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Abstract

To investigate the decay extent of wooden components in the ancient buildings of Danxia Temple, the absorption peak intensities, changes in chemical components, and cellulose crystallinity of red oak (Quercus sp.), birch (Betula sp.), and maple (Pterocarya sp.) wooden components were determined and analyzed using Fourier-transform infrared spectroscopy. The results are as follows: (1) The absorption peak intensities representing cellulose and hemicellulose decreased or disappeared obviously in the decayed red oak wood (DROW); on the contrary, those representing lignin increased. The indexes of the content of cellulose, hemicellulose, and cellulose crystallinity also decreased; on the contrary, those of the content of lignin increased. Those results indicated that cellulose and hemicellulose in DROW were largely degraded by brown-rot fungi. (2) The absorption peak intensities representing cellulose and hemicellulose decreased both in the decayed birch wood (DBW) and the decayed maple wood (DMW), whereas those representing lignin increased. The indexes of the content of cellulose, hemicellulose, and cellulose crystallinity also decreased, whereas those of lignin increased. Those results showed that cellulose and hemicellulose in DBW and DMW were seriously harmed not only by insects but also by brown-rot fungi. By comparison, the extent of fungal damage was lower in DMW than in DBW.

The Danxia Temple, with wooden structures, was first built in about 824 A.D. and is located in Liushan Town, Nanzhao County, Nanyang City, Henan Province, China. It is a national cultural relics protection unit and is one of the eight famous temples in Henan Province. It is of great historical, scientific (as an example of historical material civilization), and artistic value. However, under the influence of factors such as ambient temperature, humidity, microorganisms, insects, light, and so on, some wooden components, especially the roots of wooden pillars, have been degraded to different degrees.

It is well known that degradation inevitably leads to the change of anatomical structures (Murace et al. 2006; Win et al. 2008; Padhiar and Albert 2011, 2012; Sanghvi et al. 2013; Koyani and Rajput 2014; Bhatt et al. 2016; Bari et al. 2019, 2020; Diandari et al. 2020; Yang et al. 2020a), chemical components (Ferraz and Durán 1995; Arias et al. 2010; Bari et al. 2019, 2020; Girometta et al. 2020; Sun et al. 2020; Yang et al. 2020b), and reduction in physical and mechanical properties (Choi et al. 2006; Brischke et al. 2019; Chang et al. 2019; Gao et al. 2019; Li et al. 2019; Bari et al. 2020). These reductions eventually affect the safety and the life span of the ancient buildings. The three main chemical components of wood, i.e., cellulose, hemicellulose, and lignin, have corresponding characteristic infrared absorption spectra. The absorption peaks of 1,735 cm−1, 1,374 cm−1, and 897 cm−1 correspond to functional group C=O stretching that represents hemicellulose, C-H stretching that represents holocellulose, and C-H stretching that represents cellulose, respectively in Fourier-transform infrared (FTIR) spectroscopy characteristic spectra (Table 1). Those of 1,600 cm−1 and 1,508 cm−1, 1,460 cm−1, 1,424 cm−1, and 1,260 cm− correspond to the aromatic skeleton stretching, C-H deforming, CH2 bending, and C-O stretching, respectively, which represent lignin (Table 1) (Pandey and Pitman 2003; Stark and Matuana 2004, 2007; Ibrahim et al. 2007; Pandey and Nagveni 2009, Li et al. 2010; Monrroy et al. 2011; Zeng et al. 2012; Tomak et al. 2013; Xu et al. 2013; Liu et al. 2017; Tamburini et al. 2017; Croitoru et al. 2018; Fahey et al. 2019; Wentzel et al. 2019; Bari et al. 2020; Dong et al. 2020; Sun et al. 2020). The shapes, positions, and intensities of the absorption peaks in FTIR spectra will change, as will the chemical components of woods. Therefore, the changes of wood functional groups and the contents of the three main chemical components can be determined according to the changes (increase, decrease, or disappearance) of the absorption peak intensities in the FTIR characteristic spectra.

Table 1. Fourier-transform infrared spectroscopy characteristic bands of the wood samples studied.
Table 1.

The FTIR spectroscopy method is helpful to evaluate the changes of chemical components of woods. The method has some advantages, such as only requiring a minimal number of samples (about a few milligrams) (Pandey and Pitman 2003, Xu et al. 2013, Yang et al. 2020b) and results in less damage to ancient buildings (Yang et al. 2020 b) compared with conventional gravimetric techniques. It is increasingly used to study the deterioration extent and evaluate current bearing capacity of wooden components in ancient buildings.

