Polyethylene Glycol (PEG) Method
Freeze Drying of Waterlogged Wood
Silicone Oil Treatment
As an organic material, wood normally decays under combined biological and chemical degradation when buried in earth; it may, however, survive prolonged exposure to extreme dryness or wetness. In shipwreck sites, the wooden components of the hull and small artifacts of wood often survive in good condition, albeit thoroughly waterlogged. The mechanisms of the organic deterioration of wood are succinctly presented in Florian (1987).
Successful conservation of wooden artifacts is dependent upon a knowledge of wood structure and types. Trees are divided into two broad categories: hardwoods and softwoods. Hardwoods are classified as angiosperms, which refers to broadleaf trees which are usually deciduous. Angiosperms are referred to as 'porous' woods because they have vessel pores. Oak and birch are typical examples of hardwoods. Softwoods, or gymnosperms, are needle-bearing trees or conifers. Gymnosperms are considered 'non-porous' because they lack vessel pores. Pines are typical examples of softwood. It is critical that conservators know the category of wood that they are treating; in many instances, it is equally important that a wooden object is identified to the species level in order to successfully conserve the waterlogged wood.
Figure 6.1. Schematic diagram of hardwood, illustrating the relative appearance of vessels and tracheids (vascular cells).
Figure 6.2. Schematic diagram of softwood, illustrating the relative appearance of tracheids.
Through the loss of moisture, freshly cut, sound wood will generally experience a radial shrinkage of ca. 3-6 percent, a tangential shrinkage of 5-10 percent, and a longitudinal shrinkage of - 0.5 percent. Fresh oak shrinks 4 percent radially and 8 percent tangentially when air dried after cutting, while waterlogged oak can shrink 12 percent radially and ca. 24 percent tangentially. Proper conservation treatments can control the amount of shrinkage experienced by waterlogged wood during drying. In practice, a particular conservation technique is often selected because it is known that the treated wood will shrink a desired amount (Patton 1988:43).
Figure 6.3. Schematic drawing, illustrating the three principal planes in wood: transverse, tangential, and radial.
The manner in which a plank of wood was originally sawn from a log will have an effect on how the plank, or an object manufactured from the plank, will shrink after undergoing any conservation treatment. Flat- or plain-sawn lumber has similar proportions of radial and tangential surfaces with arched grain patterns; these features predispose the wood to warping during drying. In order for lumber to exhibit true tangential and radial planes, it must be rift- or flitch-sawn. Because rift sawing is complex and inefficient, a modified pattern called quarter-sawing was developed. Quarter-sawing results in lumber with predominantly radial surfaces on the faces, a feature which makes the wood less likely to warp during the drying process. In some cases, the manner in which planks were extracted from a log will result in the cracking and warping of a treated wooden object, regardless of the conservation technique used.
Figure 6.4. Methods of sawing logs for lumber: flat-sawn, rift-sawn, and quarter-sawn.
In most environments the primary factors for the degradation of wood include: (1) physical action (changes in temperature, fluctuations in relative humidity, etc.), (2) insect attack, and (3) fungal decay. Fungal decay can be eliminated as long as the wood is kept in an environment with a relative humidity of less than 65 percent. In anaerobic waterlogged environments, however, wood undergoes profound chemical changes and alterations that result in a significant loss of strength while retaining overall shape and form.
In all wood, after long periods in wet soil, peat bogs, and marine sites, bacterial action causes a degradation of cell wall components. In general, water-soluble substances, such as starch and sugar, are the first to be leached from waterlogged wood, along with mineral salts, coloring agents, tanning matters, and other bonding materials. In time, through hydrolysis, cellulose in the cell walls disintegrates, leaving only a lignin network to support the wood. Even the lignin will break down over a long period of time. As a result of the disintegration of cellulose and lignin, spaces between the cells and molecules increase, and the wood becomes more porous and permeable to water. All of the deteriorated elements of the wood, including all cell cavities and intermolecular spaces, are filled with water. The remaining lignin structure of wood cells and the absorbed water preserves the shape of the wood. The loss of the finer cellulose tissue does not cause much alteration in the gross volume of wood, but the porosity is increased, and the wood absorbs water like a sponge. A waterlogged wooden object will retain its shape as long as it is kept wet. If the wood is exposed to air, the excess water evaporates, and the resulting surface tension forces of the evaporating water cause the weakened cell walls to collapse, creating considerable shrinkage and distortion. The amount of shrinkage is dependent upon the degree of disintegration and the amount of water present. The amount of water in waterlogged wood is determined by the following formula:
weight of wet wood - weight of oven-dried wood
|% water =|
weight of oven-dried wood x 100
Wood containing more than 200 percent water is considered to be degraded; it is not uncommon to find wood that contains more than 500 percent or even 1000 percent water. Waterlogged wood is often classed according to the amount of water is contains.Class I: over 400 percent water
The Class III hardwoods are the most difficult to conserve.
