Species separation of aging and degraded solid wood using near infrared spectroscopy

Applicability of near infrared (NIR) spectroscopy to identify species of degraded and aging solid wood was examined among species important to Japanese art history and archaeology. NIR spectra were obtained from wood blocks of five softwood species collected over the last 80 years from various sites and stored in the wood library of the Forestry and Forest Products Research Institute, Tsukuba in Japan. Partial least square (PLS) discriminant analysis was employed for the separation of three pairs of species, i.e., Chamaecyparis obtusa and Torreya nucifera, Chamaecyparis obtusa and Chamaecyparis pisifera, Thuja standishii and Cryptomeria japonica. The effects of spectral pre-processing and wavelength range were also evaluated. Under the limitation of sample volume, PLS discriminant analysis calibrated using second derivatives and wavelengths spanning 830 to 1150 nm could separate the samples into each pair of species in the 100 % accuracy. These results suggest that NIR spectroscopy combined with PLS discriminant analysis is a powerful technique for distinguishing species for degraded and aging wood nondestructively without any sample preparations. 118 Jpn. J. Histor. Bot. Vol. 19 Nos. 1–2 (Tsuchikawa et al., 2003b; Tsuchikawa & Yamato, 2003), soft independent modeling of class analogy (SIMCA) (Tsuchikawa et al., 2003b; Tsuchikawa & Yamato, 2003; Gierlinger et al., 2004; Adedipe et al., 2008), and partial least square (PLS) discriminant analysis (Furumoto et al., 1999; Flæte et al., 2006). These studies were, however, restricted to small clean samples with ideal highly controlled conditions because the NIR spectra are influenced by surface roughness (Hein et al., 2010) as well as the physical and chemical properties of the wood. In these papers, the NIR spectra were obtained from samples collected from a single site, or the irradiation surfaces were carefully cut in the same manner. Because wood has inherent anatomical and morphological variability within species, it is still unknown whether NIR spectroscopy is applicable for the species identification of aging and/or degraded wood that has been collected from various sites. Such conditions are common in the study of ancient wooden statues. To evaluate the applicability of NIR spectroscopy to identify the species of degraded and aging wood, three pairs of five historically important conifer species used in the construction of wooden statues in Japan was studied using NIR spectroscopy. Materials and Methods 1. Sample preparation Wood specimens of five species that were collected from various sites in Japan and stored over the past 80 years in the wood library of the Forestry and Forest Products Research Institute, Tsukuba (TWTw) in Japan were used in our analysis (Table 1). They include 18 specimens of Chamaecyparis obtusa, 19 of Torreya nucifera, 11 of Thuja standishii, 19 of Chamaecyparis pisifera, and 24 of Cryptomeria japonica. The specimens were stored in the collection room, which is *Species other than Japanese Torreya nucifera, **Species not identified. Table 1 Samples used in this study showing the years and sites of collection Sample no. Species TWTw no. Calibration/ Validation set Collection years Collection sites 1 Chamaecyparis obtusa 1168 calibration 1928 Saitama 2 Chamaecyparis obtusa 11951 calibration Saitama 3 Chamaecyparis obtusa 12166 calibration 1931 Yaku Isl. 4 Chamaecyparis obtusa 13263 calibration 1970 Chiba 5 Chamaecyparis obtusa 1345 calibration Saitama 6 Chamaecyparis obtusa 14666 calibration Tokyo 7 Chamaecyparis obtusa 15 calibration Tokyo 8 Chamaecyparis obtusa 1770 calibration Tochigi 9 Chamaecyparis obtusa 2264 calibration unknown 10 Chamaecyparis obtusa 4791 calibration unknown 11 Chamaecyparis obtusa 7991 calibration Nagano 12 Chamaecyparis obtusa 873 calibration Nagano 13 Chamaecyparis obtusa 14665 validation Chiba 14 Chamaecyparis obtusa 14668 validation unknown 15 Chamaecyparis obtusa 18791 validation 2000 Miyazaki 16 Chamaecyparis obtusa 3329 validation 1929 Chiba 17 Chamaecyparis obtusa 6371 validation 1981 Tokyo 18 Chamaecyparis obtusa 9293 validation 1961 Nagano 19 Torreya nucifera 12158 calibration 1931 Kumamoto 20 Torreya nucifera 14506 calibration 1964 Tokyo 21 Torreya nucifera 14507 calibration Tokyo 22 Torreya nucifera 14755 calibration Chiba 23 Torreya sp.