FILE 7: LEATHER CONSERVATION
IN THIS FILE:
Treatment of Brittle and/or Desiccated Leather
Smithsonian Glycerin Treatment
British Museum Leather Dressing Treatment
Polyethylene Glycol Treatments
Treatment of Wet or Waterlogged Leather
Polyethylene Glycol Treatments
Dehydration using Organic Solvents
As with all porous material, it is necessary to remove the bulk of the soluble salts present in leather recovered from marine environments. The procedure is the same as that described earlier for bone and ceramics. Prior to conservation, archaeological leather must be washed in order to remove any ingrained dirt. Ideally, leather should be washed in water alone. A variety of mechanical cleaning techniques may be required, depending upon the condition of the leather and the particular cleaning problem. Soft brushes, water jets, ultrasonic cleaners, and ultrasonic dental tools are effective mechanical cleaning tools for leather. If chemical cleaning is necessary to remove ingrained dirt, a small amount of non-ionic detergent (about a 1 percent solution) or sodium hexametaphosphate may be used. If Calgon (a commercial water softener) is used, first ensure that the pH is between 3 and 5; the addition of additives may make it unsafe to use on leather. Rinse the leather well after washing. Do not use any chemicals that will damage the leather's collagen fibers.
A safe storage solution for waterlogged leather, or any organic material for that matter, can be made by preparing a stock solution of 50 percent water/50 percent ethanol. To the solution, add 10 percent glycerin by volume and two to three drops of formaldehyde.
The conservator should always keep in mind that it is often better to leave stable stains on the leather than to damage the leather by trying to remove them. For stain removal, particularly iron staining, 3-5 percent ammonium citrate or ethylene-diamine-tetraacetic acid (disodium EDTA) is used. Commercial trades names for EDTA are Titriplex III and Disodium Deterate. Soak for two to three hours while monitoring closely, then rinse the piece in running water or standing tap water until all chemical residues are removed. Check the pH of a standing bath of water containing the leather to determine if the chemical removal is complete. Always keep in mind that chemicals used to clean rusts and mineral concretions (e.g., oxalic acid, EDTA) may produce further hydrolysis of the proteinaceous collagen fibers in the leather. Furthermore, they may remove tanning and/or coloring agents, painted decorations, and other attributes that are part of the diagnostic attributes of the leather object. Caution should be exercised when using any of these chemicals on leather, as diagnostic attributes should never be removed.
Freeze drying and solvent dehydration are the most common conservation treatments for waterlogged leather. The drying behavior of any piece of leather, however, is dependent upon its condition at deposition, the burial environment, the genus, species, health, and sex of the individual (man or beast), the location of the skin on the body, the production or tanning method used, and finally, the leather object's history. Very good results have been achieved in the conservation of bog bodies through freeze drying using 15 percent PEG 400. The PEG acts as a lubricant to minimize skin and bone shrinkage during drying.
TREATMENT OF BRITTLE and/or DESICCATED LEATHER
The following treatments that involve the addition of lubricants have been used successfully on brittle and/or desiccated leather. Glycerol, which is soluble in water and alcohol, acts as a humectant for the leather.
SMITHSONIAN GLYCERIN TREATMENT
59 percent glycerin (glycerol)
39 percent water
1 percent formaldehyde or 1% Dowicide (TM) 1
25 percent glycerin
75 percent alcohol
Immerse the artifact in the solution until the leather is pliable. (When an alcohol solution is used, it is difficult to determine when the leather is pliable because the alcohol makes the leather stiff.) Treatment may require several weeks. The treatment restores flexibility, but glycerin is hygroscopic and can support mold growth. In spite of this fact, the Smithsonian Glycerin Treatment has been used successfully.
Waterlogged leather recovered from excavations by The Museum of London is conserved in a solution of 30 percent glycerin and 70 percent alcohol (ethanol) for two weeks. The leather is then dried in three successive baths of acetone, each three hours long (glycerin is not soluble in acetone). Similar results can be achieved by using 10-40 percent glycerin mixed in 90-60 percent alcohol or water. Avoid using concentrated glycerin. While the solutions in alcohol can remove tanning agents, alcohol speeds up the conservation process and confers greater mechanical strength to the leather than will a water solution.
The glycerin treatment has also been applied to basketry, matting, sandals, etc. to restore pliability, quite often with disastrous results. It should be kept in mind that there is no reason to make something flexible or pliable if it was not particularly pliable in the first place. The glycerin treatment can be used in combination with PEG. To retreat any object that was conserved with glycerin, such as basketry, remove the glycerin with successive changes of alcohol baths.
