Improvement of conservation methods for archaeological bone material (ivory, bone and antler) – ArBoCo

Bone materials take an important place among prehistoric materials as they record, as biomatérials, a wealth of information on the past way of life in their aspect, structure as well as in their chemical and isotopic composition. Therefore, many physico-chemical studies are devoted to the investigation of archaeological bone as well as their alteration phenomena. It is also important to note that a large quantity of art objects in our museums is at least partly constituted of bone, ivory or antler. They were used from prehistoric times to the Middle Ages to manufacture, among others, sculptures and book covers. However, curators are often at a loss for the conservation of the bone material which is often integrated in complex assemblages with other materials or it has been surface treated. To consolidate its fragile structure, consolidation treatments have been applied without any systematic control and testing of interaction with the different materials. The treatment is generally achieved with a polymer to reinforce the bone structure. However, the consolidation agent can have negative effects on the remaining bone material. It can cause additional alteration processes linked with the degradation of the consolidation agent. To our knowledge, there is no conservation method available today which is adapted to the degree of preservation and to the archaeological conservation purpose (museum, excavation field, DNA analysis…) and which fully respects the preservation criteria required in the archaeological and museum context. The aim of this programme is to develop the knowledge, the methods and the conserveration treatments that correspond to these needs. The proposed programme comprises two parts: an analytical part on the characterisation of model and archaeological bone material and a second one on the application of consolidates to bone material as well as the evaluation of the effects caused by consolidation treatments on the structure, the chemical composition and the mechanical properties at different hierarchical levels. An analytical strategy using complementary analytical micro- and nanotechniques is needed. In former studies, a methodology for the characterisation of the bone mineral phase has been developed. It is now necessary to elaborate a complementary procedure for the characterisation of the organic phase at different levels and of the interface between the mineral and the organic phases at nanoscale in bone material. PIXE, XRF and nano-SIMS, SEM-EDX are used for the analysis of the chemical composition. Structural investigations are mainly realised by means of XRD, microFT-IR, synchrotron based methods such as microtomography and SAXS/WAXS and morphological observations using SEM and TEM. In addition, analysis of the state of preservation of the organic phase is developed by means of microFT-IR, HPLC, LC-MS, STXM-NEXAFS and DSC analyses. A crucial step will be the sample preparation for these analyses. Indeed, the preparation procedure will comprise microtomy and ultramicrotomy in order to keep all information on the structure at micro- and nanoscale and on the texture of the material. Last but not least, measurements of the mechanical properties of the bone material are necessary. In the second part of the programme different treatment procedures using model polymers and currently used restoration polymers are tested. In order to test their efficiency, the bone material has to be analysed concerning its composition, structure and mechanical properties before and after treatment using the newly established analytical procedure. The comparison of the analytical results will allow us to evaluate the effect of the treatments on the bone material and their efficiency. The next step of the research consists in the study of the durability of the applied treatments. The treated material has to be submitted to different accelerated aging tests (under UV light, under changing relative humidity and temperature conditions, in the presence of SO2, NOx and other atmospheric gases …). In parallel, microbiological analyses will be performed to evaluate the contribution of bacteria and other microorganisms to the decomposition of bone in soils. Studies on the conservation of ancient DNA and the effects of treatments on its preservation are also very interesting and necessary. First archaeological samples are non consolidated and consolidated pieces of Palaeolithic reindeer antlers originating from the Musée National de Préhistoire, les Eyzies-de-Tayac, Magdalenian period, Dordogne, France and of archaeological ivory from Jinsha site, 1300BC-550BC, Chengdu (Sichuan province, China). These two study sites allow us to dispose of the two cases generally found in archaeology: on one hand dry samples in a museum and on the other hand wet fragments coming from field excavations. Additionally, modern bone, ivory and antler samples are investigated in order to provide an adequate reference.