Decay level classification of wooden components in Tingbao Yang’s former residence utilizing polarization and fluorescence effects

Abstract

Decay levels of wooden components in the Yang former residence were classified using polarized light and fluorescence methods. Analysis of the decay cause was conducted based on external conditions and wood species characteristics. The polarization and fluorescence effects revealed that there were varying degrees of decay in larch (Larix potaninii var. australis), spruce (Picea brachytyla), lace-bark pine (Pinus bungeana), Masson pine (Pinus massoniana), Chinese Douglas fir (Pseudotsuga sinensis), Chinese fir (Cunninghamia lanceolata), poplar (Populus tomentosa), and elm (Ulmus pumila). The primary factors contributing to decay included the inherent low natural durability of the wood species and adverse external conditions, such as damaged roofs, missing dripping water and tiles causing water leakage, uneven indoor and outdoor ground levels, contemporary tile paving indoors, and inadequate ventilation. This study aims to establish a scientific basis for subsequent conservation strategies.

Full Article

Decay Level Classification of Wooden Components in Tingbao Yang’s Former Residence Utilizing Polarization and Fluorescence Effects

Haidi Ji,^# Yan Yang,^#,* Hui Zhang, Bin Li, and Lianlong Cheng

Decay levels of wooden components in the Yang former residence were classified using polarized light and fluorescence methods. Analysis of the decay cause was conducted based on external conditions and wood species characteristics. The polarization and fluorescence effects revealed that there were varying degrees of decay in larch (Larix potaninii var. australis), spruce (Picea brachytyla), lace-bark pine (Pinus bungeana), Masson pine (Pinus massoniana), Chinese Douglas fir (Pseudotsuga sinensis), Chinese fir (Cunninghamia lanceolata), poplar (Populus tomentosa), and elm (Ulmus pumila). The primary factors contributing to decay included the inherent low natural durability of the wood species and adverse external conditions, such as damaged roofs, missing dripping water and tiles causing water leakage, uneven indoor and outdoor ground levels, contemporary tile paving indoors, and inadequate ventilation. This study aims to establish a scientific basis for subsequent conservation strategies.

DOI: 10.15376/biores.19.3.4087-4103

Keywords: Ancestral wooden homes; Wood rot; Wooden components; Polarization; Fluorescence effects; Decay cause

Contact information: School of Architecture, Nanyang Institute of Technology, Nanyang City, Henan Province, 473000, P.R. China; * Corresponding author: yangyanrainy@163.com

INTRODUCTION

The focus of this study, located in the western section of Jiefang Road, Wancheng District, Nanyang City, Henan Province, was the ancestral home of Mr. Tingbao Yang, a distinguished contemporary Chinese architect. The buildings were constructed during the late Qing Dynasty, and include the Yang family courtyard, Xu family courtyard, Taigu Sugar Company, and the Yang family back pit.

The residence is considered one of the most well-preserved residential buildings in Nanyang City, bearing witness to the struggles of modern revolutionaries from the Xinhai Revolution to the New Democracy Revolution, and showcasing a wealth of folk cultural connotations and exemplifying the urban living style prevalent in southwestern Henan Province during the late Qing Dynasty. It provides valuable insights into life conditions, folk customs, and manners of local people during the late Qing Dynasty Republic era. The well-preserved courtyards exhibit a complete architectural style and hold high conservation value.

Although preservation efforts have been ongoing for Tingbao Yang’s former residence, the key load-bearing wooden components, such as columns, beams, lintels, purlins, and rafters, have suffered extensive deterioration, worm damage, and cracks. The degradation of wood inevitably leads to alterations in the anatomical structure of wood cell walls and degradation of chemical components (Cavallaro et al. 2017; Tamburini et al. 2017; Bari et al. 2019, 2020; Dong et al. 2020; Yang et al. 2020, 2021a,b, 2022a,b,c; Broda et al. 2022). As the degree of deterioration increases, there is a noticeable attenuation in the physical and mechanical properties (Brischke et al. 2019; Ueda et al. 2020). It is widely acknowledged that the health condition of load-bearing wooden components directly impacts the safety of a wooden frame. Any issues with the wood frame pose a significant threat to the overall stability of the building structure.

