Methods of Conserving Archaeological Material from Underwater Sites
by Donny L. Hamilton

Textile Conservation

IN THIS FILE:

Documentation
Cleaning to Remove Soil, Discoloration and Stains
Reinforcing Fragile Textiles
Disinfection
Conservation of Waterlogged Canvas and Rope
Silicone Oil Treatment of Waterlogged Textiles and Rope
Storage


The term 'textile' is applied to woven objects and also to fabrics which are the products of other kinds of interlacing of yarns, such as braiding, looping, knitting, lace making, and netting. The textile category also includes materials such as felts and non-woven materials in which the fibers gain coherence by a process other than spinning.

This brief discussion of textile conservation is limited to the natural fibers of animal and plant origin: wool, hair, silk, cotton, flax, jute, hemp, nettle, grass, etc. The animal fibers consist primarily of protein and are more resistant to decay than vegetable fibers, which are composed primarily of cellulose. Flax and cotton, for example, are very susceptible to attack by bacteria under humid conditions and seldom survive in archaeological environments. All textiles are deteriorated by light, insects, microorganisms, and air pollution which, alone or together, cause considerable loss of tensile strength and pliability. The oxygen in the atmosphere affects all organic substances to varying degrees. Prolonged exposure to normal atmospheric conditions will cause textiles to weaken and disintegrate. The speed of the deterioration varies according to environment and the nature of the fibers. The main factors that promote the decay of textiles can be categorized into three groups:

1. Organic: Because textiles are organic, they are subject to attack by molds and bacteria. Decomposition is greatest in situations that favor the growth of these organisms, such as damp heat, stagnant air, and contact of the material with vegetable matter. Attack by destructive insects may also be encountered.

2. Physical: Excessive heat causes desiccation and embrittlement; exposure to ultra-violet light causes a type of deterioration known as 'tendering,' as well as the photochemical degradation of susceptible dyes.

3. Chemical: Exposure to noxious gases can also cause tendering. In some cases, these gases are converted to acids, which are the main cause for the deterioration of some textiles.

In general, textile conservation should be left to specialists; however, many archaeological specimens can be treated even by novice conservators. Fibers that compose a fabric should be identified before any treatment is undertaken, particularly if stain removal is required. Physical tests, such as burning, quickly identify the presence of animal fibers, which do not burn readily and shrivel into a residue of carbon. These fibers usually emit the distinct odor of singed hair. Vegetal fibers burn easily to a fine ash. Many fibers and hairs can also be readily identified by microscopic examination. Animal hairs, for example, can be identified by their characteristic cuticle patterns and medullar cross sections. Simple staining tests enable the conservator to distinguish between the different kinds of fibers.

The textile materials encountered most often in archaeological sites are linen, cotton, wool, and silk. Linen is a spun and woven vegetable-based fiber derived from flax stalks and branches. Linen fibers lie close together and are durable. They withstand moderate alkaline conditions because of their cellulose content, but are readily affected by acids. Moisture easily passes through the fibers of linen, causing the material to undergo dimensional and weight changes as well as changes in the overall strength. Linen does not take dye well and is usually left in a bleached or unbleached white state.

Flax

Figure 8.1. Schematic drawing of flax.

Cotton is a vegetable fiber derived from lint on the cotton seed. It can survive in moderate alkaline conditions but is adversely affected by acids. Cotton does not transmit moisture like linen and is very absorbent in its processed state. It is this characteristic which allows cotton to take dyes well. Cotton has a very characteristic clockwise twist; for this reason, it is commonly spun in a 'Z' twist.

Cotton

Figure 8.2. Schematic drawing of cotton.

Wool

Figure 8.3. Schematic drawing of wool.

Wool consists of protein fibers. The majority of the amino acids in the protein of the wool are keratin, which contains insect-attracting sulfur. Wool fibers absorb more moisture and accept dyes better than vegetable fibers. Wool is not a strong fiber and weakens considerably when wet.

Silk

Figure 8.4. Schematic drawing of silk.

Silk is an animal (insect) fiber that is derived from the cocoon filament of the silkworm. Because it is basically protein, silk is easily affected by alkalies and various inorganic acids. Like wool, it easily absorbs moisture and will take dyes readily. These dyes, however, are not as light-fast as those on wool. Silk is as strong as a steel wire of the same diameter but is very light sensitive. Therefore, it will break down faster than wool when exposed to ultra-violet rays.

The proper treatment of textiles usually requires the use of flat, shallow pans, hot plates, and racks, or other devices that can support fragile textiles during rinsing, treatment, and drying. Treatment involves documentation, cleaning, reinforcing, sterilization, and proper storage and protection from environmental dangers.