BRITISH MUSEUM LEATHER DRESSING TREATMENT (BML)
200 gm (7 oz) anhydrous lanolin
30 ml (1 oz) cedarwood oil (acts as a fungicide)
15 gm (½ oz) beeswax (optional)
350 ml diethyl ether (B.P. 15-25°C) or 330 ml of hexane
Heat the first three items together (beeswax can be omitted, its function is to act as a polish) and then pour the molten liquid into the ether or hexane. Allow to cool while constantly stirring. Exercise extreme caution, as ether and hexane have low boiling points and are very flammable. Apply sparingly to the leather and rub well. Wait two days, then polish the treated leather with a soft cloth. Very hard leather can be soaked in a solution of one part BML: three parts Stoddards Solvent. BML darkens the leather, but it is a treatment with a good success record.
GLYCOL (PEG) TREATMENTS
Dry leather can be saturated with water or alcohol and treated with PEG 1450, PEG 540 Blend, PEG 600, or PEG 400. In the past, leather was treated in PEG which was heated to a temperature of 40-50°C. Presently, most leather treatments are carried out at room temperature because heat is generally detrimental to leather.
The PEG treatment consists of immersing leather in a dilute solution of PEG ( i.e., 10 percent) in water or alcohol and increasing the PEG concentration in 10 percent increments as it is absorbed by the leather. A final PEG concentration of 30 percent in solution is adequate for most archaeological leather. Keep the artifact immersed in the 30 percent PEG solution for several days until the leather is flexible. Once pliable, remove from the solution and clean off excess PEG from the leather with toluene or water. Allow treated leather to dry slowly under controlled conditions.
As mentioned in previous chapters, there are several types of PEG, and each has its own characteristics. PEG 540 Blend (equal parts of PEGs 1450 and 300) is slightly hygroscopic and becomes moist at high humidity; for this reason, the surface of the leather treated with PEG 540 Blend is sometimes sealed with a hard wax, i.e., a mixture of 100 gm microcrystalline wax and 25 gm polyethylene wax. PEG 3250 is very hard and is not very hygroscopic. Its main disadvantage (in some cases advantage) is that the treated leather is rigid. When using PEG 3250, form the treated specimen to its final shape while the wax is still warm and then allow the artifact to cool. PEG 1450 gives consistently good results. The various PEG treatments are more commonly used for the conservation of dry leather. A 15 percent PEG 400 solution is commonly used as a pre-treatment when the leather is to be freeze dried.
PEG-treated leather can be hygroscopic, greasy, and dark in color. There is the additional possibility that the PEG may eventually migrate out of the leather.
Bavon ASAK 5205 is a water-soluble emulsion, while Bavon ASAK ABP is solvent-soluble emulsion. The exact chemistry of Bavon is unknown. In some sources, it is described as being an alkylated succinic acid-mineral oil blend. Bavon ASAK-ABP is described as being a copolymer of polyhydric alcohol and a partial ester of an unsaturated hydrocarbon. In archaeological conservation, Bavon works as a lubricant that makes leather pliable and gives it a natural brown appearance.
Very hard, desiccated leather has been successfully softened by soaking it in a concentrated Bavon leather dressing consisting of six parts Bavon ASAK ABP to four parts 1:1:1 trichloroethane. Soak until satisfactory pliability is reached, then place the leather between blotters and glass and allow it to dry.
TREATMENT OF WET OR WATERLOGGED LEATHER
Leather, like a lot of organic material from a marine environment, undergoes some complex changes in a marine environment (Florian 1987). The difficulties in achieving natural looking, chemically stable results have long been known by conservators (Jenssen 1983). The best review of the most common treatment currently in use is presented by Jenssen (1987). The most relevant treatments are discussed. below.
Waterlogged leather should be stored in water with 0.1 percent Dowicide 1 prior to treatment. If the leather is to be treated in an organic solvent, the leather can be stored in 50 percent water/50 percent ethanol or straight ethanol; a fungicide is not required. Treated leather should not be stored in an environment with a relative humidity higher than 63 percent.
GLYCOL (PEG) TREATMENTS
Treat with the method described above for desiccated leather, using PEG 400, 540 Blend, 600, 1450, or 3350. Gradually increase the concentration of PEG in solution up to 30-100 percent. Treatments with aqueous solutions of PEG are slower processes but are less expensive than treatments involving solvent solutions. Some conservators prefer alcohol treatments, while others think that alcohol treatments cause the leather to shrink more than comparable aqueous treatments. Solvent solutions, however, produce a lighter leather with more uniform shrinkage. All the PEG treatments for waterlogged leather are satisfactory by themselves, but treatment is considerably enhanced if the leather is taken through a final step of freeze drying. The freeze-drying process is identical to that described earlier for wood. A commercial freeze-drying vacuum chamber works the best; however, very good results have been obtained using domestic chest freezers. The former takes only a week or so, while the latter may take several weeks. Progress can be determined by regular weighing of the object to determine weight loss as the leather loses moisture.
