Waterlogged archaeological wood : biodegradation and its implications for conservation

Although archaeological wood found in waterlogged environments is often described as well preserved, microbial degradation has taken place. Microscopic investigations revealed that despite different types of soil, water, sediment, pH, wood species, and age, archaeological wood was mainly degraded by erosion bacteria, even though soft rot and tunnelling bacteria decay was also occasionally observed. Erosion bacteria seem to be the only wood degrading micro-organisms active in near anaerobic environments. A weak skeleton consisting of the lignin rich compound middle lamella remains after decay and maintains the form and integrity of the historical wood, as long as it is kept waterlogged. Erosion bacteria and their attack of wood cell walls are described and illustrated in detail. Presence of active erosion bacteria in 1200 year old Viking poles, suggests that degradation is generally a slow process, that proceeds until all cellulose rich parts of the wood cell wall are utilised. The results indicate that depth of deposition in historic times as well as in reburial situations today, is an important factor for successful preservation in nature. In laboratory experiments, it was found that both clay and sandy soils had an equal protective potential, and were significantly less aggressive to wood than top soil. However, 20 cm depth of burial in waterlogged soil did not prevent significant attack by soft rot fungi, but different cover sheets had some positive effect Long term effects of soil and cover protections are still unknown. From observations on aerobic decay patterns in archaeological materials, valuable information can be obtained about the history of these objects before waterlogging. In active conservation situations, polyethylene glycol (PEG) in impregnation baths give rise to microbial growth at the liquid/air interface during immersion. However results indicate that the microbes present are not degrading the wood during treatment and therefore do not represent a threat for historical objects. Knowledge on microbial degradation in wood, particularly waterlogged archaeological wood, is important for the development of passive and active conservation strategies in future in the interdisciplinary areas of conservation and archaeology