WATERLOGGED WOOD CONSERVATION
The conservation of waterlogged wood is a two-fold process that involves (1) the incorporation of a material into the wood that will consolidate and confer mechanical strength to the wood while the water is being removed (e.g., PEG- or sugar-bulking treatments), and (2) the removal of the excess water by a method which will prevent any shrinkage or distortion of the wood (e.g., solvent- or freeze-drying). The most common techniques for treating waterlogged wood are discussed below. In any treatment involving wood recovered from a salt water environment, it is necessary that the bulk of the soluble salts be removed first. If the salts are not removed prior to treatment, they will cause a white bloom on the conserved wood and may adversely affect any remaining iron components in the wood and even other material in the same room or environment in which the treated wood is stored.
POLYETHYLENE GLYCOL (PEG) METHOD
Polyethylene glycol (PEG) is a synthetic material that has the generalized formula H2OCH (CH2OH2) CH2OH. The low molecular weight PEGs (300 - 600) are liquids, the intermediate members (1000-1500) are semi-liquids or have the consistency of Vaseline, and the higher molecular weight PEGs (3250-6000) are wax-like materials. The various PEGs are now designated by their average molecular weight. What was once called PEG 1500 is now called 540 Blend (it is equal parts PEG 300 and PEG 1500), PEG 1540 is now called PEG 1500, and PEG 4000 is now called PEG 3250. Although the PEGs have some of the physical properties of waxes, they are distinguished from true waxes by the fact that they are freely soluble in alcohol (ethanol, methanol, isopropanol), as well as water. PEG 4000, which has a melting point of 53-55°C, was once the most commonly used PEG because it is the least hygroscopic; its large molecules, however, prevent it from penetrating dense wood. Now PEG 1500 and the 540 Blend are more commonly used.
The PEG conservation process was the first reliable method for treating waterlogged wood that was also relatively simple to perform. This method removes excess water while simultaneously bulking the wood. After preliminary cleaning to remove all surface dirt, the waterlogged object is placed in a ventilated vat containing a solution of PEG and solvent (water or alcohol). The vat temperature is gradually increased until, after a period of days or weeks, it has reached 60°C. During this time, the PEG percentage of the solution increases as additional increments of PEG are added while the solvent evaporates. During the conservation process, the wax slowly permeates the wood, displacing the water. At the end of the operation, the wooden object is covered with 70-100 percent molten PEG, depending upon the nature of the wood. The object is then removed, the excess wax wiped off, and the object is allowed to cool. After cooling, any excess wax on the surface of the object is removed with a hot-air gun or with hot water.
In most instances, the wood to be treated is placed in a vat of water containing a small increment of PEG (usually 1-5 percent. The vat is kept at a constant temperature of approximately 52°C. If the solution is not heated, it will solidify when the concentration of PEG in the solution reaches 20-30 percent. Over a period of months (or even years), the PEG percentage of the solution is increased in small increments until a minimum concentration of 70 percent is reached. If this minimum concentration is reached, the wood will remain stable. In some instances, if the percentage of PEG in solution exceeds 70 percent, water may be drawn out of the well-preserved heartwood without being replaced by PEG; this will cause the wood to collapse. The size of the PEG increments is dependent upon the condition, size, and specie of the wood being treated.
An additional method of treating waterlogged wood that is only appropriate for small objects and thus is seldom used in practice involves using a container in which the PEG concentration is increased solely by the evaporation of the solvent. When performing this procedure, it is important that the dimensions of the container be such that the amount of PEG in solution will be more than enough to cover the object at the end of the process.
