MICROSCOPIC STUDY OF WATERLOGGED ARCHEOLOGICAL WOOD FOUND IN SOUTHWESTERN CHINA AND METHOD OF CONSERVATION TREATMENT

Thousands of waterlogged wood pillars beneath crop fields were discovered during the 2008 excavation of an archeological site in southwestern China. Specimens were studied with scanning and transmission electron microscopes, and 2.5 cm x 2.5 cm x 5-cm specimens were dehydrated with methanol followed by treatment with neutral phenol formaldehyde (PF) resin. The wood, identified as Pinta kesiya var. langbianensis, was severely degraded by bacterial surface erosion and tunneling of cell walls. Bacterial tunneling was more frequently observed near the cell corners with thick walls. Bacterial degradation of cell walls was accompanied by accumulation of degradation products and bacterial slime in cell lumens. Neither brown-rot nor soft-rot decay was detected in the wood. The wood samples gradually darkened after sampling, but removal of degradation products with methanol and a brief 2% oxalic acid treatment reversed the discoloration. The average specific gravity and crushing strength of the waterlogged wood were 0.25 and 7.1 MPa compared with 0.37 and 33.7 MPa of normal wood of the same species. Treatment of the waterlogged wood with neutral PF resin increased specific gravity to 0.44 and crushing strength to 12.8 MPa. The PF treatment minimized shrinkage and stabilized wood color of the waterlogged wood.