As a result, the accurate and scientific detection and diagnosis of wooden components have become a top priority in assessing ancient wooden buildings. Non-destructive testing methods (Chang et al. 2016; Dai et al. 2017; Zhang et al. 2021), such as stress wave detection, resistance meter detection, and ultrasonic detection, can be utilized to evaluate the internal health condition or material performance without destroying the original shape and structure of wooden components. However, this method still remains a macro-scale detection technique that can only offer a more comprehensive qualitative or quantitative assessment of deteriorated phenomena in severely damaged areas. It presents challenges in scientifically identifying the initiation of cell wall microstructure degradation within regions diagnosed as “healthy” at the macro level.

Polarized microscopy can be utilized to assess the distribution and composition of cellulose crystal zones within wood cell walls. The birefringence brightness of crystalline cellulose (BBCC) is directly proportional to the concentration and content of cellulose (Kanbayashi and Miyafuji 2016; Yang et al. 2021a, 2022a,b,c). Meanwhile, fluorescence microscopy can be utilized to evaluate lignin distribution and content in wood cells through the green fluorescence brightness of lignin (GFBL). Higher GFBL indicate greater concentration and content of lignin (Kanbayashi 2016; Liu et al. 2017; Kiyoto et al. 2018; Yang et al. 2021a, 2022a,b,c). This technique requires only a limited quantity of samples from wooden components, while avoiding noticeable damage. Consequently, it is increasingly utilized for evaluating the material health status of wooden components in ancient buildings.

In this study, various microscopic observation techniques, including bright-field light, polarized light, and fluorescence methods, were utilized to analyze the extent of cell wall damage as well as the distribution and content of cellulose and lignin in the wooden components of Tingbao Yang’s former residence. Additionally, decay levels would be categorized based on cellulose and lignin content. Furthermore, an extensive analysis of decay causes was conducted by examining internal characteristics exhibited by different wood species alongside external conditions. The objective of this research was to establish a foundation for scientifically selecting subsequent conservation strategies.

EXPERIMENTAL

Materials

Sixty-one samples were randomly collected from various wooden components, including columns, beams, lintels, purlins, rafters, and other wooden components in Rooms 1 to 23 of Tingbao Yang’s former residence (Fig. 1) located in Jiefang Road, Wancheng District, Nanyang City, Henan Province. The sampling was conducted using an increment borer (10-100-1027, Haglöf AB, Mora, Sweden). The samples consisted of visually decayed wood as well as moth-eaten and undamaged wood. Control samples were obtained from normal positions within these wooden components. Table 1 provides specific details on the sampling locations and the names of the wood species.

Fig. 1. Present state of Yang Tingbao’s former residence

Table 1. Material Sampling Data

The predominant wood species identified by the authors (Yang et al. 2024) include larch (Larix potaninii var. australis), spruce (Picea brachytyla), lace-bark pine (Pinus bungeana), Masson pine (Pinus massoniana), Chinese Douglas fir (Pseudotsuga sinensis), Chinese fir (Cunninghamia lanceolata), poplar (Populus tomentosa), and elm (Ulmus pumila).

METHODS

Pretreatment of Wooden Component Samples

Some samples were seriously damaged by decay fungi, such that the material had become soft. In order to increase the strength of the samples and facilitate better sectioning, all the samples were embedded with polyethylene glycol (PEG). The processing procedures for wooden components were conducted in accordance with the following steps (Yang et al. 2021a, 2022a,b,c): (1) Air evacuation: The small samples were placed in a vacuum dryer to eliminate air from within the wood samples. (2) Infiltration with PEG (molecular weight = 2000): The sample was sequentially immersed in aqueous solutions of PEG at concentrations of 20%, 40%, 60%, and 80% before finally being subjected to a concentration of 100%. Each gradient underwent a processing duration of 48 h at a temperature of 60 °C, with two infiltrations utilizing the 100% PEG solution. (3) Embedding in PEG: The samples were positioned at the base of an embedding mold and subsequently covered with an aqueous solution of 100% PEG. A plastic embedding box was then employed to enclose the setup. (4) Freezing: The embedding box was placed inside a freezer for approximately 10 min.