It has already been noted that PEG is soluble in both water and various alcohols. Water is generally used in PEG solutions for large objects, as it is considerably cheaper than an equal volume of alcohol. When using PEG in water it is necessary to add a fungicide, such as Dowicide 1 (ortho phenylphenol), at .05 to .1 percent of the weight of the PEG used. During the conservation of the 17th-century warshipWasa, a fungicide consisting of seven parts boric acid and three parts sodium borate (1 percent of weight of PEG) was used (Barkman 1975:82). For smaller objects, it is often more convenient to use alcohol in the PEG solution. This considerably reduces the overall treatment time, and the finished product is lighter in both weight and color. To further reduce treatment time, the additional step of dehydrating the wood in at least three baths of ethanol before placing it in the first PEG/alcohol solution is recommended. However, it is not critical that all the water be removed from the wood prior to treatment, as PEG is soluble in both water and alcohol. Alcohol treatments save time but are less cost-effective and always pose the risks inherent in heating alcohol. Since all alcohols are fungicidal, no fungicide is required when using alcohol in PEG solutions.
Before a decision is made to conserve wood with PEG, it is important to consider the fact that PEG is corrosive to all metals, especially iron. For this reason, PEG treatments should not be used on wood that will be in contact with any metal (e.g., gun stocks).
Treating small waterlogged wood artifacts with PEG in the laboratory is a simple and straightforward process. Small vats (stainless steel or glass) are readily available and they can be placed in a thermostatically controlled oven to maintain the correct temperature; furthermore, only a small amount of PEG is required. In contrast, when large pieces of wood are treated, there is a considerable investment in PEG (sometimes measured in the tons). A substantial vat must also be constructed with the capability to heat and circulate the solution. Laboratories that intend to conserve large pieces of waterlogged wood must be prepared to make major investments in both equipment and chemicals. Of all of the wood conservation methods discussed in this section, any of the various PEG treatments with water is the most utilized because of its reliability and low cost.
Figure 6.5. Setup for treatment of small specimens of waterlogged wood.
The sucrose (sugar) method of conserving waterlogged wood was developed as an alternative to more expensive methods (Parrent 1983, 1985). The procedure is identical to that described for PEG, except that sucrose is used. Wood to be conserved should be carefully cleaned by rinsing in baths of fresh water to remove all ingrained dirt and to remove the bulk of any soluble salts that are present. Once the wood is cleaned, the following procedure is recommended:
If sugar is selected as the treating medium, refined white sugar (pure sucrose) should be used. The brown- colored, coarse-grained unrefined sugar (Type A sugar) should be avoided, as it is much more hygroscopic than the white. Each time the relative humidity rises, the surfaces of wood treated in unrefined sugar will become wet. This hygroscopicity is analogous to that encountered when using the medium molecular weight PEGs. The Type A sugar-treated wood, however, remains dimensionally stable.
Maintaining artifacts treated by sugar in a controlled atmosphere will ensure the continued success of the conservation procedure. Artifacts conserved with this method require no more or no less care than those treated with other preservatives. This method constitutes an acceptable means of conserving waterlogged wood and is the least expensive of the methods currently available. Sucrose-treated wood, however, has a dull muted color, and small hair line cracks will frequently form on the surfaces. The treatment will produce dimensionally stable wood and is a viable alternative when the overall cost is a major consideration. The required equipment is the same as discussed above for PEG treatments.
The treatment consists of replacing the water in wood with a natural rosin, in this case, pine rosin (also called colophony). This treatment was developed to conserve well-preserved hardwoods that cannot be penetrated by the higher molecular weight PEGs (McKerrel and Varanyi 1972; Bryce et al. 1975).
The following procedure is recommended:
In a sealed container at a thermostatically controlled 52°C, a saturated solution of rosin in acetone is 67 percent rosin. To ensure a saturated solution, an excess amount of rosin should be placed in the container. This results in a thick, viscous layer of rosin that will settle to the bottom of the container. The object being treated should be suspended or supported above this undissolved rosin. Objects 5-10 cm thick should be left in the solution for four weeks, while objects less than 5 cm thick should be left in the solution for two weeks. These treatment durations are only rough approximations; each piece of wood should be evaluated based upon its own characteristics.
