Conservation and restoration of waterlogged wood

Mary Rose, an English warship, was raised from the ships underwater site on October 11, 1982. Since then there has been extensive conservation and preservation on the vessel. This part of the vessel is almost exclusively wood, making conservation imminent. The extracted vessel is on exhibit at the Mary Rose Museum in the United Kingdom.[2]

Wood is an organic material and decays under biological and chemical degradation when buried or submerged above water or soil. Wood, an organic material produced by plants, are chemically composed of: carbohydrates (cellulose and hemicellulose), lignin and other components (aliphatic acids, alcohols, proteins and inorganic substances) in a smaller amount. The most important composition of the plant is the cellulose. The cellulose accounts for the majority of the cell, about 40% to 50% of the wood's total mass. Hemicellulose represents the second most important carbohydrate and accounts for 20% to 30% of the wood's cell.[3] In cases of extreme wetness or dryness, wood can be preserved until the extreme environment is disrupted. Conservation of wooden waterlogged objects is dependent on the natural wood type and biological structure. Wood is separated into two categories, hardwoods and softwoods. The category the waterlogged wood falls into can determine the amount of porous absorption. Hardwoods are classified as angiosperms. Angiosperms are considered porous woods because of the vessel pores. Softwoods are gymnosperms and are considered non-porous because of the lack of vessel pores. The type of wood and the availability of vessel pores largely affect how conservators treat and preserve waterlogged wood.[4] Waterlogged wood objects can be found in a range of excavations sites. For example, waterlogged wood is something an archaeologist might stumble upon during an excavation either from the wood being submerged or near water or being in moist soil over time.

Inconsistent relative humidity and temperature provide the perfect environment for mold and fungi growth on waterlogged wood. Growth of mold or other bacteria requires immediate attention to control damage. Natural cellulosic materials are the best environment for mold spores to grow and mature. "Mold is a microorganism that produces enzymes that convert the cellulose in fibers to soluble sugar that is metabolized as food. Proteins are generally less susceptible, but keratinophilic fungi will feed on, and damage, these fibers as well".[6] Similar to wood found in nature, mold and other feeding organisms are attracted to the material, this includes pests and naturally occurring fungi and bacteria. Mold is a high risk for waterlogged wood even with controlled relative humidity and temperature due to the resilient nature of the organisms and the perfect environment waterlogged wood provides as a food source to the growth of spores.

Diagram showing shipworm boring into a piece of wood.

Within maritime environments one of the largest pest threats to waterlogged wood includes shipworms (Teredinidae). Shipworms bore into wood that is immersed in seawater.[7] Within a museum exhibit or storage environment and post treatment of waterlogged wood objects, there are a number of wood-boring pests that feast on wood. Two of which include the wood-boring/powderpost beetle and termites. Wood-boring beetles can include several different types within the board category. Each type of beetle could target a different type of wood. Evidence of pest deterioration includes powder-like frass near entry and exit holes within the wood. Termites also feast on and live below the surface of the wood so they can be more difficult to detect. Termites discharge six-sided fecal pellets and, if found on a wooden object, could be used to determine the type of pest damage[8]

Physical deterioration of waterlogged wood can happen immediately through the evaporation of the water. If this happens rapidly, the cells in the wood can shrink and collapse. This damage might be inevitable depending on accessibility to treatment. Additionally waterlogged wood should never be handled extensively or put on exhibit for long periods of time.

Waterlogged wood should be in the environment that it is found in until a stable treatment proposal is found. Most waterlogged wood is such that the cells in the material fill up with water. Depending on the wood type and how long the wood has been in water, removal of the water from the cells too rapidly (in the form of natural dehydration or other) could apply major stress to the cell walls and cause them to collapse and deteriorate. Waterlogged wood, prior to treatment, should be kept in a similar environment of hydration. To limit the growth of mold, waterlogged wood should be stored with proper relative humidity that does not encourage growth. Since moisture of the waterlogged wood is imminent, waterlogged wood is best stored for preventative conservation submerged in a solution to what the wood is waterlogged with. Similar to the preservation of zoological specimen, temporarily preserving waterlogged wood in the environment it was in helps minimize the severity of change. Waterlogged wood will retain its structure as long as it is wet. If the wood is exposed to air, and water evaporates, there is a change in surface tension that forces the evaporating water out of the weakened cell walls which cause the walls to collapse. This force ends up shrinking and ultimately distorting the object.[9]

The purpose of the polyethylene glycol is to remove water from the wood while simultaneously bulking the deteriorated wood cells. PEG is applied to the surface, either by spraying or immersing the object in a solution of PEG in water. Over the course of the treatment, PEG is slowly increased in concentration, coating the interior walls of the wood cells with PEG, which will provide support once the artifact is dried. PEG compounds with different molecular weights may be used depending on the wood type, level of deterioration, and scope of project.