1. Wash leather in a 1 percent solution of Lissapol, castile soap, soft soap, or saddle soap; never use commercial detergents on leather as they may extract tanning materials.
2. If iron stains are present, they can be removed by immersing the leather in a solution of tap water and 3-10 percent disodium EDTA (pH 4) or 3-5 percent ammonium citrate (pH 5) for a maximum of one hour (shorter if possible). EDTA has been reported to damage fibers of leather but is relatively safe if used selectively and cautiously. Ammonium citrate is recommended over disodium EDTA by the Canadian Conservation Institute.
3. If calcareous material is present, place the leather in a 2 percent hydrochloric acid solution for up to one hour.
4. Rinse the leather thoroughly in running water for 30 minutes to lower the pH to 3-6 or to the pH of the rinse water.
5. Dehydrate the object in acetone. Use two or more successive hour-long baths.
6. Air dry until the leather is supple, then place between absorbent tissue and glass to dry for 24 hours.
7. Apply Bavon leather dressing (below) with a brush. Flex and manipulate leather during the application of the Bavon.
Bavon Leather Dressing
1 liter of stabilized 1:1:1 trichloroethane
1 gram Dowicide 1
50 grams anhydrous lanolin
20 grams Bavon ASAK-ABP
Waterlogged leather recovered from excavations by The Museum of London is conserved by placing it in a solution of 30 percent glycerin and 70 percent alcohol (ethanol) for two weeks. The leather is then dried in three three-hour long baths of acetone (glycerin is not miscible in acetone and is, therefore, not removed in the acetone baths). Good results can also be achieved with a solution of 10-40 percent glycerin mixed in 60-90 percent alcohol or water.
Freeze drying is the best method for conserving waterlogged leather. The leather is first immersed in a 15 percent solution of PEG 400, which acts as a humectant and prevents excessive shrinkage (a fungicide, such as 1 percent boric acid should be mixed with the PEG solution). After immersion, the object is frozen to -20 to -30°C. Like biological specimens, a quick freeze is best. This can be achieved by immersing the leather in acetone with dry ice (frozen CO2). The piece is then placed in a freeze-drying chamber under vacuum for a period of two to four weeks. Some acceptable results have also been had using non-vacuum freeze drying in a (preferably frost-free) freezer chest.
USING ORGANIC SOLVENTS
This treatment involves the replacement of water in leather with a water-miscible organic solvent. In most cases, a sequence of solvents with decreasing polarity is used, e.g., a series of baths of x percent water/x percent of isopropanol, followed by a bath of 100 percent isopropanol, a bath of 100 percent ethanol or methanol, 100 percent methyl ethyl ketone, 100 percent acetone, and finally 100 percent ether. Slow desiccation of glutinous collagen fibers allows their surfaces to become less sticky and less brittle and thus more flexible. This example is a very conservative method of treatment. In most instances, fewer baths are used and for some leather, drying only through acetone is necessary. The following solvent dehydration treatment, described by Plenderleith and Werner (1971:34), allows the leather to dry out in a flexible condition without undue shrinkage.
1. Remove iron stains with 5-10 percent disodium EDTA (pH 4).
2. Rinse the leather in clean water, brush lightly.
3. Remove excess water by soaking the object in methyl ethyl ketone or acetone.
4. Immerse the object in carbon tetra-chloride with a fungicide, such as oxide of naphthenate.
5. Dry between blotting paper and glass plates.
6. Apply leather dressing and work the leather into shape if necessary.
Of the treatments discussed above, solvent-drying treatments followed by the application of a leather dressing are the most effective. Controlled air drying from an aqueous state never works. The contractile forces of the escaping water draw the protein fibers together, causing the leather to harden and shrink. Some conservators prefer freeze drying with a pre-treatment of PEG 400. These are the two most common treatments, and both can give acceptable results, but there is always the problem of the leather feeling greasy, overly stiff, and/or too dry and brittle when treated by any of the methods now in general use.
The process of treating waterlogged leather with silicone oil is very similar to the treatment outlined for waterlogged wood. In the treatment of waterlogged leather, however, best results are obtained when the catalyst is painted onto the surface of the leather after it has been saturated with silicone oil and a crosslinker.
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Copyright 2000 by Donny L. Hamilton, Conservation Research Laboratory, Texas A&M University.
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