Evaluation of the state of preservation of waterlogged archaeological wood based on its physical properties: basic density vs. wood substance density

The state of preservation of waterlogged archaeological wood was evaluated on the basis of the maximum moisture content (MMC), the basic density (BD) and the wood substance density (WSD) determined in water and helium. The degree of wood degradation was compared under the criteria: the loss of wood substance (LWS) and the loss of wood substance density (LWSD). Studies were conducted on the wood samples differing in species, degree of degradation, age and place of origin. The physical properties of wood were determined for the material containing mineral compounds and the material without minerals. The properties of the latter, in which the minerals are replaced by water, were calculated from the mass and volume of the wood containing minerals as well as the content and density of the ash obtained after burning the sample. The study revealed the effect of minerals on the tested parameters and wood degradation indices. A strong relationship between BD and MMC was confirmed for both the wood containing minerals and without them, by contrast a substantially weaker correlation between BD and WSD was observed. It was found that the assessment of the state of wood preservation conducted on the basis of LWS and LWSD yielded different results. In addition, it was revealed that both indices of wood degradation might be unreliable. The main drawback of the LWS-based assessment is associated with a wide range of basic density of fresh wood. In turn, the LWSD mainly indicates the changing ratio of the carbohydrates/lignin content, but fails to provide information on the loss of wood substance. This may hinder the comparison of the wood sampled from different sites and subjected to different decay mechanisms. Nevertheless, the WSD-based assessment of the state of preservation of waterlogged archaeological wood might be a valuable complementary method to the BD- and/or MMC-based assessment, which is routinely carried out in many conservation centers. (C) 2014 Elsevier Ltd. All rights reserved.