Sectioning of Wooden Component Samples

The sections were prepared following the steps outlined in GB/T 29894 (2013) and Yang et al. (2021a, 2022a,b,c). (1) Firstly, the embedded samples were carefully sectioned using a microtome (HistoCore AUTOCUT, Leica Biosystems, Wetzlar, Germany) to obtain transverse, radial, and tangential sections with a thickness of approximately 10 µm. (2) Subsequently, the sections were subjected to baking at 60 °C for about 60 min to eliminate excess water and prevent curling. (3) Dehydration was then performed by immersing the slices sequentially in ethanol solutions of increasing concentrations (50%, 75%, 95%, and finally 100%), each treatment lasted for a duration of 10 min. (4) To remove any remaining fat content from the slices, they were defatted using dimethyl benzene solution for a period of 3 min. (5) Finally, neutral gum was applied as a sealing agent after all treatments. It is important to note that no red O dye was used on any of the slices to avoid interference with polarized light and fluorescence observations.

Microscopic Observation under Bright-field Light, Polarized Light, and Fluorescence

A polarizing microscope can convert ordinary light into polarized light, so as to analyze the birefringence of crystals. The crystalline region of cellulose in wood cell wall has birefringence, so the cellulose concentration can be determined according to the intensity of birefringence, so as to qualitatively assess the degradation of wood cellulose (Cui et al. 2016). Filter blocks with different wavelengths of fluorescence microscope can stimulate the fluorescent material in wood to emit fluorescence of different colors. Lignin in wood cell wall can produce green fluorescence when irradiated by blue filter block. Therefore, the lignin concentration can be determined according to the intensity of green fluorescence, and the degradation of wood lignin can be qualitatively assessed (Cui et al. 2016). The microstructures of the prepared slices were observed using bright-field light, polarized light, and fluorescence under a standing fluorescence microscope (ECLIPSE Ni-U, Nikon, Tokyo, Japan). The degree of cell damage was assessed through bright-field observation. The qualitative measurement of cellulose crystal distribution and content in the cell wall was conducted using polarizing light. Higher birefringence values indicate higher concentrations and contents of cellulose. The qualitative measurement of lignin distribution and content in the cell wall was performed using fluorescence (Blue Monochromatic filter block: excitation wavelength 450 to 490 nm; Block 510 nm; emission wavelengths: 515 nm). Higher brightness levels of green fluorescence and red fluorescence correspond to higher concentrations and contents of lignin (Yang et al. 2021a, 2022a,b,c).

Wood degradation level was determined based on the BBCC as well as the GFBL. If these values are similar to those of the control wood, then it is defined as slight decay; If they decrease by 1/3 to 1/2 compared to the control, then it is defined as medium decay; If they decrease by more than 1/2 compared to the control, then it is defined as serious decay.

RESULTS AND DISCUSSION

Decay Levels of Larch Wooden Components

Figure 2 illustrates the microstructure of polarization and fluorescence effects in larch wooden components, along with their corresponding control wood.

Fig. 2. Microstructure of polarization and fluorescence effects in larch wooden components

It is noteworthy that tracheids in Nos. 42 and 47 remained predominantly intact, while those in No. 38 exhibited evident structural damage when examined under bright-field light. Under polarized light, the BBCC of tracheids in Nos. 42 and 47 exhibited a noticeable presence, a little weaker compared to the control wood. Notably, there was an evident decrease in the BBCC within the middle layer (S₂) and inner layer (S₃) of tracheid in No. 47, indicating that a certain level of degradation had occurred. Moreover, tracheid in No. 38 displayed a greater reduction in the BBCC, suggesting a higher degree of cellulose degradation. Under fluorescence, the GFBL of tracheids in Nos. 42 and 47 exhibited noticeable intensity, suggesting that the lignin within these two wooden components remained intact or only slightly damaged by wood decay fungi. However, an evident reduction was observed in No. 38, particularly within the compound middle lamellas (CML) and cell corners (CC), indicating a more extensive degradation of lignin. Based on comprehensive analysis, it can be concluded that No. 47 exhibited slight decay, No. 42 displayed moderate decay, while No. 38 demonstrated severe decay.

