How do you assess waterlogged wood and decide on a conservation treatment?

Deterioration

Under the best conditions (an anaerobic context with waterlogged sediments), wood will be remarkably well preserved. Markings, coatings, tool marks, and other use wear will be evident on the surface, even though the surface may be soft and spongy. It is critical to record the wooden object thoroughly and completely before undertaking any conservation work, since there is always the chance that the surface may change during conservation treatments.

During all initial stages of examination and cleaning, you must always keep the wood wet, ideally immersed. Waterlogged wood is particularly sensitive to changes in temperature and moisture. Any drying at all will lead to irreversible cell collapse and shrinkage, which will destroy the original form of the object. The ideal storage environment is wet, dark and cool. Storage

In general, you want to keep the wood immersed in water, and if you must take it out of the storage tank for cleaning or investigation, keep a slow water flow, mister or other moisture source handy to keep the surface damp. You can also keep the exposed surface covered with damp rags or open cell foam sheets. Do not use rapidly flowing or splashing water, as this may erode delicate surfaces. Tap water is sufficient for most storage purposes. If the wood is coming from a saline environment, it should not be put directly into fresh water – the differential in salt content between the salt water in the wood cells and the fresh water in the tank will cause osmotic shock that can damage the cell walls and increase the degradation of the wood. You can prevent this by slowing increasing the fresh water content of the tank by 25% increments over several weeks.

Biological growth is a common problem in long-term storage tanks. It is important to remove burial sediments as soon as possible. In an ideal world, you should not use biocides or fungicides to control algae, bacteria, or fungi that may grow in the tank and on the wood. Chilled water, or cold storage will help control growth, as will frequent water changes, and constant water circulation with filtration. (A simple filter can be built out of a garden-pond submersible pump, a perforated 5-gallon paint bucket, and polyester filter material). But even chilled water must be changed frequently and the containers thoroughly rinsed, since some fungi can grow at very low temperatures. Constant circulation will also help prevent the growth of anaerobic bacteria which can grow in small stagnant pockets of water under and between tightly packed objects, or in sealed containers. These bacteria will generate iron sulphides, which can stain the wood black. Small containers that can be sealed to prevent evaporation can be treated by adding 10% ethanol or rubbing alcohol, but be careful to use this technique only in locations where sparks and flames are not a hazard.

Snails, goldfish or koi have also been used to keep down biological growth in long-term storage tanks. Equipped with standard pond filters and air bubblers, a population of fish can keep tank and wood surfaces free from algae, fungi, and bacteria. Snails can suffer population boom and bust cycles, which may cause long-term cleanliness problems in the tank.

Ultimately, all waterlogged wood will require conservation for its long-term preservation. While you can keep wood in water indefinitely, there will always be on-going degradation, and the cost of maintaining the storage tanks and their contents will add up over the long run.

Assessment

The treatment of waterlogged wood focuses on two basic problems: the natural pore spaces of wood have been increased by degradation (either biological decay or chemical hydrolysis), which has reduced the physical strength of the wood structure; and all those pore and decay spaces are filled with an excess of water which is partly supporting the weakened structure, but is not physically or chemically stable. There may be other problems as well, such as mineral staining (usually from metal fasteners) that change the appearance of the object, and contribute to the breakdown of the wood.

Treatment

Since dimension changes are possible during treatment of waterlogged wood, careful recording of size prior to treatment is extremely important. Photo by L. Young, used by permission of Alexandria Conservation Services.

The basic premise of treating waterlogged wood is the removal of excess water while preventing the damaged wood structure from collapsing. This is usually accomplished by introducing some bulking or supporting material while the wood is still wet, and then drying it in a controlled fashion. In all cases, the fine details of a treatment will be determined by the degree of wood degradation, the intended disposition of the object, and the resources available for treatment. Other components of wood treatment may include the removal of foreign materials such as metal staining. Treatments are usually assessed based on their ability to dry the wood without dimensional change, preserve associated materials and provide aesthetically acceptable results.

Since dimension changes are possible during treatment of waterlogged wood, careful recording of size prior to treatment is extremely important.

Bulking Agents

Water-soluble:

Sugar: There has been considerable experimentation using natural sugars (sucrose) and artificial sugars (mannitol, lactitol) to replace the missing wood cellulose, which is itself a long-chain sugar polymer. Results have been varied, and the precise conditions which lead to a successful treatment are difficult to define or reproduce. While sucrose is non-toxic and relatively cheap (and is therefore a good option in some developing countries), there are considerable problems with preventing fermentation of the early stage (low concentration) solutions, and calculating the proper final concentration and time of treatment. The artificial sugars do not ferment, but are much more expensive.

Polyethylene Glycol (PEG): PEG is the standard treatment for most waterlogged wood, and has been studied extensively by conservation scientists. It is non-toxic, and available through most chemical suppliers. It can be applied by immersion, or by spraying. It is a wax-like long-chain polymer that comes in a variety of molecular weights (MW); -common grades used in conservation are PEG 200 or 400 (viscous liquids), and PEG 3350 or 4000 (flaky solids). The low and high molecular weight PEGs are often used in combination – the PEG Wheel in PEG.