The goals of this study were to determine the changes in chemical components and investigate the degradation extent and mechanism of wood components with FTIR analysis in the ancient buildings of Danxia Temple. The results of this study provide reliable data support and guidance for later protection and repair of wooden components of cultural relics.

Materials and Methods

Materials

Three samples were collected from wooden components in the ancient buildings of Danxia Temple in Nanzhao County, Nanyang City, Henan Province, China. The first sample, identified as red oak wood (Quercus sp.), was obtained from the root of the wooden pillar in the Tianwang Hall. The second sample, identified as birch wood (Betula sp.), was obtained from the root of the wooden pillar in the Pilu Hall. The third sample, identified as maple wood (Pterocarya sp.), was obtained from the root of the wooden pillar in the Tianran Ancestor's Hall. The samples were taken with an increment borer (10-100-1027, Haglöf AB, Mora, Sweden,). The first sample was degraded seriously by fungi, whereas the second and third samples showed signs of insect attack visible to the naked eye (Fig. 1). Control samples were obtained at the location of 1 m from the living tree roots about 10 years old and 30-cm breast diameter by increment borer (10-100-1027, Haglöf) and they were dried before FTIR testing. The wood powder needed for FTIR analysis was processed using an agate mortar.

Figure 1.Figure 1.Figure 1.
Figure 1. Locations of sampling in Danxia Temple. (a) Sample no. 1. (b) Sample no. 2. (c) Sample no. 3.

Citation: Forest Products Journal 71, 3; 10.13073/FPJ-D-21-00015

FTIR testing

For FTIR analysis, oven-dried wood powder and spectrum KBr were fully mixed in a ratio of 1:150, poured into an agate mortar, and ground into powdered form while keeping the wood powder and KBr in dried condition by using a high-wattage lamp throughout the whole process. The powder mixture (about a few milligrams) was compressed into sliced samples using a powder tablet press machine (FW-5A, Uncommon Technology Development Co., China).

FTIR spectra were generated using a FTIR spectrophotometer (ALPHA2, Bruker Inc., Germany) in the range of 4,000 to 400 cm−1 to provide detailed information on the functional groups present in the sample surface (Yang et al. 2015, 2020c). Scans were run at a resolution of 4 cm−1 and the spectra were obtained using attenuated total reflectance. To avoid the effect of experimental error, a minimum of five scans per specimen was run to calculate their average.

Analysis of the changes in chemical components

To eliminate the differences of wood powder content and the influence of operation error on the experimental results in the experimental process, the stable absorption peak of 1,508 cm−1 representing aromatic skeleton stretching was taken as the standard, and the ratio of the absorption peak heights of cellulose or hemicellulose to lignin (1,508 cm−1 ) was used to characterize the contents of chemical components (Pandey and Pitman 2003, Li et al. 2010, Xu et al. 2013, Liu et al. 2017).

The absorption peak height was calculated as follows: the tangent of the lowest point on both sides of the absorption peaks was taken as the baseline, and the vertical line was drawn from the end of the absorption peak to the baseline. The distance from the intersection of the vertical line and the baseline to the top of the absorption peak was the absorption peak height (Pandey and Pitman 2003, Li et al. 2010, Xu et al. 2013, Bari et al. 2020; Dong et al. 2020).

The content of lignin was characterized by the values of I1508/I1735 and I1508/I1374, whereas that of hemicellulose was characterized by I1735/I1508, that of cellulose by I897/I1508, and that of holocellulose by I1374/I1508 and I1159/I1508 (Li et al. 2010, Xu et al. 2013, Liu et al. 2017, Bari et al. 2020, Sun et al. 2020). The percentage of change in the amount of chemical component is calculated using Equation 1, whereas the retained amount is from Equation 2. In the calculation results, “+” indicates increase and “−” indicates decrease (Dong et al. 2020).

Analysis of change in cellulose crystallinity

The index of cellulose crystallinity in wood was characterized by the values of I1374/I2900 and I1429/I897 (Monrroy et al. 2011, Tomak et al. 2013, Yang et al. 2020b). The change amount and the retained amount of cellulose crystallinity is referred to Equations 1 and 2.

Results

FTIR analysis of the red oak wooden components

As can be seen from Figure 2, the absorption peak positions of decayed red oak wood (DROW) in the region from 1,800 to 800 cm−1 had no displacement, but the intensities had obvious changes. Compared with nondecayed red oak wood (NDROW), the absorption peak intensities of 1,735 cm−1, 1,374 cm−1, and 897 cm−1 were obviously decreased (Fig. 2), showing that cellulose and hemicellulose were largely degraded by decay fungi. In addition, the absorption peak intensities of 1,159cm−1 and 1,058cm−1, which represent holocellulose, disappeared (Fig. 2), indicating that the degradation of holocellulose was more evident at these two absorption peaks. The decreases and disappearances of the absorption peak intensities eventually resulted in the decrease of the contents of cellulose and hemicellulose.

