Conservation of Archaeological Wood from the Fizine Site

Archaeological wood survives in places with no oxygen, most often in the silt of varied waters and bogs, or in places not suitable for the growth of microorganisms (very dry environments and permafrost). Archaeological wood discovered in wet environments is known as waterlogged wood. It has been preserved as microorganisms had been unable to fully decompose it owing to the lack of oxygen. Since it was often stuck in the silt for several centuries or even millennia, decomposition has nonetheless taken place at a slow rate, which is the reason why such wood is highly sensitive. It can be preserved to variable degrees, depending on the conditions to which it was exposed. Better preserved objects are still firm to the touch, whereas less preserved objects are soft to the touch, very similar to a biscuit soaked in tea or milk. The shape of such objects has been partially preserved also because they were saturated with water.. If any of waterlogged wooden objects was left in the air after its discovery, it would dry out quickly and change shape significantly. Specifically, mostly lignin that serves as support for the tree, is largely preserved, whereas the cellulose and hemicelluloses, which form the binding, have been completely or partially decomposed.

The conservation of this kind of object takes place in a very special way. The object must be kept in water almost till the end of the procedure, which can last several years (even more than ten for larger and thicker objects). If this is not possible, it should then be kept wrapped in foil and wet cloth in the coldest possible environment till the beginning of the treatment, in order to prevent the growth of microorganisms and the decomposition of the wood.
The procedure starts with cleaning of the surface with an attempt to remove as much sediment and any organisms (e.g. algae), which have accumulated on the surface of the object, as possible. The object is then subjected to deep cleaning as well. This is done by objects being soaked in deionized, demineralized or distilled water. During this process, water-soluble salts and acids, which could disturb further procedure, are released from the wood into the water.

The water must be changed regularly to accelerate the process and to reduce the possibility of microbial growth.
As soon as it is confirmed with pH and conductivity measurements that the object has been also suitably deep cleaned, a consolidant is prepared. In our workshop, we use predominantly a melamine resin solution. It is a water-soluble resin mixed with water and some other water-soluble additives. The object is soaked in this solution for few months (or even more than a year). During this procedure, a diffusion process takes place, which enables the resin to penetrate the wood. By checking how deeply the resin has already penetrated the object, the latter could be damaged, hence we carry out such analyses only occasionally, or rather leave the object in the solution till the resin is still fresh. The solution contains small resin molecules that react with each other in the course of time. We also exploit this attribute at the end of the consolidation process, when crosslinking the melamine resin molecules with each other by heating them. In this way, the resin forms a new framework for the wood – by consolidating it. During this process we must take care that the object does not start to dry out.

There follows the last, longest phase – drying. This is carried out at room temperature, but is considerably slowed down by wrapping the object in a vapor-permeable foil. Consequently, the water evaporates much more slowly, preventing the object from being mechanically damaged and deformed. Even though the wood is reinforced, it is still very feeble. We monitor the water evaporation by weighing the objects. As soon as the mass of the object stabilizes, the drying process is accomplished. The object is then protected with a coating and is ready for exhibition.

Text and photos: Katja Kavkler, Institute for the Protection of Cultural Heritage of Slovenia; Institute for the Protection of Cultural Heritage of Slovenia, Restoration Centre, Natural History Department