200 or 400 is introduced first to penetrate the smaller spaces and bond to the cell walls, while the PEG 3350 or 4000 is introduced later to fill the larger spaces in the deteriorated wood. There are different opinions as to whether this should be done in two separate applications, or if it can be done in one blended bath. The degree of degradation of the wood determines the final concentration of each grade used, and the length of treatment. Arguments against using PEG include the long treatment times, and the potential for a dark, greasy wood surface, but both can be controlled by proper application. PEG also remains hygroscopic (attractive to atmospheric moisture), but some argue that this helps buffer the treated wood from swings in relative humidity.

Solvent-based treatments:

Acetone-Rosin: This treatment involves immersing the object in a heated bath of colophony resin dissolved in acetone. It was a popular treatment from the 1950s, now mostly discontinued because of serious hazards from the heated acetone baths. It also seemed to work well only on a limited number of species of wood.

Silicone Oils: This is a relatively new procedure using cross-linking silicone oils. The wood is immersed in baths of acetone to replace the water, then immersed in the silicone polymer under vacuum. When the polymer has penetrated fully, it is exposed to a cross-linking agent, and allowed to dry. The cross-linked polymer cannot be removed from the wood. The process (also known as plastination) has long been used to preserve forensic and medical specimens, but the application to waterlogged archaeological artifacts is relatively recent. Many conservators feel that it requires more study to evaluate its long-term effects. It may never be appropriate for large objects (longer than one meter) because of its expense and the toxicity of the cross-linking agent.

Other polymers: A variety of other solvent-based polymers have been tried, but none have ever worked well, or they developed long-term problems.

Dehydration: the most critical stage of treatment is dehydration. If the first phase of treatment does not replace water with some other solvent, you must take great care. As water droplets evaporate, they shrink. Water has a very high surface tension, and water in the wood cells is bound to the cell walls by hydrogen bonds – as the water evaporate, the shrinking water droplet pulls the cells walls inwards with it, causing cell collapse and irreversible shrinkage. The various bulking agents, such as PEG, help to prevent this, but will not stop it completely.

Air-drying: in some very limited cases, it is possible to air-dry wood without introducing any bulking agents. This is usually only successful where the wood is very robust, with little degradation. The rate of drying and the environment in which the object is dried must be carefully controlled to prevent drying too fast. It usually requires a chamber where both temperature and relative humidity can be tightly controlled and monitored, and usually takes several months.

Freeze-drying: freeze-drying, either under vacuum or at ambient air pressure, has been proven a gentle method of drying. Because under certain conditions ice can sublime directly to water vapor without passing through a liquid phase, you do not have the water-tension shrinkage problems. This can occur most efficiently under a vacuum (thus the creation of vacuum freeze-driers, used commercially to prepare foods and pharmaceuticals), but it can also happen in a very low-temperature freezer (“freezer burn” of frozen meats is an example of ambient pressure freeze-drying), or in Arctic environments. PEG is commonly used as a pretreatment for freeze-drying because it will form a solution of ice and PEG that does not expand like pure water on freezing, and the PEG is left behind to bulk the cell walls when the water is removed.

Solvent-drying: This involves replacing the water in the wood structure with a solvent of lower surface tension. One theory behind this treatment using solvent replacement is that solvents such as acetone have a much lower surface tension than water, and should not cause as much shrinkage as the wood dries. In reality, the acetone can replace ALL the water in the wood, including that chemically bonded inside the cellulose molecules, so shrinkage occurs anyway.

Additional concerns

Recently, researchers in the treatment of waterlogged wood have focused their attention on the role of elemental sulphur and iron sulphides in the long-term deterioration of archaeological wood. Sulphur compounds are often deposited in the wood during burial, especially in polluted and anaerobic environments, and they are difficult to remove. As the sulphur compounds are exposed to oxygen and moisture during and after treatment, they become oxidized and sulphuric acid is produced, which attacks the wood. The reaction is catalyzed by iron, which is also commonly deposited in waterlogged wood. So far the phenomenon has been recorded primarily in timbers from large ships such as Batavia, Mary Rose and Vasa, but there are implications for the treatment, storage and long-term stability of all waterlogged wood. No simple test for sulphur content has been determined yet. It is usually necessary to send a sample to a laboratory for analysis.

Storage after treatment

No matter how it is treated wood that has previously been waterlogged should be kept in a stable environment. The relative humidity should be maintained between 45-60% and large or rapid changes in humidity should be avoided. Light levels should be controlled and dust excluded as much as possible.

Wood that has been treated with a bulking agent or other treatment materials may experience a significant gain in weight during the process. Depending on how they will be displayed and/ or stored fragile wood fragments may require some form of external support system to prevent long-term damage or deformation.

Sourche from; http://www.sha.org/research_resources/conservation_faqs/treatment.cfm

 

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