Figure 2.Figure 2.Figure 2.
Figure 2. Infrared spectrum of the red oak wooden component: region from 1,800 to 800 cm−1.

Citation: Forest Products Journal 71, 3; 10.13073/FPJ-D-21-00015

Compared with NDROW, the absorption peak intensities of 1,600 cm−1, 1,508 cm−1, 1,460 cm−1, 1,424 cm−1, and 1,336 cm−1, which represent lignin, increased, especially at 1,508 cm−1, 1,424 cm−1, and 1,460 cm−1 (Fig. 2), suggesting that the content of lignin increases in DROW. The absorption peak intensity of 1,245 cm−1, which represents lignin, disappeared, but two new peaks were added at 1,260 cm−1 and 1,232 cm−1 (Fig. 2). Although the absorption peak of 1,112 cm−1 disappeared, a new absorption peak was added at 1,120 cm−1 (Fig. 2). The increases of lignin absorption peak intensities and the formations of the new peaks indicated that the lignin contents in DROW increased. In general, the increases in lignin contents also indicated a great increase in cellulose decomposition (Pandey and Pitman 2003, Pandey and Nagveni 2009).

As a rule, brown-rot fungi can attack both cellulose and hemicellulose and retain lignin; whereas white-rot fungi can consume cellulose, hemicellulose, and lignin, especially lignin (Guo et al. 2010); the selective removal of cellulose and hemicellulose suggests that the oak wood was decayed by brown-rot fungi.

Table 2 shows that the content of lignin in DROW increased by about 10-fold in I1508/I1735 and threefold in I1508/I1374, respectively, whereas those of cellulose and hemicellulose decreased. These findings agree with those previous studies (Yang et al. 2020a, Sun et al. 2020) and are also consistent with the visual spectrogram (Fig. 1). The index value of the cellulose crystallinity in I1374/I2900 decreased by 45.00 percent and that in I1424/I897 decreased by 21.05 percent (Table 2), indicating that the crystalline zones of cellulose were greatly degraded by brown-rot fungi (Monrroy et al. 2011). The reduction of the cellulose crystallinity naturally would also result in a reduction in physical and mechanical properties of wood components.

Table 2. Ratio of Fourier-transform infrared spectroscopy characteristic peak height of wood samples.
Table 2.

FTIR analysis of the birch wooden components

Compared with nondecayed birch wood, the absorption peak intensities at 1,735 cm−1, 1,374 cm−1, and 1,158 cm−1 in decayed birch wood (DBW) were clearly reduced (Fig. 3), indicating that both cellulose and hemicellulose were seriously degraded by rot fungi, resulting in a decrease of the contents of cellulose and hemicellulose; whereas those at 1,651 cm−1, 1,598 cm−1, 1,508 cm−1, 1,460 cm−1, 1,424 cm−1, 1,336 cm−1, 1,245 cm−1, and 1,120 cm−1 in DBW increased significantly, especially at 1,508 cm−1, 1,460 cm−1, 1,424 cm−1, and 1,336 cm−1 (Fig. 3), indicating an increase of lignin content in DBW. According to the results of Guo et al. (2010), DBW might be severely damaged by brown-rot fungi.

Figure 3.Figure 3.Figure 3.
Figure 3. Infrared spectrum of the birch wooden component: region from 1,800 to 800 cm−1.

Citation: Forest Products Journal 71, 3; 10.13073/FPJ-D-21-00015

The content of lignin in DBW increased by 67.27 percent in I1508/I1735 and 60.00 percent in I1508/I1374, respectively, whereas those of cellulose and hemicellulose decreased evidently by 40.22 percent in I1735/I1508, 37.50 percent in I1374/I1508, and 30.56 percent in I1159/I1508, respectively, as shown in Table 2. The results further showed that cellulose and hemicellulose were seriously degraded and harmed not only by insects but also by brown-rot fungi. The results are consistent with the visual spectrogram (Fig. 3). The index value of the cellulose crystallinity in I1374/I2900 decreased by 58.33 percent and that in I1424/I897 decreased by 34.42 percent in DBW (Table 2), indicating that the crystalline zones of cellulose were also greatly degraded by brown-rot fungi (Monrroy et al. 2011).