In some cases, when conserving very well-preserved hardwood, the conservator might consider submerging the wood in a 10 percent hydrochloric acid (HCl) bath after washing the object and before dehydrating the wood in Step 2 (above). Pre-treatment with hydrochloric acid improves the penetration of the rosin into the object by breaking down the organic acids in the wood. Caution must be exercised, however, as hydrochloric acid may result in a checked surface, which is more subject to cracking post-treatment. In addition, hydrochloric acid is supposed to bleach the wood to a more natural or original color, but the bleaching is only temporary and rarely affects the final color of the treated piece. (Hydrochloric acid pre-treatment can also be used to improve the penetration of PEG into wood, although the finished product may be more prone to checking and shrinkage.) To make the pre-treatment bath, mix one volume of HCl to nine volumes of water. The duration of pre-treatment is very variable, but objects 5-10 cm thick should be left in the acid for approximately four days, while objects less then 5 cm should be left in the acid for about two days. After pre-treatment, it is necessary to rinse the wood in running water for three to five days to thoroughly remove all traces of the acid before continuing to Step 2 (above). This pre-treatment is optional and is often eliminated because of the potential damage to the object.
In practice, ethanol is often used instead of accetone as a solvent for the rosin (especially when treatment is carried out in a PVC pipe). Room-temperature treatments, both in acetone and isopropanol, are also commonly employed. If room temperature treatments are used, the treatment time is increased considerably to ensure complete saturation of the object with the rosin solution.
The advantages of the acetone-rosin treatment include the fact that treated wood is light in weight, dry, strong, and can be glued and repaired easily. Because rosin does not react with any of the metals, the acetone-rosin treatment can be used on compound wood and metal objects. It is considered by many to be the treatment of choice for all composite wood/metal artifacts. Disadvantages include the flammability of acetone and the high cost of materials, which make this treatment practical only for small objects. In addition, the treatment would not be an ideal choice in cases where it is necessary to flex the treated wood (i.e., when reconstructing a composite object) because the wood will splinter and break if it is flexed too much.
In general, the only problems that have resulted from using acetone-rosin have occurred when an old solution was used in which the acetone had already absorbed a considerable amount of water from the atmosphere. It is important that dry acetone or alcohol be used. Despite the inherent dangers of the treatment and the relative expense, the acetone-rosin treatment should be used more frequently by conservators, particularly for small pieces. This treatment has one of the better success records in wood conservation and produces the most dimensionally stabilized wood after the PEG 400 and 540 Blend treatments but without the hygroscopic problems of PEG (Grattan 1982b).
This method is similar to the process used for drying out biological specimens. If necessary, the wood should be cleaned prior to treatment. The waterlogged object is first immersed in successive baths of alcohol until all the water has been replaced by the alcohol. Isopropanol or ethanol is usually used. This is followed by successive baths of acetone. If necessary, the dehydration progress can be monitored by measuring the specific gravity of each bath. When all water has been replaced by acetone, the object is immersed in successive baths of dimethyl ether to replace all the acetone with ether. When this has been accomplished, the object is dried very quickly by placing it in vacuum to rapidly volatize the ether. Ether is used because it has a very low surface tension of 0.17 dyne/cm compared to 0.72 dyne/cm for water. This means that when the ether evaporates, the surface tension forces are so low that there is no appreciable collapse of the weakened cell wall. If desired, 10-20 percent dammar resin, colophony rosin, or a mixture of the two may be dissolved in the final bath of ether to consolidate the wood and to protect it from warping due to changes in relative humidity. PVA may be used in place of the resins on some pieces.
This method has proved to very successful, producing a very natural-looking wood that is light in both weight and color. The dehydration process is very efficient, but the alcohols and ether must be water-free. For many objects, a dehydration of only alcohol and acetone is effective. Due to the high cost of the materials, this method is practical only for the treatment of small objects. The alcohols and especially the ether are highly flammable, and extreme caution should taken when conserving wood with this method.
In essence, this treatment is analogous to the dehydration method described above but with a temporary bulking agent added. The water in the wood is completely displaced by a water-miscible alcohol, which is then displaced by camphor. The camphor fills the cavities and cell walls of the wood, then slowly sublimates (goes directly from a solid state to a gas) without exerting any surface tension on the cell walls. Consequently, the wood does not collapse, shrink, or distort. The treatment results in a very aromatic, lightweight, and light-colored wood. Camphor can be dissolved in any of the alcohols. The following procedure is recommended.
