The degradation of conserved archaeological wood in the presence of pyrite - An analytical protocol

The StAr Project (Development of Storage and assessment methods suited for organic archaeological artefacts) within the framework of Joint Programming Initiative on Cultural Heritage and Global Change (JPI-CH) Conservation, Protection and Use, seeks to establish an analytical assessment protocol suited for museums, using well-established analytical techniques to observe the extent of degradation of archaeological wood containing unstable inorganic compounds. The assessment of the post-treatment condition of conserved archaeological wooden artefacts would allow identification of early stages of degradation, leading to earlier and more cost effective mitigation measures to be applied. The role of pyrite oxidation in the degradation of archaeological collections is well documented. In the presence of oxygen, pyrite oxidizes producing sulphuric acid and hydrates of iron(II) sulphate. Sulphuric acid causes extensive depolymerisation of cellulose and hemicellulose, reducing mechanical strength, while the iron(II) sulphate adsorbs water from the atmosphere, further enhancing oxidation. Additionally, the high amounts of iron(II) and iron(III) ions also degrade polyethylene glycol (PEG), which is often used as a conservation agent to support the wood cells before drying. This paper presents our methodological efforts in analyzing wood samples from several objects and excavation sites, treated by different conservation treatments, both standard and newer (PEG, and the polyester-based ‘Nucléart’, respectively). Combining powder X-ray diffraction (XRD) with scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), infrared spectroscopy and pH measurements, we sought to elucidate the state of pyrite oxidation and wood degradation. XRD patterns revealed the presence of complex inorganic mixtures, often dominated by the presence of gypsum, and iron sulphate in different oxidation and hydration states along with unoxidized pyrite. Moreover, IR spectra of wood revealed degradation of the holocellulose fraction, while lignin appears less affected. Key words: Archaeological wood, pyrite, oxidation, sulphate, degradation