FTIR analysis of the maple wooden components

Compared with nondecayed maple wood, the absorption peak intensities at 1,735 cm−1 and 1,374 cm−1 in decayed maple wood (DMW) was distinctly reduced (Fig. 4), indicating that cellulose and hemicellulose were degraded seriously by rot fungi. In addition, the absorption peaks at 1,158 cm−1 and 897 cm−1 in DMW disappeared (Fig. 4), indicating that the degradation of cellulose and hemicellulose was more obvious at these two absorption peaks. The decreases and disappearances of the absorption peak intensities resulted in a decrease of the contents of cellulose and hemicellulose.

Figure 4.Figure 4.Figure 4.
Figure 4. Infrared spectrum of the maple wooden component: region from 1,800 to 800 cm−1.

Citation: Forest Products Journal 71, 3; 10.13073/FPJ-D-21-00015

Moreover, the absorption peak intensities at 1,650 cm−1, 1,598 cm−1, 1,508 cm−1, 1,460 cm−1, 1,424 cm−1, 1,336 cm−1, and 1,120 cm−1 in DMW increased significantly, especially at 1,508 cm−1, 1,460 cm−1, 1,424 cm−1, 1,336 cm−1, and 1,120 cm−1 (Fig. 4). The absorption peak at 1,245 cm−1 in DMW disappeared, but two new peaks were added at 1,260 cm−1 and 1,232 cm−1 (Fig. 4). The increases of absorption peak intensities and the formation of new peaks indicated the increase of lignin contents in DMW. According to the result (Guo et al. 2010) that brown-rot fungi can consume both celluloses and retain lignin, it was speculated that DMW was severely damaged by brown-rot fungi.

Table 2 shows that the content of lignin in DMW increased by about eightfold in I1508/I1735 and 14-fold in I1508/I1374, respectively, whereas those of cellulose and hemicellulose decreased evidently in I1735/I1508, I1374/I1508, I1159/I1508, and I897/I1508. The results further indicated that cellulose and hemicellulose in DMW were seriously degraded and harmed not only by insects but also by brown-rot fungi. The results are consistent with the visual spectrogram (Fig. 4). The index value of the cellulose crystallinity of I1374/I2900 decreased by 85.29 percent and that of I1424/I897 decreased by 47.67 percent (Table 2), indicating that the crystalline zones of cellulose were greatly degraded by brown-rot fungi.

Discussion

The wooden components, especially the eaves pillars, in ancient Danxia Temple buildings have been exposed to the air for a long time, so they were affected by multiple factors such as environmental temperature and humidity changes, microorganisms, insects, ultraviolet rays, and so on. Studies in the paper have shown that lignin has an adhesive role and is retained at higher levels; those of cellulose, which has a framework role, and hemicellulose, which has an interstitial filling role, were consumed seriously by brown-rot fungi, whether red oak, birch, or maple wooden components.

In addition to the decay caused by brown-rot fungi in the birch and maple wooden components, they were also attacked by insects, the damage being visible to the naked eye. Some studies (Graham 1967, Guo et al. 2010) have shown that fungi can attack woods and make them soft, decaying them and making it easier for insects to gnaw; for example, the dampwood termite can attack decayed woods but not nondecayed woods. The combination of fungi and insects accelerated the destruction of DBW and DMW. Even if the insects do not attack the decayed woods, these insects hollowed out the decayed woods, built nests, and lived in nests, eventually destroying the wooden components of the ancient buildings. Further study (Guo et al. 2010) has shown that the decayed woods can emit products that have a smell similar to that of pheromones, such as vanillic acid, hydroxybenzoic acid, coumaric acid, and so on, that attract insects. For example, termites and ants can use pheromones to mark their feeding paths, thus invading woods. The degradation of wooden components in the ancient buildings of Danxia Tample will eventually lead to a decrease in their strength and bearing capacity.

Conclusions

The absorption peak intensities, the changes in chemical components, and cellulose crystallinity of red oak, birch, and maple wooden components in the ancient buildings of Danxia Tample were analyzed with FTIR spectra. For the red oak, birch, or maple wooden components, part of absorption peak intensities that represent cellulose and hemicellulose decreased and disappeared obviously, whereas those representing lignin increased. The indexes of the contents of cellulose and hemicellulose and the cellulose crystallinity also decreased, whereas those of lignin increased. Those results indicated that cellulose and hemicellulose in the three woods were largely degraded by brown-rot fungi. In additional, both DBW and DMW were seriously harmed by insects, in addition to the attack of brown-rot fungi. The research of decay extent in this paper can supply scientific data for later protection and repairs in Danxia Tample.

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