Decay Levels of Spruce Wooden Components

Figure 3 illustrates the microstructure of polarization and fluorescence effects in spruce wooden component and its control wood. A comparative analysis reveals that the tracheids in No. 14 exhibited a higher incidence of perforations when observed under bright-field light, indicating more severe damage to its structural integrity compared to the control wood. Furthermore, examination through a polarizing microscope demonstrated an evident reduction in the BBCC of tracheids as opposed to the control wood, suggesting a greater extent of degradation in cellulose arrangement. Under fluorescence observation, there was slight attenuation of the GFBL of tracheids compared to the control wood, indicating a certain degree of lignin degradation. Based on analysis, it can be concluded that No. 14 exhibited severe decay.

Fig. 3. Microstructure of polarization and fluorescence effects in spruce wooden components

Decay Levels of Lace-bark Pine Wooden Components

Figure 4 illustrates the microstructure of polarization and fluorescence effects in lace-bark pine wooden components. Under bright-field light, the tracheids in Nos. 1 and 44 remained predominantly intact. However, under polarized light, there was a significant weakening of the BBCC of the tracheids, accompanied by a loosening of the overall arrangement of crystalline cellulose. This observation suggests that these two wooden components had undergone a higher degree of cellulose degradation. Furthermore, fluorescence analysis revealed a noticeable reduction in the GFBL of tracheids, indicating more extensive lignin degradation. Based on the analysis, it can be concluded that Nos. 1 and 44 exhibited severe decay.

Fig. 4. Microstructure of polarization and fluorescence effects in lace-bark pine wooden components

Fig. 5. Microstructure of polarization and fluorescence effects in Masson pine wooden components

Decay Levels of Masson Pine Wooden Components

Figure 5 illustrates the microstructure of polarization and fluorescence effects in Masson pine wooden components along with the control wood. Under bright-field light, the tracheids in Nos. 59 and 60 appeared predominantly intact. However, when examined using a polarizing microscope, the BBCC of tracheids in No. 60 was generally weaker compared to that of the control wood, indicating a slight degradation of cellulose in No. 60. Conversely, the tracheids in No. 59 exhibited a yellowish BBCC with minimal intensity, indicating a greater degree of degradation of cellulose in No. 59. Fluorescence microscopy revealed that a GFBL of the tracheids in both Nos. 59 and 60 was similar to that in the control wood. Based on comprehensive analysis, it can be concluded that No. 59 exhibited severe decay while No. 60 showed slight decay.

Decay Levels of Chinese Douglas Fir Wooden Components

Figure 6 illustrates the microstructure of polarization and fluorescence effects in Chinese Douglas fir wooden components. The tracheids in all samples remained predominantly intact structures when observed under bright-field light. However, examination through a polarizing microscope revealed an evident reduction of the BBCC of tracheids in Nos. 17 to 21 and 28, indicating noticeable degradation of cellulose. Additionally, there was a noticeable decrease in the BBCC of tracheids No.16, suggesting partial degradation caused by decaying bacteria activity. Furthermore, No. 41 exhibited observable BBCC, implying slight degradation. Under fluorescence, the GFBL of tracheids in all samples remained relatively prominent. However, there was a noticeable reduction in GFBL in the CML and CC of tracheid in No. 41. This observation suggests that there has been some degradation of lignin in No. 41. Based on analysis, it can be concluded that Nos. 17 to 21 and 28 exhibited severe decay, while No. 16 showed moderate decay and No. 41 exhibited slight decay.