be appropriate for some leather, if it is fully tanned and in good condition. Any liquid used should be applied sparingly. For degraded leather and semi-tanned hide (either brain or smoke tanned) aqueous solutions should be avoided if possible as they can reverse many of the chemical bonds created during the tanning process.

Stabilization
Consolidation, desalination, deacidification, corrosion inhibitors; etc.

Structural treatments
Degraded leather can sometimes require consolidation—especially where it has lost the cohesive strength between fibers . In the case where the leather is friable and powdery, it can be consolidated with various adhesives that can be applied either in a spray (using a nebulizer) or with a brush.

Breaks/tears or losses to a leather or hide piece can affect its stability as well as distracting from the legibility of the piece. In some cases, the breaks can be knit back together by applying adhesive just along the break edges—especially if the break does not need to support its own weight; however, most breaks require a bridge applied to the back (or front depending on the type of object and its construction) of the break. Bridge materials vary greatly and are based on both the weight and condition of the leather or hide adjacent to the break. Bridge materials include Japanese tissue, nylon netting, nylon gossamer, spun bonded synthetics, and goldbeater’s skin. Adhesives used also can vary widely and include heat-set films, solvent- and water-based adhesives (for example BEVA 371, Lascaux 360HV/498HV, Aquazol and Butvar B-98, Elvace 40705, Jade 403, methyl cellulose, and arrowroot paste). Care must be taken if a water-based adhesive is selected that it does not denature the tannins present.

Where losses are present, it is sometimes appropriate to fill these. In other cases, it is more appropriate to leave the losses visible due to cultural sensitivity and/or the history of the piece. Many materials have been used as fills including Japanese tissue, polyester felt, and other spun bonded synthetic materials. Silicon molds of the surface of leather with similar grain and wear patterns (not accessioned or historically important leathers) can be used to cast fills using acrylic paint and Japanese tissue.

Over time, environmental and physical stresses placed on leather can cause distortions. Depending on how extensive the damage and how structurally sound the component materials, leather and hide objects can sometimes be restored to their original shape. This is almost always achieved with gentle, controlled humidification to increase flexibility, followed by the use of wadding or weights to hold the object in the correct shape until all introduced humidity has been dissipated. [AKS3] Depending on the type of object, permanent supports may be required. For example, with shoes or moccasins, custom cushions can be made using Tyvek and polyester batting. For garments, either storage pillows or custom mounts can be used to support the leather/hide and prevent further distortions. For full hides that sit flat, custom trays can be made with pillows supporting areas where the hide is does not lay flat.

Aesthetic reintegration
Where loss compensation is required strips of non woven polyester fabric (e.g. Reemay®) and Japanese Tissue paper, coated with conservation-grade acrylic adhesives can be used to fill losses. Water colours can be used to colour matching of missing areas. Losses can also be sorted filling with new leather, matched according to the grain, thickness and colour, using conservation-grade adhesives to insert new patches.  

References

Aldehyde Tannage. ND. Bogazici University Chemistry Department. [www.chem.boun.edu.tr/webpages/courses/leathertechnology/deri26.htm] (accessed 03/21/13).

Anon. 1806. Retrospect of philosophical, mechanical, chemical and agricultural discoveries: Being an abridgment of the periodical and other publications, English and foreign relative to arts, chemistry, manufactures, agriculture and natural philosophy, vol. 1. London: J. Wyatt.

Bishop, J. L., E. T. Freedley, and E. Young. 1864. History of American manufactures, from 1608 to 1860: Exhibiting ... comprising annals of the industry of the United States in machinery, manufactures and useful arts, with a notice of the important inventions, tariffs, and the results of each decennial census. Philadelphia: Edward Young & Co.

Cameron, E., J. Spriggs, E. Wills, 2006. Chapter 22: The Conservation of Archaeological Leather. In: Kite and Thomson (eds.), Conservation of Leather and Related Materials, pp. 244-261.

Cronyn, J. M. 1990. Elements of Archaeological Conservation. New York, New York: Routledge.

CCI Notes 8/2: Care of Alum, Vegetable, and Mineral Tanned Leather. 1992. Canadian Conservation Institute. [2] (accessed 10/2/19).

CCI Notes 8/4: Care of Rawhide and Semi-Tanned Leather. 1992. Canadian Conservation Institute. [3] (accessed 03/25/13).

Falcao, L and M. E. Araujo. 2010. Tannins Characterisation in New and Historic Vegetable Tanned Leathers Fibres by Spot Tests. Journal of Cultural Heritage 12: 149-156.

FitzGerald, W. N. 2007. The carriage trimmers' manual and guide book. Mendham, NJ: Astragal Press. ISBN 1931626235

Florian, M. E. 2007. Protein Facts: Fibrous Proteins in Cultural and Natural History Artifacts, pp. 87-94.

Haines, B. 2006. Chapter 3: The Fibre Structure of Leather. In: Kite and Thomson (eds.), Conservation of Leather and Related Materials, pp.11-21.

Identifying Chrome Tanned Leather. 1998. Cool Conservation OnLine. [4] (accessed 03/21/13).

May, E., and M. Jones. 2006. Conservation Science: Heritage Materials, pp. 92-119.

Procter, H. R. 1922. The principles of leather manufacture. New York: D. Van Nostrand Company.

Ravenel, N. 2011. Notes on Laura Brill's unpublished analytical work on patent leather at the Shelburne Museum in 2009. Shelburne Museum, Shelburne, VT.

Seeley, N. and A. Sutherland. 1991. Enamelled, Japanned and patent carriage leathers. In Conservation of leather in transport collections. London: United Kingdom Institute for Conservation. 24-26. ISBN 9781871656138

Shao, Y. 2005. Chapter 3: Chemical Analysis of Leather. In: Fan (ed.), Chemical Testing of Textiles, pp.47-71.

Smith, C. W. 2003. Archaeological Conservation Using Polymers: Practical Applications for Organic Artifact Stabilization, pp. 60-69.

Thomson, R. 2006. Chapter 1: The Nature and Properties of Leather. In: Kite and Thomson (eds.), Conservation of Leather and Related Materials, pp. 1-3.

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

Volken, M. 2001. Practical Approaches in the Treatment of Archaeological Leather. In: Wills, B. (ed.) Leather Wet and Dry: Current Treatments in the Conservation of Waterlogged and Desiccated Archaeological Leather, pp.37-43.

Further reading

Fogle, S. and T. Raphael. 1992. Leather conservation terminology. Leather Conservation News. 4(1):39-51.

Fran Ritchie & Bethany Palumbo (2023) Lascaux Adhesives in Objects Conservation: Three Practical Case Studies on Leather, Skin, and Entomological Specimens, Journal of the American Institute for Conservation, 62:3, 199-212, DOI: 10.1080/01971360.2022.2093538

Use this section for: additional references or resources not cited in the text, external links, etc. For references please follow the JAIC style guide.
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