Sucrose (Sugar) Treatment of Waterlogged Wood
The first step in the sucrose treatment process, as in any treatment of waterlogged wood, is freshwater rinsing and careful mechanical cleaning. Of course, all waterlogged woods should be immersed in water immediately upon their removal from the archaeological context, as even a few minutes of exposure to air can cause irreparable damage (Cronyn 1990: 261-262). While the first impulse of field archaeologists may be to place a wooden artifact from a marine site into fresh water, so as to immediately begin the desalination process, this action could in actuality prove detrimental. This is because the difference in salinity between the water inside the wood and that outside creates an osmotic pressure differential, triggering the outward movement of the salt solution from within the wood. The tracheid walls of the wood cell structure form a semi-permeable membrane which prohibits fresh water from the outside from entering the cell as fast as salt water can leave it, which results in capillary tension collapse similar to that which occurs if the wood is allowed to air-dry. This effect is basically the same as that which shrivels one's toes after a long bath, but in the case of wood the distortion is permanent (Rodgers 1992: 22-23).
This danger can be eliminated simply by subjecting the artifact to a gradual introduction to fresh water. Typically a 50% fresh-salt water mix is used for the storage of newly-recovered waterlogged wood. The desalination of the wood can proceed at a slow pace, changing the water about every week or so, each time with a 50% mix. After two or more freshwater additions, the salinity should be around four parts per thousand; it is then safe to immerse the object in 100% fresh water (Rodgers 1992: 23). Successive baths of tap and desalinized water will result in the total (or virtually total) desalination of the object.
If the wooden artifact is from a freshwater context, then this concern is of course negated. Desalinized water baths, however, will still aid in the cleaning of the artifact. Mechanical cleaning will probably be required but this should be done with utmost caution, as even those specimens that look well-preserved are often delicate and easily subjected to damage. This is particularly true of the outer surface, which often exhibits tool marks or other features interesting to archaeologists. While cleaning may be required to expose this archaeological surface, it is best to record any such features before active treatment begins, as they can become obscured by most treatment procedures or even prolonged wet storage (Cronyn 1990: 257).
Mud, clay, or silt can usually be removed by holding the object under a running stream of water, or with a gentle sprayer. Persistent sediments or bits of concretion can usually be dealt with by cleaning with a soft-bristled brush, or careful use of a dental pick (cf. Cronyn 1990: 253; Rodgers 1992: 24). It is important to continuously wet the object to prevent damage from air-drying (capillary tension collapse can begin to occur on the cellular level even if the exterior of the artifact appears damp). It is also a good idea to wrap the artifact in a wet cloth so that only the immediate area being cleaned is exposed, thus reducing the chance of accidentally marking the object's surface.
Barnacles and shell growth can sometimes present a challenge during the cleaning process, and may have to be removed with a scalpel and forceps. Rodgers (1992: 24) points out that the under-surface of such growth can sometimes exhibit details of the wood's original surface, and should be inspected before disposal.
Once the object is cleaned and recorded (and any appropriate analyses-species identification, specific gravity, moisture content, etc.óhave been made), it should be placed back into desalinized water storage to await active treatment. It should be remembered to check storage tanks for algal or other biological growth, especially if there is a delay before the start of active treatment (the addition of an appropriate biocide may be prudent in such a case).
It is assumed that there has been some thought and discussion upon the reason why sucrose is the preferred treatment for this particular artifact(s). Sucrose bulking is one of several traditional methods of conserving waterlogged wood, including PEG (polyethylene glycol) bulking, acetone-rosin treatment, alcohol-ether and camphor-alcohol solvent treatments, and freeze-drying (often in conjunction with PEG bulking) (Hamilton 1996: 27, 29-32; Cronyn 1990: 257-261). Both PEG and sucrose are bulking agents; when using these treatments the conservator's goal is to incorporate the bulking material into the waterlogged wood, consolidating it and providing the mechanical strength necessary to prevent capillary tension collapse on the cellular level, and thus distortion and shrinkage of the artifact, as the water is removed. Solvent drying (acetone-rosin, alcohol-ether, and camphor-alcohol) and freeze-drying procedures are designed to remove excess water from the wood while preventing shrinkage and distortion, either with or without the addition of a bulking agent (Hamilton 1996: 27).