The camphor-alcohol method comes very highly recommended, but like the alcohol-ether method, it is economically impractical for the treatment of large objects and the solution is highly flammable.
Freeze drying is used with some regularity of small pieces of wood, but the only limitation is access to the properly sized freeze-drying container (Ambrose 1970, 1975; Rosenquist 1975; McCawley et al. 1982; Watson 1982). In the past, the main problem that presented itself was the tendency for the surface of the wood to check and crack. This is caused by the ice crystals expanding and damaging the cell walls. Ambrose (1970) found that if the wood is pre-treated by soaking it in a 10 percent solution of PEG 400 until it is saturated, the formation of ice crystals during the freeze-drying process is essentially eliminated. This pre-treatment has become a standard element of the freeze-drying method for wood, as well as for leather. In addition to inhibiting the formation of ice crystals during freeze drying, the PEG introduced into the object during pre-treatment will act as a humectant after treatment and prevents the wood from undergoing excessive shrinkage.
More recently, Watson (1987:274-275) observed that a 20 percent or higher PEG solution will dehydrate and kill any microorganisms present in the solution through osmosis. He recommends using 20 percent PEG 400 for mildly degraded wood and 10 percent PEG 400 + 15 percent PEG 4000 for more degraded pieces. For severely degraded wood, the PEG 4000 may be increased to up to 25 percent, but treatment time is increased when PEG 4000 is used. If a PEG solution of less than 20 percent is used, a fungicide, such as 1 percent borax/boric acid or Dowicide 1, should be mixed with the PEG solution to stop any slime or mold from growing in the solution during pre-treatment.
Following pre-treatment with PEG, the wood is frozen in a domestic freezer. After freezing, it is recommended that the wood be placed in a freeze-drying chamber at a temperature of -32 to -40°C, and a vacuum applied after the temperature of the wood reaches -25°C. During the process, the frozen ice crystals sublimate, and the water vapor is frozen onto the condenser coils. This continues until all the water is removed, which can be determined by weighing the piece being treated. The treatment is completed when the weight loss stabilizes. After treatment, the wood should be stored in a relative humidity of 45-60 percent. Freeze drying as described here and in the chapter on leather is essentially the same when treating any waterlogged organic material. (See Watson 1987 for additional details.)
Although the freezing can be done in a chest freezer, like biological specimens, a quick freeze is best. This can be achieved by immersing the wood in a container with acetone and dry ice (frozen CO2). Some acceptable results have been achieved using non-vacuum freeze drying in a domestic freezer (particularly frost-free freezers). When a domestic freezer is used, the pre-treated wood is placed in the freezer on an open rack to allow air to circulate around it, and left there until it is dried. In this non-vacuum process, treatment times are in terms of months, as opposed to weeks in a vacuum freeze drier (McCawley et al. 1982).
Of all of the treatments discussed in this section, freeze drying is the most expensive due to the high cost of freeze dryers. Because of the size limitations of most freeze dryers, and the substantially higher costs when investing in equipment capable of treating larger objects, freeze drying is restricted to small objects in most laboratories.
SILICONE OIL TREATMENT
Since 1993, Dr. C. Wayne Smith of the Conservation Research Laboratory and the Archaeological Preservation Research Laboratory at Texas A&M University has been conducting research in the use of polymer media for the stabilization and conservation of organic materials. Waterlogged wood, glass, leather, woven basketry, and cork have been successfully conserved with polymer media, as well as artifacts such as corn cobs, which have been nearly impossible to conserve while maintaining the diagnostic features of the samples. The conservation of animal hides, biological tissues, and archaeological and histological bone samples has also been successful. Electron microscopic and chemical analysis of organic samples, which have been stabilized by the displacement of free water and air with silicone polymers exhibit some unique qualities over water-stored and air-dried specimens. An informal survey of university laboratories and departments has indicated that there are numerous areas where silicone bulking and related technologies would have almost immediate beneficial impact. The same holds true for museum artifact conservation, archival work, and in industrial applications.
