Alkalinization

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The application of an alkaline agent that leaves an alkaline reserve in the paper, often in the form of an alkaline salt, that is capable of reacting with future air-born acidic pollutants as well as acids that will occur from the degradation of the cellulose itself. According to aging tests, papers that are to remain free of acidic products should have a 3% alkaline reserve.

Certain aqueous treatments from the previous chapter may be addressed again in this chapter. However, they will be evaluated for their alkaline reserve capacity in this chapter.

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Purpose


Alkalinization or the introduction of an alkaline reserve is used to retard future acid hydrolysis of the cellulose of the paper so that it retains it flexibility and brightness for a longer period of time after treatment. It is also intended to stabilize the paper against further acid attack.

Factors to Consider

Philosophical Issues. Advantages and disadvantages of treatment

Chemical and mechanical quality of the paper

Degree of inherent instability or “vice” of the paper and expected degradation projection Early papers may not need an alkaline reserve depending on prior aqueous treatments performed, such as washing in an alkaline bath. While later papers, such as those containing alum-rosin sizing with a lot of inherent vice may require an alkaline reserve. Ground wood papers may not be treated at all with a high pH calcium hydroxide solution because it will remove hemicelluloses from the paper substrate that are essential for the physical stability of this type of paper. The condition of the paper, how degraded it is may preclude any alkaline (or aqueous) treatment whatsoever.

Sources of acidity prior to treatment

  • Inherent sources of acidity such as fiber finish, sizing, pigment, etc.
  • External sources, such as environment and specific storage environment.

Effectiveness of prior washing and neutralization if undertaken

Housing and storage conditions after treatment

Likelihood of reintroduction of acid due to poor environment

Effectiveness of buffered housing material in protecting against environmental acidity

Wet strength of paper to in an aqueous solution depending on previous aqueous treatment or the condition of paper

Results of spot testing

Before any aqueous or non-aqueous treatment takes place the conservator should perform spot tests to determine the sensitivity of the paper and media to the solution being used. All solutions to be used should be tested. Spot testing gives an approximation of the behavior of the artifact in a bath. Tidelines caused by spot testing are often difficult to remove unless the object is immersed in the solution immediately. Therefore, it is advisable to spot test when the decision to wash is fairly certain.

It is advisable to test book pages in the unprinted margins as well as in the printed area because the wetting out properties of printed-paper are quite variable within a printed sheet.

All solutions to be used should be tested at the temperatures they will be used at. If variable wetting out is observed with the testing solutions, it is advisable to spot test with ethanol/isopropanol and water as well so that pre-wetting can be considered an option.

For mass alkaline reserve (or deacidification) treatments testing methods may depend on more streamlined methods. (See Variations section.)

Physical or Chemical Sensitivity of Paper

Alteration of paper appearance, tactile qualities and dimension

Alkalinization treatments, like washing, will alter the paper appearance, its tactile qualities and dimensions (PCC). The degree to which this occurs will vary base on the choice of treatment protocol.

Shifts in color or tone. Alkaline solutions continue to remove soluble material from the paper. In addition, the solutions tend to cause greater changes in the pH of the paper, which in the case of ground wood papers and chemical pulps. For ground wood pulps the pH of the paper should stay between 7 and 7.5 otherwise there is considerable darkening of the substrate. PH in papers treated with the aqueous solutions of magnesium or calcium bicarbonate increases as they dry due to the interaction with oxygen in the air so color change may not be manifest in the bath, but later upon removal.

Changes, bleeding or loss of colorants and brighteners in the paper may occur during the solvent or aqueous bath, or upon drying.

Opacity of the paper may change. It may increase with the use of a non-aqueous solvent system or if the aqueous system used has a high concentration of an alkaline salt. Or an aqueous alkaline treatment may remove more material from the paper substrate and lead to a decrease in opacity.

Print impression may be compromised if using an aqueous method. It may also occur when books are packed too closely in a non-aqueous mass treatment system.

The paper surface will be more open after treatment. Aqueous alkaline solutions of high pH help open the paper sheet and aid in elution of materials. Upon drying it retains this characteristic unless it is sized. (YK has noticed that paper treated with Bookkeeper absorb moisture quickly and efficiently.)