All of these treatments have drawbacks. Most of the solvents are quite flammable, creating a potential hazard, and they are often impractical for use on large or heavily decayed artifacts. Freeze-drying can be prohibitively expensive, and also is usually limited to the treatment of relatively small objects. PEG, the most common treatment used, can be very effective but it dramatically darkens and increases the weight of the final product. Additionally, all of these solvents and chemicals cost money, and can become quite expensive to purchase and transport, especially for a large-scale excavation (even more so for one in a remote area or third-world country).
Sucrose, on the other hand, is cheap and readily available, and of course non-toxic and non-volatile. It is lightweight, and will not artificially darken the color of the treated artifact. Expediency is often its most attractive attribute; for example, 16th century ships timbers excavated in Cuba-where funding is scarce, but sugar abundant-have been successfully treated with sucrose, which was seen as the only realistic method available (Roger Smith, personal communication, 11 March 1995). Another important consideration is that PEG and therefore PEG-treated wood is corrosive to all metals, especially iron. Sucrose is an ideal bulking agent, therefore, to use on composite wood-iron items whose parts will be in contact once in storage or on display (Rodgers 1992: 32; Hamilton 1996: 28).
While the use of sugar to treat wood dates to 1904 (see Parrent 1985: 83 for an account of experimental treatments throughout the first half of the 20th century), Jim Parrent in the early 1980s developed the methodology currently in use by most conservators, as outlined in his master's thesis and subsequent articles (Parrent 1983; 1985). The overview of treatment that follows is derived from his recommended procedure (1985: 93-94; subsequently re-printed in Hamilton 1996: 28-29). In general, the procedure is quite similar to that used with PEG.
Once the wooden artifact has been desalinized and cleaned as described above, an appropriate sucrose solution should be prepared. The conservator should be thoroughly familiar with the state of preservation of each individual object after the continuous examination entailed during the rinsing/cleaning process. It is important that the initial sucrose solution is low enough to prevent the dehydration of well-preserved wooden artifacts, or areas of well-preserved wood on or within an otherwise degraded piece. These dense, better preserved areas, especially in tight-grained hardwoods, tend to resist penetration of treatment mediums with high molecular weight (such as some varieties of PEG). Upon drying this usually causes localized warping, checking, andóin severe casesósplitting (Parrent 1985: 89). The low molecular weight of sucroseó342.3 góallows it to penetrate all regions of a wooden artifact, even one of well-preserved hardwood. But the conservator should correctly gauge the degradation of each individual object to determine the most appropriate initial sucrose concentration in order to minimize this problem. As a rule of thumb, the more decayed an object is, the greater concentration of sucrose may be used in the ambient solution. An extreme example was a highly degraded, soft, and spongy canoe fragment successfully treated with an initial sucrose solution of 20% (followed by weekly 10% increases) by the North Carolina Underwater Archaeology Unit (Bright 1987: 90). But if in doubt, the conservator should start with a decreased amountó1% is safe for any piece. Regardless, the initial solution normally should be between 1 and 5 % weight to volume (i.e., 1 to 5 grams of sucrose per 100 ml of water).
The sugar used should be refined white sugar (pure sucrose). Unrefined sugar (type A), recognized by its brown color and coarse grains, is not an acceptable material. While Parrent (1985: 88-89) demonstrated that unrefined sugar performs as well as refined sugar in shrinkage prevention, it causes problems related to the long-term stability of treated artifacts. Wood treated with unrefined sugar is much more hygroscopic than wood treated with refined sugar; what this means is that rises in relative humidity will cause the treated wood surface to become wet (this is also a problem encountered with medium molecular weight PEGs) (Hamilton 1996: 29). Type A sugar also seems to cause the finished wood to have a somewhat browner color (Parrent 1985: 89).
Two other steps are necessary when mixing the initial sucrose solution. A representative piece of wood in a particular vat should be weighed; this will allow the conservator to determine, through subsequent weighing, when it and any others in that container have reached equilibrium with their ambient solution. It is also important to add an appropriate antimicrobial agent, such as Dowicide (or Lysol), to the initial solution. This will allow for an even and complete penetration of the artifact. Biotic inhibitors are necessary because sucrose in concentrations of less than 40% tends to ferment. If this occurs, the bulking capacity of the sugar is negated, and the production of acetic acid with further damage the artifact (Rodgers 1992: 35-36). Fermentation can be readily detected by foam production and a distinctive odor.
