Abstract
Archaeological wood is a kind of ‘new material’ that has deteriorated due to long-term degradation. The existing wood science theory and evaluation methods are not fully applicable to archaeological wood. Moreover, current physical-mechanical evaluation methods are inadequate for fragile archaeological wood due to their insufficient accuracy and the large sample amount required, causing difficulties in many necessary physical-mechanical repeatability tests. In light of these limitations, the representative samples on Nanhai No. 1, a merchant shipwreck in the Song Dynasty, were selected as the research objects in this paper. The shipwreck is a typical waterlogged wooden artifact. A quasi-nondestructive physical-mechanical evaluation technique for archaeological wood was developed with the thermomechanical analyzer (TMA). This study used TMA to evaluate the bending strength of representative waterlogged archaeological samples of Nanhai No. 1 shipwreck and sound wood with the same species. Besides, the thermal linear expansion coefficients in the ambient temperature range were obtained. The sizes of the samples used in the tests were only 2 mm × 8 mm × 0.3 mm and 1 cm × 1 cm × 1 cm, respectively. Bending strength results of archaeological wood by the TMA method conformed to the tendency that the bending strength decreases with the increase of decay degree. In addition, the longitudinal linear expansion coefficients of archaeological wood reached 80%–115% of those in the transverse grain direction, which were about 10 times higher than those of the sound wood. The linear expansion coefficients of archaeological wood in three directions were similar. Based on the results of Fourier transform infrared analysis (FT-IR), the significant differences in the physical-mechanical properties of the archaeological wood and the sound wood were induced to be mainly ascribed to the decomposition and the loss of hemicellulose in the archaeological wood. The cell wall substrate could not stabilize the cellulose skeleton, which led to the instability of the tracheid structure of the archaeological wood. This study provided a proven quasi-nondestructive method for the preservation state evaluation of waterlogged archaeological wood (WAW) from the Nanhai I shipwreck and other similar waterlogged wooden relics.
Highlights
The wooden cultural relics usually refer to shipwrecks, wooden production tools, wooden beds, wood carvings, coffins, and other artifacts left over from the social activities of humans
Some cell walls are separated from the intercellular layer (Figure 5a,d,g), indicating that the waterlogged archaeological wood selected in this study had deteriorated to a certain extent
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Summary
The wooden cultural relics usually refer to shipwrecks, wooden production tools, wooden beds, wood carvings, coffins, and other artifacts left over from the social activities of humans. These relics contain rich historical information and are precious physical evidence for studying ancient science, technology, art, and culture. According to the protection experience of the discovered marine shipwrecks, such as the Vasa shipwreck in Sweden [2] and the British Mary Rose shipwreck [3], the assessment of wood preservation is the premise for the scientific protection of these relics. Its cell wall structure and properties have seriously deteriorated, making it a “new” material different from sound/recent wood [5]. The existing wood theory is not suitable for this material
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