DOCUMENTATION
A thorough documentation, both photographic and written, should record all pertinent information about the textile to be treated. The various features and properties that the conservator should look for are:

  1. The nature of the fibers
  2. The spin of the yarn (i.e., 'Z' or 'S' twist)
  3. The number of wefts and warps per inch (or centimeter)
  4. The type of weaving and of dye (water-fast or soluble)
  5. Paint
  6. Metal threads, and any other features that may be diagnostic or of interest.

CLEANING TO REMOVE SOIL, DISCOLORATION AND STAINS
A great number of substances can be removed from textiles by simply washing the artifact in water. De-ionized water is always preferred. Add 0.4 to 1 percent ammonium hydroxide (0.4 percent for animal fiber, 1 percent for vegetable fibers) to the water for greater cleansing power. If necessary, add 1 percent neutral non-ionic detergent (such as Lissapol N) to remove more stubborn soil. During the washing and cleaning process, the fabric can be bleached in a 4 percent hydrogen peroxide solution. For more stubborn stains (including mildew, mold, black sulfide stains, and organic stains), soak the artifact in one of the following solutions:

Stain Solution 1
1 liter de-ionized water
60 ml 30 percent hydrogen peroxide
2.5 g sodium silicate dissolved in 100 ml hot de-ionized water

Stain Solution 2 (for more stubborn stains)
1 liter de-ionized water
300 ml 30 percent hydrogen peroxide
20 g sodium silicate
5 g sodium carbonate
5 g sodium hydroxide

Soak for ½ to 1 hour or until the stain is removed then place the artifact in a closed bag to oxidize.

Sodium silicate and metasilicate are stabilizers, which are added to control the decomposition of hydrogen peroxide (H202) to water (H20). Whenever possible, a simple solution of hydrogen peroxide (and possibly a stabilizer) is recommended for stain removal because as the hydrogen peroxide loses oxygen, it bleaches and converts to water. Therefore, there is no danger of continuing chemical action. Hydrogen peroxide can be used on all vegetable fibers and its bleaching effect is permanent. Do not use hydrogen peroxide on hair or any fiber that was not white when originally in use. The addition of different sodium or alkali compounds are used to control the pH and to increase the cleaning power of the solution.

Copper corrosion stains can be treated with 1-5 percent ammonium hydroxide. For silver corrosion stains, first soak the stain with potassium cyanide then apply a few drops of iodine. Remove the resulting silver iodide product with a 5 percent solution of sodium thiosulfate. Textiles with iron rust stains can be treated with any of the following chemical solutions:

  1. 5 percent hydrochloric acid [muriatic acid (HCl)]
  2. 5 percent oxalic acid H2C2O4
  3. 10 percent hydrofluoric acid (HF)
  4. 5 percent EDTA disodium
  5. 5 percent acetic acid (CH3COOH)
  6. 5 percent formic acid (HCOOH or CH2O2)
  7. 2-10 percent ammonium citrate

Oxalic acid and hydrofluoric acid solutions are the most effective for removing iron stains from archaeological textiles. Extreme caution must be taken, however, when handling hydrofluoric acid. EDTA disodium and ammonium citrate solutions are often recommended because their higher pHs (>2.5) potentially do less damage to fibers. These solutions are effective but rather slow. After any chemical treatment, intensive rinsing in de-ionized water will remove all residue from the textile which may adversely affect the fibers over time.

For textiles that cannot be cleaned with water (such as textiles with water-soluble dyes), dry cleaning using organic solvents, such as perchlorethylene or trichlorethylene, or petroleum solvents, such as white spirits, is recommended. The advantages of solvent cleaning are:

  1. Unlike water, solvents do not soften textile fibers, minimizing the risk of shrinkage and loss of shape.
  2. Dyes not fast in water may be undisturbed in solvent.
  3. Solvents are more efficient than water for dissolving grease.
  4. In general, solvents are volatile and dry quickly.

The costs of solvent cleaning are much higher, and the problems of toxicity and inflammability must be taken into consideration.