It may also be desirable to add a chemical deterrent at this stage to discourage rodent or insect attacks on the wood after treatment. While often mentioned as a potential drawback to sucrose treatment, Parrent (1985: 90) believes that such vermin do not actively seek out sugar-treated wood, based on three years of continuous treatment and storage without such incidences. The controlled environments in most museum settings should preclude problems of this nature, though this may not be the case in third-world storage or display facilities. In such situations it may be prudent, then, to take precautions; and the insecticide should be added to the initial sucrose solution to insure full penetration. The conservator should choose an insecticide that will not produce harmful fumes or be readily absorbed through skin contact during the application process, and one that will not require any special safety precautions when handling the final treated object. Additionally, it should have a very low molecular weight so as not to interfere with the bulking process. Parrent recommends a combination of small concentrations (0.1% and 0.05% respectively) of sodium fluoride and coumarin. The former can be absorbed through the skin and appropriate safety precautions should be taken, though at those concentrations risk is minimal (Parrent 1985: 90, 93).
Once the initial solution has been prepared, the wooden artifact(s) must now soak; time is a prominent factor in all wood conservation. Once the wood has reached equilibrium with its solution, the sugar concentration should be increased by 1 to 10%. It is best to start with a lower increase, not more than 5%, unless the wood is highly degraded. In such a case the incremental increases may be more frequent in addition to being larger (up to 10%). The safest action, however, is to increase the concentration in increments of only 1 to 5% (again, weight to volume) after every soaking period (often one to two weeks or more at a time) during which the wood has reached equilibrium with its new solution. Parrent (1985: 84) gradually heated the solution to a temperature of 50∞C in some of his treatments, but this step does not seem necessary as he does not include it in his recommended procedure.
Once a concentration of 50% has been achieved, it is safe to start increasing the mixture 10% at a time. Of course, if the conservator is in doubt, small increments may be used throughout the entire process. These incremental increases continue until the wood has reached equilibrium with the desired final concentration; Hamilton (1996: 29) states this should be 70% though Parrent (1985: 84) continued increases until reaching concentrations of 100% in his experiments.
Regardless, upon reaching the desired concentration, the wood should be removed from the solution and slowly air-dried in a humidity chamber or other humidity-controlled environment. The slow drying at high humidity lessens the possibility of osmotic collapse from too fast a drying period, and allows the artifact(s) to dehydrate at a more uniform pace, which reduces stress incurred when different parts of an object dry at different rates (Rodgers 1992: 36). The humidity can be gradually lowered as the wood dries. This controlled-drying process is important for sucrose conservation as it is for all wood treatments.
The final consideration in the conservation of wooden artifacts with sucrose is proper storage conditions. As with all conserved artifacts, maintaining a controlled environment will ensure the continued success of the treatment procedure. Sucrose-treated wooden objects should be stored in atmospheric conditions of less than 70% humidity and, ideally, 30-40%. The wood should never be exposed to a relative humidity of over 80%, as the resultant condensation could leach out sugar from the wood and cause structural damage.
This essay was written by Chuck Meide in 2002.
|Bright, Leslie E.|
|1987||Candied Canoes of North Carolina. In Underwater Archaeology Proceedings from the Society for Historical Archaeology Conference, edited by Alan B. Albright, pp. 89-91. Society for Historical Archaeology, Savannah, Georgia.|
|Cronyn, Janet M.|
|1990||The Elements of Archaeological Conservation. Routledge, London.|
|1996||Basic Methods of Conserving Underwater Archaeological Material Culture. U.S. Department of Defense, Legacy Resource Management Program, Washington, D.C.|
|Parrent, James M.|
|1983||The Conservation of Waterlogged Wood Using Sucrose. Unpublished Master's thesis, Department of Anthropology, Texas A&M University, College Station.|
|1985||The Conservation of Waterlogged Wood Using Sucrose. In Proceedings of the Sixteenth Conference on Underwater Archaeology, edited by Paul Forsythe Johnson, pp. 83-95. Special Publication Series No. 4, Society for Historical Archaeology, Boston, Massachusetts.|
|Rodgers, Bradley A.|
|1992||The East Carolina University Conservator's Cookbook: A Methodological Approach to the Conservation of Water Soaked Artifacts. Program in Maritime History and Underwater Research, Department of History, East Carolina University, Greenville, North Carolina.|
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