Biological-degradation, lignin performance and physical-chemical characteristics of historical wood in an ancient tomb

Biodeterioration assessments of archaeological woods, also comparisons with recent wood, provides an effective strategy for its conservation in cultural-heritage. This research aimed to analyze some wooden structures of an ancient tomb, situated in West-Azerbaijan province of Iran adjacent to Lake-Urmia. The anatomical characteristics of historical wood (HW) was investigated to identify species and trace xylem micro-morphological variations in detail. Chemical-changes of deteriorated wood were examined using X-ray diffraction (XRD) and ATR-FTIR spectroscopy, and its wood-water interactions were compared with the recent new-wood. Macro/micro-morphological and mycological assessment of historical poplar wood (Populus alba) confirmed signs of soft-rot fungi, and recent attacks by wood-boring (Xylophagous) insects probably due to local climate-change. Type-I microcavities and advanced soft-rot decay caused cell-walls depletion or deformation, which was visually observed in HW by discoloration and transverse-cracks. Based on Optical- and SEM-microscopy, lignin preservation was observed in the middle-lamella and especially the vessels rich in guaiacyl-lignin (G-type) units. However, the fiber walls were susceptible to fungal degradation due to syringyl-lignin (S-type). Histochemical changes increased the porosity, hygroscopicity, and especially water-absorption (1h-720h) of HW compared to the corresponding new-wood, introducing the severity of damage and treatability criteria for conservationists. Lignin increment, loss of cellulose-crystallinity in XRD (70.5 to 59.9 %) and FTIR ratios (1280/1200, 1317/1336), and also preferential-degradation of polysaccharides were confirmed through chemical analyses of soft-rot decayed wood. These findings, in addition to helping monitor potential risks of wooden artifacts, especially non-durable species, facilitate preventive and protective management of archaeological-heritage in both wet and dry environments.