The PEG treatment is often paired with freeze-drying, as the eutectic point of most PEG solutions is below the freezing point of water. This allows the free waterlogging water to sublimate in the process of freeze-drying, which minimizes damage to the wood by preventing ice from forming within the cells and expanding, causing more shrinking and warping. Some artifacts are too large to fit in a commercially available freeze-drier (see the treatment for Vasa or Mary Rose[10] or the Bremen cog[11]).

Identical to PEG treatment process but sucrose is used instead of PEG solution. The cells of the wood are replaced by sucrose, rather than water.

Acetone-Rosin Treatment is sometimes used on dense wood that cannot be penetrated by PEG. This would include softwoods that are nonporous. The goal of this treatment is to replace cells of wood with natural rosin. Rosin is a natural resin that is produced within some woods, for example pines naturally produce resin and are considered a softwood. As a volatile solvent treatment option, the impact on the health and safety of the workplace must be considered.

This treatment follows the similar treatment of the PEG solution but instead of replacing the cells with another solution, the cells are replaced with alcohol and when the alcohol evaporates the cell and overall object is dehydrated.

Camphor treatment is similar to the Alcohol-Ether treatment but instead of a quick evaporation of the alcohol inside of the cells, the camphor alcohol slowly replaces cell walls with camphor which goes from a solid to a gas state over time, keeping walls of cells bulked.

Treatment must also take into account the type of water the wood was found in. Soluble salts must be removed from the waterlogged wood. This can be done by a desalination process. The removal of soluble salts, desalination, of wood extracted from a marine environment is crucial. Desalination is done with clean water, with disinfectant added to prevent the development of harmful organisms. The disinfectant fungicide, algaecide, orthophenyl phenol; however, the most commonly used and recommended because of its lesser toxicity is a mixture of boric acid and borax. The desalination process takes a long period of time and is necessary that the water is changed until the concentration of excreted of soluble salts reach its maximum.[12]

January 2016 a mid 18th century ship was discovered on the waterfront of Alexandria, Virginia. Conservation efforts include keeping the wood submerged and wet since the wooden framed was waterlogged. the conservation history of this ship is ongoing. When the waterlogged wooden frame was originally removed from the archaeological site, the timber frames were stored in fresh water vats until June 2017. The framed structure was then packaged and sent to the Conservation Research Laboratory at Texas A&M University for conservation. Documenting and conservation included laser scanning, modeling, X-ray, and wood degradation analysis before the treatment using polyethylene glycol and vacuum freeze drying[13]

Mary Rose being sprayed with water in a Plymouth conservation facility, March 1984.

After finding the vessel Mary Rose objects and what was left of the ship were placed in a passive storage which slowed down the process of immediate deterioration removing the vessel from its extreme environment. In 1994, an elaborate three phased conservation treatment began on Mary Rose. From 1993 to 2003 was the first phase that consisted of the wood getting sprayed with low-molecular-weight PEG to replace the water in the cellular structure of the wood. The second phase from 2003 to 2010, raised the molecular-weight of the PEG solution with the purpose to strengthen the outer surface layers. The third phase in 2016, included controlled air drying.[14][15]

Vasa on exhibit, from the picture you can tell that a large majority of the ship was preserved.

The Vasa is a Swedish 1628 ship found archaeologically and was very well preserved. Completely submerged for 333 years allowed for the wood to be completely waterlogged but pollution in the 20th century off of the city Stockholm was able to kill any microorganisms that would have feast on the wreck, like the shipworm. Vasa was sprayed with a PEG solution for 17 years followed by a period of drying which is ongoing.[16]