To preserve waterlogged wood artefacts various materials and procedures have been used over time, some fairly efficient and stable and others with deleterious effects on wood.
Archaeological wood is often found in a poor state of conservation since it has been subjected to physical, chemical and biological degradation. Depending on the degree of degradation, the artefacts can undergo conservation treatments to stabilise the wooden structure and avoid shrinkage and collapse. It is possible to preserve them through the application of single or combined conservation materials and freezing processes. The waterlogged wood can be impregnated with consolidation agents to provide superficial and/or internal strengthening. To prevent microbial growth, preservatives (biocides) can be added to the treatment solution. Most of the consolidants used in the past are no longer recommended due to their ability to degrade wood components such as cellulose and lignin and weaken its structure. To establish new conservation methods one has to take into account the properties of the consolidation agent and its interaction with the wood and other existing components (e.g., metal decoration and nails). It is important to consider a consolidant that: remains stable over long periods; is reversible; penetrates and distributes evenly throughout the wood; prevents dimensional changes and maintains the original appearance of the artefact; is compatible with wood, metals or other consolidants that might be present; has low or no toxicity; and prevents biological degradation and acidification of the wood. The most relevant consolidation agents assessed in the ArCo project and some complementary methods are described:
Potassium aluminium sulphate - KAl(SO4)2∙12H2O - is a salt that was frequently used as conservation material in Scandinavia in the 1850–1950’s. Alum is soluble in water and can penetrate the wood when in solution, replacing the free and bound water and reinforcing the wooden structure through formation of alum crystals. In brief, the waterlogged wood is immersed in a supersaturated alum solution (~100 °C) that recrystallizes inside the wood.
Since alum was considered the most suitable material to conserve highly degraded wood, the artefacts from the Oseberg find that were made of pine, birch and maple were treated with this salt. After impregnation with alum the wood was air dried and linseed oil and lacquer were applied, to reduce the adverse effects of moisture. Despite this consolidant stabilising the wooden structure and preventing shrinkage, alum treated objects are currently in great need of re-treatment: the alum crystals caused rupture of wood cells; some areas became more fragile and contain cracks and voids due to the uneven distribution of alum i.e., the inner core remained un-impregnated since alum does not penetrate in depth; the wood has become acidic and brittle due to the decomposition of alum during heating and consequent formation of sulphuric acid; migration and re-crystallization of salts occurred due to the dissociation of alum into ions during treatment and changes in humidity, resulting in salt precipitates on the wood surface.
The consolidant polyethylene glycol - H(OCH2CH2)nOH - was first used in the 1960’s and has since been applied widely in conservation treatments. This compound is able to replace the existing water inside the wood and maintain long-term stability of both the material and the wooden structure. Polyethylene glycol (PEG) exists in a range of molecular weights that are used in accordance with the degradation state of wood, through single or dual impregnation steps. In the one-step process, highly degraded wood is immersed in water and impregnated with increasing concentrations of high molecular mass PEG (10 to ~100%); when slightly degraded wood is treated the impregnation is done with low molecular mass PEG. To avoid biological deterioration preservatives are added to the treatment solution and, depending on the size of the object and density of the wood, air-drying or freeze-drying succeeds. In the two-step process, waterlogged wood is first impregnated with low molecular mass PEG, to allow the consolidant to penetrate further in wood and reduce shrinkage of the better preserved inner zone, and then with high molecular mass PEG, to stabilise the more degraded outer zone. PEG’s degree of penetration in wood depends on its molecular mass: low molecular mass PEG penetrates deeper than high molecular mass PEG. Furthermore, the water-soluble and hygroscopic properties of PEG behave according to their molecular mass: solubility in water and hygroscopicity increase as the molecular mass decreases. Despite low molecular mass PEG being suitable to treat the wood core it has the disadvantage of providing less mechanical strength, being more hygroscopic and consequently adsorbing water when relative humidity fluctuates. Another problem that arises when using PEG as consolidant is the artefact’s alteration of appearance; the surface of wood turns darker and becomes sticky after treatment. However, the most important factor to take into consideration is the ability of PEG to corrode metals and increase ionic mobility inside the wood, leading to accelerated degradation. Therefore, an alternative must be found to treat artefacts containing a considerable amount of inorganic compounds.
In the attempt to create the ideal consolidation agent and conserve waterlogged wood artefacts over generations, researchers have been developing new conservation materials. As mentioned above, a large number of prerequisites need to be fulfilled so that an appropriate conservation method can be established. Above all, it is important to keep the wooden artefact stable and re-treatable. However, prior to the application of a new consolidation material it is important to remove existing metal ions from wood and neutralise the acidic environment, in order to reduce the amount of deterioration sources. In this context, alternatives to preserve archaeological wood are presented:
The monomer styrene is often combined with unsaturated polyesters to form a radiation-curing resin used to impregnate archaeological wood. This method was developed in the late 1960’s and has been applied on small wooden artefacts in urgent need for intervention. For instance, wood from marine environments after treatment with PEG can develop salt precipitates and cracks; these mineral concretions can be treated with in situ application of styrene-unsaturated polyester resins. The freeze-dried wood is impregnated with the liquid resin under nitrogen pressure and polymerized through gamma ray irradiation. In this manner, the resin penetrates the wood in depth and involves and fixes the salt crystals inside the wood. The wood structure is well-strengthened and stabilized and maintains insensitive to fluctuating relative air humidity. However, polyester resins are only used in worst case scenarios and if no better solution is available since the method is irreversible.
Experiments have been performed using dicarboxylic acids - HO2C(CH2)nCO2H - as consolidant agent for waterlogged wood, in particular azelaic acid (n=7) and sebacic acid (n=8). These compounds are chemically resistant and insensitive to changes in relative humidity due to its hydrophobic nature. However, azelaic acid becomes water-soluble and can be used as wood consolidant when heated to 70°C; the wood is immersed in an azelaic acid solution (70–75°C), cooled down to allow crystallization and air-dried. Although this method was successful in replacing the water inside the wood and providing good mechanical strength, the treatment solution was acidic and azelaic acid interacted with existing metals and mineral salts. As an alternative, disodium sebacate (C10H16Na2O4) was synthesized from sebacic acid and used to control acidification, since this compound is partially soluble in water and has basic properties.
The idea of treating archaeological wood with consolidants that resemble materials existing in nature is promising. The materials used in bio-inspired conservation are non-toxic and chemically similar to wood components such as cellulose. It is important to create consolidants that provide good mechanical strength and, at the same time, leave an open structure for potential re-treatment of wood. For this reason, studies with crystalline cellulose and chitosan have been carried out. Crystalline cellulose whiskers can be obtained through treatment of pure cellulose with sulphuric acid, resulting in a product that is more resistant to acidification and moisture. These cellulose whiskers can form stable suspensions in water and be used to treat waterlogged wood, filling in some cracks and voids and forming an open structure after freeze-drying. However, this material is not able to penetrate in depth and the wood requires an additional impregnation step with a different consolidation agent to guarantee mechanical strength. Thus, significant focus has been put on chitosan, a derivative of chitin that is naturally available in crustacean shells (e.g., crabs and shrimps). This bio-polymer can be dissolved in a range of organic acids and be used to impregnate waterlogged wood; chitosan penetrates archaeological wood and attaches to cellulose remains, forming a stable structure after freeze-drying. In addition, chitosan can potentially bind to metal ions present in wood. These bio-inspired materials are of special interest when combined with other consolidation and/or neutralising agents. However, it is important to develop materials that function both as strong wood consolidants and acid neutralisers.