Paper may expand or contract along both the x and y axes during alkalinization treatment depending on the methods used. Expansions of the paper when undergoing aqueous alkalinization treatment may be controlled by the methods used for drying the paper. Mass alkaline treatments have been optimized to keep the text block and binding intact and functional after treatment therefore they are non-aqueous treatment (How polar are the solvents used in them?)

The drying process has a major effect on the visual, tactile and dimensional characteristics of the paper. In aqueous treatments drying is another variable in the treatment protocol that can be manipulated to retain or regain characteristics of the paper that may no longer be visible in the wet sheet. For mass solvent-based alkalinization treatments the drying process is ideally meant to return the book to its pre-treatment state visually. Of course, chemically it will have been altered.

Alteration in surface of the paper

Sized papers will loose the gelatin sizing in aqueous alkalinization solutions and result in a more open paper substrate as the bulky space-filling proteins are removed.

Papers with a closed surface, such as highly calendared papers will retain the alkaline salts on the surface of the paper as a white powdery deposit, especially in those alkalinization systems that deposit closer to 3% alkaline reserve in the paper.

Other surface coatings, such as clay-coatings, may be affected adversely by alkaline treatments, both aqueous and solvent-based. Complete loss of textual and image material can result.

Physical or Chemical Sensitivity of Media

Alkaline solutions are know to cause changes in tone and hue of media as well as its physical characteristics.

Solubility or Softening of Media/Breakdown of Binder

Printing inks, pigments and colorants are all soluble in a particular solvent—water or one of the many non-aqueous solvents used in the mass alkalinization treatments. Careful testing is required.

Printing inks, especially the softer printing inks used for engravings, may undergo a saponification reaction in the presence of a high pH (9?) calcium hydroxide alkalinization bath that will make them more soluble in water. By the same token this alkaline treatment may remove the binder of the printing ink and result in ink that is friable.

Color change of the media.

Colorants or dyes in the printing inks may be pH sensitive and should be tested for potential changes both while wet and after drying. The pH of a paper after alkalinization with magnesium or calcium bicarbonate increases and therefore certain color changes are only manifest upon drying and the formation of carbonates in the paper substrate.

Printing ink may appear grey as alkaline salts form a powdery deposit on the surface of the ink. This is usually visible on large initials of older printed books where the paper substrate is more compressed. The ink not only forms a barrier for the deposition of some of the alkaline salt into the interstices of the paper but also provides the perfect contrast to the white salts that form upon the interaction of the alkali with air.

Choice of Alkalinization Method and Alkalinization Solutions for the Conservation Laboratory

Below are number of standard of alkalinization methods that leave a significant alkaline reserve in the paper that are used in conservation laboratories by conservators. Mass treatments are discussed in the next section.

Solutions for alkaline treatments

Prior to alkalinization, the text blocks have been washed according to the techniques outlined in the “Washing and Neutralization” chapter. No further degradation products have been eluted from the paper.

Calcium hydroxide adjusted deionized water (pH 7— 8.5).

Advantage:
- Calcium adjustment minimized the risk of removing calcium ions from the paper support.
- Easy to make up calcium hydroxide and add to water.
- Can increase pH of wash solutions with successive application to gradually modify paper support.
- Higher alkalinity opens up the paper support so that more effective elution of the degradation products is possible.
- Effective removal of gelatin size. (Freeman research at Buffalo: but not as much as ammonium hydroxide.)
- Leaves a small amount of alkaline salts in the paper that may help mitigate future formation of acids.
Disadvantage:
- Not easy to achieve pH range and harder to maintain once achieved.
- Requires more
- Certain printing ink and other ink components may be pH sensitive.
- Once the fiber structure is more open and alkaline it remains more easily accessible to moisture. Resizing should be considered to fill up the gaps in the paper substrate.
- May cause graying of the paper support.
- More effective removal of gelatin size.

Calcium bicarbonate.

Advantages:
- pH of bath not alkaline (between pH 6 and 7) and therefore to wash paper and media that may be adversely affected by alkaline conditions.
- Continued elution of degradation products.
- May also be used as a washing bath (SR) and therefore could be used to minimize the paper's exposure to wet conditions by washing, neutralization and the deposition of same alkaline salts in the paper substrate.
- Post-treatment pH is not as high as magnesium bicarbonate treatment.
Disadvantages:

Magnesium bicarbonate.

Ethanol-modified magnesium bicarbonate

Application

Still bath

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