REINFORCING FRAGILE TEXTILES
Quite often, the only practical method of reinforcing fragile textiles is to fasten them to a synthetic mesh of terylene, light cotton fabric, fiber glass, or other substance. Particularly fragile textiles are sometimes mounted between sheets of plastic or glass. In most cases, a heat-sealable adhesive, such as polyvinyl acetate, polyvinyl alcohol, Acryloid B-72, or their emulsions, are used to coat the backing which is then ironed (heat-sealed) onto the textile. Any breaks in the threads either in the warp or weft of the material should be affixed with drops of glue to prevent additional unraveling

The conservator also has the option to consolidate and reinforce fragile textiles with various synthetic resins. Since water softens and makes textile fibers pliable, emulsions and water-soluble resins are preferred for textile consolidation. Water-based adhesives also give the conservator more 'working time' than solvent-based adhesives. The most frequently recommended resins for textile conservation are:

  1. polyvinyl alcohol (water-soluble, dries clear with minimum shrinkage)
  2. polyvinyl acetate (V7) (note: resin shrinkage during drying may distort fibers)
  3. ethulose (ethyl-hydroethyl cellulose) (water-soluble, very pliable)
  4. polymethacrylate
  5. Acryloid B-72, 5 percent in toluene

A mixture of 0.15 percent ethyl-hydroethyl cellulose, 0.6 percent polyethylene glycol (PEG) 400, and 0.2 percent fungicide will consolidate fragile fabrics and restore moisture to dry, brittle fibers. A solution of 20 percent lanolin in toluene can be applied to fibers that have a tendency to shred or lint.

DISINFECTION
For wholesale treatment of mold and insect infestation, place the infested objects in a closed container with thymol crystals. The crystal can be vaporized by holding the container over a light bulb. After treatment with thymol crystals, spray the objects with a 0.5-1 percent Lysol solution. This treatment will remedy most problems. Carbon disulfide can also be used as a fumigation agent.

Disinfectant solution can be prepared in the lab by mixing 0.1 percent Dowicide 1 (ortho-phenylphenol), 68 percent ethanol, and 30 percent de-ionized water. This solution is lethal to most bacteria, fungal spores, and surface mildews. Dowicide 1 has a maximum solubility of 0.1 percent in water and 46-58 percent in the various alcohols. Alternatively, a solution of 2 percent DowicideA and sodium ortho-phenylphenate can be used. Dowicide A has a maximum solubility of 120 percent in water and approximately 350 percent in alcohol. For the majority of textile disinfectant needs, Lysol disinfectant spray will suffice. Lysol spray consists of 0.1 percent Dowicide1, 79 percent ethanol, 8 percent n-alkyl, and 0.035 percent n-ethyl morpholinium ethylsulfates (deodorizers and scents). The conservator should keep in mind that topical treatments are not long lasting.

CONSERVATION OF WATERLOGGED CANVAS AND ROPE
After a series of tests by the Conservation Division of the Western Australia Museum on artifacts recovered from a 19th-century shipwreck, the following sequence of treatment was proposed for the conservation of canvas (and other similar fabrics) and rope.

  1. Immerse artifact in 10 percent hydrochloric acid to remove all adhering encrustation and some iron corrosion and stains.
  2. Rinse artifact in running water. Watch for any dyes that may be adversely affected.
  3. Soak artifact in acetone to remove any tar, pitch, tallow, or other acetone-soluble substances. Watch for any dyes that may be removed.
  4. Soak artifact in 5 percent oxalic acid to remove the bulk, if not all, of the iron stains. Treatment time will vary from a couple of hours to a couple of days.
  5. Immerse artifact in 5 percent EDTA disodium to remove any remaining iron stains. Soaking time will vary from a couple of hours to up to three days.

    Note: Both Steps 4 and 5 may be required for particularly stubborn iron stains. In other instances, either Step 4 or Step 5 may be used.

  6. Bleach artifact in a 5 percent hydrogen peroxide solution. Particularly stubborn stains may be treated for short periods in a stronger (10-20 percent) solution. Perform this step only on cloth, canvas, and textiles that were originally white. Never use hydrogen peroxide on animal fibers.
  7. Rinse artifact thoroughly in de-ionized or distilled water.
  8. Dehydrate artifact in acetone and air dry.
  9. If necessary, consolidate the material with a suitable synthetic resin. In some cases, the textile may have to be treated with heat-sealable resin and either dry mounted or sealed to a backing of another fabric, such as light cotton or a synthetic mesh.


SILICONE OIL TREATMENT OF WATERLOGGED TEXTILES AND ROPE
Waterlogged textiles and rope are currently being conserved using silicone oil. The results have been quite promising. See the Archaeological Preservation Research Laboratory Reports.

STORAGE
Store textiles in an environment that limits their exposure to atmospheric pollutants and ultra-violet light. Relative humidity should be kept at a maximum of 68 percent. (Environments with a relative humidity of over 70 percent encourage mold growth.) Ideally, textiles should be stored in a dark place with a low temperature of 10°C and a relative humidity of 50 percent or less. Moths and other insects should be deterred by keeping moth balls (paradichlorobenzene) in the storage area. This is particularly critical when storing wool.

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