* 15979 calibration China 24 Torreya nucifera 18403 calibration 2000 Okayama 25 Torreya nucifera 19683 calibration 2002 Tsushima Isl. 26 Torreya sp.** 20112 calibration unknown 27 Torreya nucifera 3321 calibration 1929 Chiba 28 Torreya nucifera 4332 calibration 1981 Saitama 29 Torreya nucifera 471 calibration 1950 Chiba 30 Torreya nucifera 4772 calibration unknown 31 Torreya nucifera 1191 validation 1928 Saitama 32 Torreya nucifera 11938 validation Saitama 33 Torreya nucifera 13247 validation 1970 Chiba 34 Torreya nucifera 13662 validation 1987 Tokyo 35 Torreya nucifera 14504 validation Tokyo 36 Torreya sp.* 15978 validation China 37 Torreya nucifera 852 validation Miyazaki 38 Thuja standishii 12441 calibration unknown 39 Thuja standishii 14537 calibration Chiba 40 Thuja standishii 24337 calibration 2008 Nagano 41 Thuja standishii 4790 calibration unknown 42 Thuja standishii 595 calibration 1950 Saitama 43 Thuja standishii 875 calibration Nagano 44 Thuja standishii 9295 calibration 1961 Nagano 45 Thuja standishii 12186 validation 1924 unknown 46 Thuja standishii 14639 validation Tokyo Sample no. Species TWTw no Calibration/ Validation set Collection years Collection sites 47 Thuja standishii 22003 validation 2005 Gifu 48 Thuja standishii 649 validation 1951 Nagano 49 Chamaecyparis pisifera 11952 calibration Saitama 50 Chamaecyparis pisifera 13264 calibration 1970 Chiba 51 Chamaecyparis pisifera 1347 calibration Saitama 52 Chamaecyparis pisifera 14390 calibration 1979 Tokyo 53 Chamaecyparis pisifera 14563 calibration Chiba 54 Chamaecyparis pisifera 22065 calibration 2005 Ibaraki 55 Chamaecyparis pisifera 24266 calibration 2008 Nagano 56 Chamaecyparis pisifera 24588 calibration 2008 Miyagi 57 Chamaecyparis pisifera 3330 calibration 1929 Chiba 58 Chamaecyparis pisifera 4792 calibration unknown 59 Chamaecyparis pisifera 874 calibration Nagano 60 Chamaecyparis pisifera 8881 calibration The Netherlands 61 Chamaecyparis pisifera 21822 calibration 2004 Hokkaido 62 Chamaecyparis pisifera 12169 validation unknown 63 Chamaecyparis pisifera 14391 validation 1979 Tokyo 64 Chamaecyparis pisifera 24196 validation 2008 Nagano 65 Chamaecyparis pisifera 24293 validation 2008 Nagano 66 Chamaecyparis pisifera 651 validation 1951 Nagano 67 Chamaecyparis pisifera 9294 validation 1961 Nagano 68 Cryptomeria japonica 1173 calibration 1928 Saitama 69 Cryptomeria japonica 1346 calibration Saitama 70 Cryptomeria japonica 14590 calibration Chiba 71 Cryptomeria japonica 14593 calibration Yaku Isl. 72 Cryptomeria japonica 14594 calibration Yaku Isl. 73 Cryptomeria japonica 14914 calibration 1991 Ishikawa 74 Cryptomeria japonica 1769 calibration Tochigi 75 Cryptomeria japonica 19512 calibration 2002 Amami Isl. 76 Cryptomeria japonica 4785 calibration unknown 77 Cryptomeria japonica 6427 calibration 1983 Saitama 78 Cryptomeria japonica 6428 calibration 1983 Saitama 79 Cryptomeria japonica 6429 calibration 1983 Saitama 80 Cryptomeria japonica 817 calibration Akita 81 Cryptomeria japonica 9290 calibration 1967 Akita 82 Cryptomeria japonica 9291 calibration 1967 Shizuoka 83 Cryptomeria japonica 96 calibration 1996 Tokyo 84 Cryptomeria japonica 10275 validation unknown 85 Cryptomeria japonica 14591 validation Chiba 86 Cryptomeria japonica 14592 validation unknown 87 Cryptomeria japonica 18328 validation 1988 Akita 88 Cryptomeria japonica 3322 validation 1929 Chiba 89 Cryptomeria japonica 4787 validation Tochigi 90 Cryptomeria japonica 9289 validation 1962 Miyazaki 91 Cryptomeria japonica 9292 validation Aomori