Archaeological objects made of organic materials such as wood, leather and textiles can undergo significant degradation during their burial period. In waterlogged sites, degradation is mainly caused by microorganisms which use these materials as a food source. During burial, the objects may also absorb iron and sulfur, which contribute to degradation after the object is exposed to air. Even after they are conserved, deterioration doesn't stop. Conserved archaeological wood and leather will undergo natural degradation over time. The rate of deterioration can increase if iron and sulfur are present in the object and not removed during conservation treatment - as is often the case, since they are difficult to remove.
Leather samples were investigated in order to try to link state of preservation to indicators, either chemical or physical. This proved to be more difficult than imagined. A part of the problem was the interference of remaining tannins, which overlapped with signals from collagen, making it difficult to interpret chemical changes in infrared spectra. Dynamic vapor sorption, which measures moisture content (MC) at different levels of relative humidity (RHs), showed some relation to macroscopic properties, such as stiffness and extent of powdering. Definite trends between state of preservation and MC were not possible to discern, however.
Although leather was investigated to some degree in this project, the main focus was on archaeological wood. We aimed to relate the chemical deterioration in the wood, using different analytical methods, to the type of iron- and sulfur-compounds it contained. However the typical high variability of archaeological wood samples made comparisons to wood condition very challenging. Despite this, we can draw some interesting conclusions, briefly described here.
Initial analyses using infrared spectroscopy and X-ray diffraction (XRD) showed that the consolidant dominated all spectra. Therefore time was used to develop a protocol which allowed separation of the combination of wood + inorganic content from the consolidant. Water was compared to dichloromethane (DCM). Infrared spectra of DCM-rinsed samples demonstrated that more inorganic content was preserved than in water-rinsed samples, allowing for their identification using XRD.
We saw that wood samples had highly variable inorganic content. Inorganic compounds identified were both ‘inert’ (quartz, calcium carbonate, gypsum), and potentially chemically unstable (pyrite and various iron sulfate hydrates). Generally, the lower the pyrite content, the greater the content of iron sulfate hydrates, indicating that the pyrite oxidized, producing these compounds, along with acids. Infrared spectroscopy showed that all archaeological woods were extensively degraded. However it was not possible to establish trends with iron salt contents. This is likely because some wood signals overlap with those from the inorganic compounds, which prevents interpretation of wood condition.
Overall, the work confirmed the complexity of wood as a material, especially when it is also degraded and contains impurities. In order to evaluate the potential risk of degradation due to the presence of unstable salts, further work should focus on developing methods for multivariate comparisons of parameters.
We also investigated the chemical state of preservation of degraded PEG 2000 from France which had been recycled several times. Here the amount of moisture uptake in PEG samples correlated well with extent of degradation. We found this is a simple way to screen PEG condition, compared to more advanced techniques (such as HPLC, etc.). We plan to undertake further work investigating PEG degradation in treated archaeological wood.
Other work in StAr investigates ways to avoid mould growth when objects are stored in water, before conservation. Storage in water can last from several months to years. Mould can disfigure the object permanently. So far, we have tested several potential additives. Two of the tested additives were found to function satisfactorily (ethanol and azelaic acid) while other additives showed poor results (mineral salts, citric acid, lysozyme, hydrogen peroxide, calcium hydroxide, sodium carbonate and mixtures of sodium carbonate / sodium bicarbonate), either because they were ineffective, or because they caused damage to the wood.
Further experimentation using archaeological wood from Biskupin has shown that storage solutions for using a combination of azelaic acid and ethanol effectively maintained much lower levels of bacterial activity over 6 months, compared to control solutions. Solutions were most effective when the wood has been well-cleaned of soil deposits. Further experiments are planned for longer periods and using different fatty acids and other compounds such as tannins.
Academic and practical: We have found promising methods for long term storage in solutions containing waterlogged organic archaeological finds, which reduce biological growth and are environmentally safe. Further research will be carried out by partners in France.
Identifying relevant parameters and analytical techniques to evaluate material stability after conservation treatment: Here we certainly increased our own knowledge on treated archaeological wood which holds potentially unstable salts. We found the work to be extremely challenging, since special protocols had to be developed and tested in order to be able to obtain information held within samples, such as wood condition and identification of the salts present. The mechanisms of degradation however are not elucidated. For example, in some samples, pyrite had oxidized to acidic hydrates of iron sulfate, while in other samples, the pyrite held in the wood has not oxidized. It was also difficult to evaluate the effects on wood condition of the acidic salts formed after pyrite oxidation. This is due to the natural high variability of archaeological wood, which often defies attempts to elucidate degradation trends, even when several complementary analytical methods are used. We are however, optimistic, and see that our research results will certainly add to existing knowledge on the topic.
We also began to investigate methods to determine PEG degradation. This is important to understand, as it has been used as a wood consolidant for more than 50 years. Finding straightforward ways to monitor PEG condition is a general challenge, but we have promising results from sorption studies. More time-consuming and specialized analytical methods in further work will be used to better understand the phenomena presented in sorption data, so that in future, we can quickly analyze and evaluate PEG extracts from wood.
Research results from this project will also be used to structure further work on the specific case of the Klåstad ship, which is currently undergoing active degradation in regions around nail holes. We aim to obtain enough information to develop methods for its long term preservation.
Public awareness: In Oslo, we mainly focused on informing conservation professionals about the ongoing research. The project has also been presented at several international scientific conferences.
Economic: Inappropriate conservation treatments are expensive to deal with, increase risk of object damage and may involve expensive modifications of building ventilation. Finding appropriate solutions for object display and storage must be knowledge based. An immediate impact of this project lies with the potential to find solutions for the long-term preservation of the Klåstad ship, often called Norway’s Fourth Viking Ship. Tourism represents a vital sector for the economic resources of many towns, including Tønsberg, so good preservation strategies will extend the life of this object.
Although much work has been done on conservation treatments of archaeological collections, there are few studies of two critical situations, pre- and post-treatment phases.
Archaeological organic artefacts (AOA) are generally found in a waterlogged state which must be maintained until treatment as they cannot support air drying. One of the topics in StAR deals with developing strategies that permit storage of organic archaeological finds for long periods in the waterlogged state, without compromising their scientific evidence. Several methods of controlling storage conditions will be tested on real archaeological wooden and leather samples. Specific monitoring protocols for these wet artefacts will be adapted. Analysis of the organic materials before and after their storage period are planned. Experiments are foreseen in a true archaeological excavation context (Biskupin site in Poland).
The second topic aims to set up efficient and practical methods to assess degradation over time of AOA after treatment. Understanding impact of environmental conditions and conservation treatments allows earlier identification of potential degradation, which in turn offers better protection of objects and more cost-effective mitigation measures. The main objective is to establish an assessment protocol on treated and untreated archaeological materials, artificially or naturally aged. These practical assessment methods must be well suited for museums/storage facilities and will be validated by advanced analytical techniques.
This project further intends to reinforce interdisciplinary approaches among archaeologists, conservators and conservation scientists in order to improve the sustainability of protection practices. The expected results will involve field practices both for the stabilization of waterlogged artefacts for excavation/archaeometry situations and for degradation assessments in museums. In the JPI, Norway is involved in the second topic.