Structural Degradation of Third Millennium B.C.E. Archaeological Wood from Shahr-i Sukhta, Iran
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Structural Degradation of Third Millennium B.C.E. Archaeological Wood from Shahr-i Sukhta, Iran Mohsen Mohammadi Achachluei1 , Gholamreza Vatankhah1,2 1 Faculty of Conservation, Art University of Isfahan, P.O.Box 1744, Isfahan, I.R. IRAN 2 Adjuct Professor, Art Conservation Program, Queens University, Kingston, ON, Canada Abstract Detection of structural changes has a great importance in conservation and treatment of wooden artifacts found in burial environments. In this study, two wooden samples excavated in Shahr-i Sukhta, an important prehistoric site in eastern Persia, were tested by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). FTIR results showed that decay of lignin occurred. This was accompanied by cellulose degradation. SEM images showed detachment of wood fibers in cross sections and cell wall breakage in tangential sections of the wood samples. These results indicated that the structural change in cellulose and lignin has caused detachment of wood fibers and cell wall breaking. This is the sign of low physical characteristics of wood which has direct relevance to cellulose decay and lignin degradation in middle lamella of wood cells. Cellulose chain breaking over time, has produced stresses in the compound middle lamella layer of the cell wall, that became accelerated in the middle of the lamella layer. Consequently, wood fibers were detached, and disintegration and powdering of wood surfaces occurred. Introduction Characterization of structural degradation has a great importance in the procedure of conservation of wooden artifacts. Wood is an organic material and its main components include cellulose and lignin in a cellular structure like reinforced concrete where the polysaccharides represent the steel and lignin the concrete (Fig. 1). Structural changes of each component lead to different forms of decay in wooden objects [1].
Figure 1. (A) Cross section of a wood cell. (B) Model of cell wall structure of conifer tracheids and hardwood libriform fibers: L Lumen, CML compound middle lamella, ML middle lamella, P primary wall, S secondary wall, S1 outer, S2 central, and S3 inner layer of the secondary wall, W warty layer [1].
Changes over time in dry archaeological environments are very different from structural changes in wet environments. Dry environments refer to those which do not have enough water to saturate wood fibers [2, 3]. Shahr-i Sukhta , one of most important prehistoric archaeological sites of eastern Iran, is a dry archaeological site. Shahr-i Sukhta stands 12-18m above the ground of the region. It is located 56 km northwest of the city of Zabol in Sistan va Baluchistan province in Iran. Shahr-i Sukhta was located on the delta of the former Hirmand River. It was the centre of the most important civilization in the region in the 3rd millennium B.C [4]. One hundred twenty hectares of the site contains human remains which date back to 3200-1800 B.C. Dating information has been obtained from stratigraphy data and radiocarbon dating [5]. Shahr-i Sukht
