The separation of softwood and hardwood in historical wooden statues of the Nazenji-temple in Japan using NIR spectroscopy

Hisashi Abe,Mitsuharu Iwasa,Tomoyuki Fujii,Atsuko Ishikawa,Hiroaki Kaneko,Yohei Kurata,Hiroshi Wada,Ken Watanabe,Shuichi Noshiro

doi:10.1163/22941932-bja10038

Abstract

Abstract The applicability of near-infrared (NIR) spectroscopy to the identification of wood species of archaeologically/historically valuable wooden artifacts in a non-invasive manner was investigated using reference wood samples from the xylarium of the Forestry and Forest Products Research Institute (TWTw) and applied to several wooden statues carved about 1000 years ago. Diffuse-reflectance NIR spectra were obtained from five standard wood samples each of five softwood species (Chamaecyparis obtusa, Cryptomeria japonica, Sciadopitys verticillata, Thujopsis dolabrata, Torreya nucifera) and five hardwood species (Aesculus turbinata, Cercidiphyllum japonicum, Cinnamomum camphora, Prunus jamasakura, Zelkova serrata). A principal component analysis (PCA) model was developed from the second derivative spectra. The score plot of the first two components clearly showed separation of the wood sample data into softwood and hardwood clusters. The developed PCA model was applied to 370 spectra collected from 21 wooden statues preserved in the Nazenji-temple in Shizuoka Prefecture in Japan, including 14 made from Torreya spp. and 7 made from Cinnamomum spp. In the score plot, the statue spectra were also divided into two clusters, corresponding to either softwood (Torreya spp.) or hardwood (Cinnnamomum spp.) species. These results show that NIR spectroscopy combined with PCA is a powerful technique for determining whether archaeologically/historically valuable wooden artifacts are made of softwood or hardwood.

Highlights

The identification of wood species in objects of archaeological/historical significance is important both for repairing and maintaining the artifacts themselves as well as for gaining an understanding of their historical and cultural backgrounds
Using the principal component analysis (PCA) model developed from reference samples, spectra from wooden statues were projected onto the score plots of the model (Fig. 1)
Most of the plots fell within the softwood and hardwood clusters of reference samples, and the separation of softwood and hardwood was still recognizable in the score plots of the wooden statues

Summary

Introduction

The identification of wood species in objects of archaeological/historical significance is important both for repairing and maintaining the artifacts themselves as well as for gaining an understanding of their historical and cultural backgrounds. For over 20 years, we have been determining wood species of wooden statues by collecting minute, naturally detached fragments and observing them with mostly optical and partly scanning-electron microscopy (Kaneko et al 1998, 2003, 2010). These studies have shown that in Japan only a limited number of species were used for carving ancient wooden statues. One problem with this approach, is that it is sometimes difficult to obtain samples that are sufficient in both quality and quantity for making a definitive identification. It is important to develop alternative methods in which artifacts can be examined non-invasively

Methods

Results

Conclusion