A simplified version of the silicone bulking process, which is applicable for the treatment of small wood artifacts and other organic material is as follows:
This silicone oil treatment results in a very naturally colored wood that undergoes little to no dimensional changes. The wood is stable and does not require the close environmental controls like some other treated woods. The conservator must keep in mind that this treatment is not reversible; however, few of the other treatments outlined in this chapter are reversible.
Reversibility in Artifact Conservation
Ideally, reversibility is a desirable aspect of any conservation process. In reality, however, the issues of reversibility have been grossly overstated and, in many cases, misrepresented. For example, it is impossible to remove all of the polyethylene glycol from a conserved piece of waterlogged and badly deteriorated wood. During the process of treatment, some of the PEG is chemically bonded to the remaining lignin and cellular structures of the wood, preventing the complete removal of the polymer. In addition to chemically bonding, additional PEG will simply be trapped in cellular voids. Even the best methods that treat wood with PEG cause intracellular damage during treatment. In essence, the process of conserving the wood may undermine the structural integrity of the wood. The process of removing PEG causes additional damage to the already weakened physical structure of the wood. More times than not, the process of re-treating heavily waterlogged, damaged timbers causes more damage than should be desirable.
Too often, the theoretical state of reversibility of an artifact outweighs important issues that include the degree of artifact degradation, effects of attempted treatment reversal, the real potential for successful 100 percent reversibility, and the best interest of the artifact. In contrast, silicone oil treatments for even heavily waterlogged and damaged wood do not cause the cellular distortion that has been associated with PEG treatments. After treatment, thin sections of polymer-treated wood samples are so well preserved that in most cases, post-treatment genus and species identification are possible.
To date, it has been demonstrated that silicone and polymer treatment processes are reversible. In reality, however, the potential for loss of diagnostic attributes is too high. This suggests that a great deal of research needs to be completed in the development of reversible processes. Expected longevity, short time frame for conservation, and ease of curation, however, are invaluable aspects of silicone and polymer processes that make them a serious consideration for the treatment of many artifacts.
The greater issue which conservators need to address is the long-term well-being of an artifact assemblage. Many of the conventional processes which are routinely used for the conservation of artifacts have a relatively short life expectancy. This is why reversibility has always been an issue. In the case of PEG-treated artifacts, permanent curation in climate- and temperature-controlled environments only prolongs, to some degree, the life expectancy of the artifact. Water miscibility and chemical changes within this bulking media inevitably cause slow degeneration within the artifact.
The longevity of silicone and polymer processes is not an issue. Extensive testing and nearly 25 years of data collected by the silicone and polymer industries has demonstrated that the minimum half-life of the polymers used in conservation is at least 200 years. Ease of treatment in using these new technologies, too, is another consideration. Actual treatment times for the conservation of very delicate glass beads recovered from excavations of the Uluburun shipwreck (ca.1300 B.C.) is approximately 20 minutes. Once completed, the beads require only a brief curing period before they can be handled.
The last consideration to bring to the whole issue of reversibility is that strict adherence to traditional technologies is a good way to never discover new, and hopefully better technologies. This manual does not suggest that silicones are the panacea for all conservation needs. Rather, it suggests that these new technologies will, and are, having an impact in archaeological conservation simply because reversibility has never been an absolute fact using traditional processes. Silicone and polymer processes are merely an additional set of tools in the conservator's tool kit. Research has indicated that the following decades will hold exciting new advances in the science of archaeological conservation. Conservation sciences have a responsibility to seek out, define and refine tomorrow's technologies.
There are several other treatments for conserving waterlogged wood, such as bulking with paraffin in a solution of hexane, but they are not extensively used. What is most important to realize is that the problems of conserving waterlogged wood may be overcome with a number of treatments. The decision to select a treatment may be dependent upon one or many factors: a particular color of wood or enhanced grain is desired; the resulting wood must be glueable, flexible, or rigid; the wood is part of a compound wood/metal artifact; or the resulting product cannot be sensitive to fluctuations of humidity and can withstand storage in adverse conditions. All of these issues may be of concern to the conservator, and there are ways of treating waterlogged wood that provide the desired result for each of these cases. All of the treatments outlined in this chapter are applicable to different situations and all should be considered acceptable alternatives to the conservator.