JPI kulturarv - Ageing Study of Treated Composite Archaeological Waterlogged Artifacts

Wooden artifacts recovered from archaeological sites are threatened by various chemical, biological and physical degradation processes. Conservation measurements have to be taken to stop the decay and strengthen the objects. This requires a secure knowledge on the nature of the deterioration processes and evaluation of suitable preservation methods. The ArCo project focused on the role of unstable salts in wood degradation processes. Such salts may be formed inside the wood by ions migrated into the wood during burial, or they may have been applied after excavation during conservation treatments. Most representative for the first group are iron salts, for the latter one alum (potassium aluminum sulfate dodecahydrate). The salts may decompose during time or being oxidized by air, resulting in swelling and cracking of objects and leading to acid formation. In the framework of ArCo collaborated scientists and conservators from Denmark (National Museum), France (Arc-Nucleart), Italy (University of Pisa and ARCHA, a private laboratory), and Norway (Museum of Cultural History). Investigated were samples from a Gallo-Roman shipwreck found near Lyon, from a 16th century cargo vessel excavated near Nice, from two privateer ships from St Melo, sunk in the 18th century, and from the Nydal Viking ship, excavated in Denmark. All those are affected by the formation and alteration of iron salts. Samples from the Oseberg collection, Norway, represented the consequences of alum treatment. The chemical processes which causes damages proceed very slowly: in case of the alum-treated objects it took seventy years to discover the consequences of the conservation method. In the ArCo project the condition of the objects were studied, including severe chemical analyses, and then accelerated aging experiments were carried out. Samples of the objects were placed in a climate chamber and exposed to cycles of changing temperature and humidity, simulating the stress factors in a museum environment. After that the objects were analysed again, and alterations of the salts and the wood were studied. In a second part of the project the same procedure was performed with objects treated with different consolidation and neutralization agents. Synchroton-based infrared spectroscopy was carried out on ArCo related objects at BESSY in Berlin, in co-operation with our Italian colleagues. Results were presented at the Bessy user meeting in December 2014. A wider perspective on the use of SR-FTIR including examples from ArCo related research was presented at NFR's Synchroton User Meeting, Stavanger, in January 2015. The decay of iron salts and alum led to formation of sulphuric acid. In the scope of ArCo a new neutralizing agent, sodium sebacate, was developed and tested. Analytical results were presented in a joint oral presentation at the TECHNART conference in Catania (April 2015), the MaSC - Users' Group for Mass Spectrometry and Chromatography, May 2015 in Chicago, and the condition.2015 conference in Gdansk, Poland, also May 2015. A full scientific article on analytical results is published in the Microchemical Journal. A guest scientist from Pisa, D. Tamburini, joined the Oslo group for a 3-month research stay. A significant part of his PhD thesis - submitted September 2015 - is dealing with analyses of ArCo objects. A new webpresentation of ArCo and the problems around the preservation of archaeological wood was released (http://www.khm.uio.no/english

Museums and preservation workshops have been confronted for the last years with a phenomenon of "leprosy" which questions treatments used to conserve wooden wet archaeological objects. The rashes which appear on objects after treatment are due to the pres ence of unstable salts. In contact with air, these salts oxidize leading to swelling and cracking of the wood and causing a significant acidification of the material (pH < 2). The most known case of acid attack of the wood is the famous Swedish warship Va sa (17th century), displayed in Stockholm: approximately 2 tons of acid are present in the whole of the ship. Similar problems exists with the Oseberg collection, the most important archaeological Viking find in Norway, treated by alum salts in the earl y 1920s. Other European countries did not escape to this problem. ArCo project proposes a preventive way that consists in choosing the most suitable treatment to limit and - if possible - to stop the oxidation of salts. To choose a suitable preservation Method is quite hard because the long-term behavior of unstable concretions and the different interactions between consolidants/bulking agents and wood is not yet well known. Therefore, the present program aims in developing protocols to assess availabl e treatments. Samples treated in fdifferent ways as well as untreated Reference samples will be aged in climatic chambers under various conditions. A full diagnostic of the degradation level of composite objects will be performed before and after artifici al ageing treatments and in-situ monitoring will be carried out during weathering. This will enable us to evaluate different conservation methods and give recommendations to choose the most suitable preservation strategies.