Chemical degradation of wooden components in traditional dwelling No. 9, East Gate Lane, Gulian village: Assessed by polarized light microscopy, fluorescence microscopy, and histochemical staining

Abstract

To accurately elucidate the microscopic degradation characteristics and deterioration mechanisms of wooden components in traditional northern Chinese dwellings, this study investigated wooden components retrieved from the Ming-Qing residential building at No. 9, East Gate Lane, Gulian Village, Qi County, Shanxi Province. Polarized light microscopy, fluorescence microscopy, and histochemical staining techniques were employed to qualitatively assess the degradation degrees of cellulose and lignin in six wood species: elm, poplar, willow, birch, Chinese larch, and Chinese pine. Internal and external deterioration drivers were analyzed, and corresponding conservation strategies were proposed. The results showed that all tested wooden components exhibit moderate to severe chemical degradation. The deterioration of wooden components is synergistically driven by interspecific differences in natural decay resistance, rainwater erosion, ultraviolet radiation, building structural damage, and microbial infestation. Elm components are mainly attacked by white-rot fungi, while the other wood species are predominantly affected by soft-rot fungi. Poplar and willow are more susceptible to insect damage. This study provides a scientific basis and technical support for the microscopic diagnosis, precise restoration, and preventive conservation of Ming-Qing wooden architecture in the Jinzhong region.

Full Article

Chemical Degradation of Wooden Components in Traditional Dwelling No. 9, East Gate Lane, Gulian Village: Assessed by Polarized Light Microscopy, Fluorescence Microscopy, and Histochemical Staining

Haidi Ji  ,# Tongyu Xu,# Xianghe Liu,# Ziyan Luo, Bin Li, and Yan Yang *

To accurately elucidate the microscopic degradation characteristics and deterioration mechanisms of wooden components in traditional northern Chinese dwellings, this study investigated wooden components retrieved from the Ming-Qing residential building at No. 9, East Gate Lane, Gulian Village, Qi County, Shanxi Province. Polarized light microscopy, fluorescence microscopy, and histochemical staining techniques were employed to qualitatively assess the degradation degrees of cellulose and lignin in six wood species: elm, poplar, willow, birch, Chinese larch, and Chinese pine. Internal and external deterioration drivers were analyzed, and corresponding conservation strategies were proposed. The results showed that all tested wooden components exhibit moderate to severe chemical degradation. The deterioration of wooden components is synergistically driven by interspecific differences in natural decay resistance, rainwater erosion, ultraviolet radiation, building structural damage, and microbial infestation. Elm components are mainly attacked by white-rot fungi, while the other wood species are predominantly affected by soft-rot fungi. Poplar and willow are more susceptible to insect damage. This study provides a scientific basis and technical support for the microscopic diagnosis, precise restoration, and preventive conservation of Ming-Qing wooden architecture in the Jinzhong region.

DOI: 10.15376/biores.21.4.9351-9372

Keywords: Traditional dwelling; Gulian village; Chemical degradation of wooden components; Polarized light microscopy; Fluorescence microscopy; Histochemical staining

Contact information: School of Architecture, Nanyang Institute of Technology, Nanyang, China;

#These three authors contributed equally to this work; *Corresponding author: [email protected]

INTRODUCTION

Traditional villages are tangible carriers of agrarian civilization, preserving historical heritage and architectural wisdom. Strengthening the protection and inheritance of traditional villages and buildings is a critical pathway to advance the creative transformation and innovative development of traditional Chinese culture (Li et al. 2024). Gulian Village, located in Qi County, Jinzhong City, Shanxi Province, is a national-level historical and cultural village and a traditional Chinese village. First established in the early Hongwu reign of the Ming Dynasty, it was originally named Kulue Village. Following the construction of a fortified village enclosure in the 20^th year of the Jiajing reign (1541), it was renamed Kulue Fortress. It adopted the name Gulian Fortress in the early Qing Dynasty, and earned the prestigious moniker “Silver Gulian” after the Qianlong reign period. Currently, it preserves over 40 Ming-Qing merchant residences and public buildings. It is predominantly of brick-wood structure with exquisite brick, wood, and stone carvings. This village is a rare physical example for systematic research on the architectural systems, craft inheritance, and cultural connotations of traditional dwellings in Jinzhong and northern China, holding significant historical and cultural value (Li 2017).

Fig. 1. Past and present of Traditional Dwelling No. 9, East Gate Lane, Gulian Village

Traditional Dwelling No. 9, East Gate Lane, originally the residence of Gao Yuanlie, sits at the southern entrance of Pawnshop Lane in East Gate Lane, Gulian Village. It consists of a residential courtyard and a family ancestral hall (Fig. 1a). Located at the core of the Ming-era Gulian Fortress, its initial construction is dated to the Hongwu–Yongle period by inference, aligned with the center-outward expansion rule of ancient Chinese village planning. As the village’s sole architectural complex integrating structures from both dynasties, it holds great historical and cultural significance. The western residential section is a two-courtyard complex facing south, and its inner courtyard dates to the Ming Dynasty. The entire complex is roughly rectangular. Both the main gate and the second gate are exposed-column overhanging gatehouses. This architectural form is common and representative in traditional Chinese architecture. It uses exposed columns as structural supports and features outward-extending roof eaves, integrating structural function, practicality, and artistic decoration (Fig. 1b). This section contains 26 rooms. The eastern section served as the family ancestral hall with 15 rooms. A pair of flagpoles originally stood in front of its gate (Fig. 1a), but they have been destroyed. Currently, only the western residential courtyard of Traditional Dwelling No. 9 remains, and its overall layout is relatively well preserved. However, severe structural damage has been observed. The east and west wing rooms of the first courtyard have been destroyed. The courtyard walls, the roof system of the inverted house, the eastern section of the main hall, and the southern part of the east wing have collapsed. Although the main gate, archway, and west wing remain relatively intact, they also show varying degrees of deterioration (Fig. 1c). These damages pose a serious threat to the structural safety and long-term preservation of the building. Currently, Traditional Dwelling No. 9 remains chronically vacant, with its collapsed and deteriorated structural elements yet to receive timely stabilization and repair. In accordance with the follow-up conservation and development plan for Gulian Village, the renovation of this courtyard complex is expected to commence in the near future.

The load-bearing structure of the residential building at No. 9 Dongmenli Courtyard is predominantly wooden, and its structural damage is primarily caused by the deterioration and degradation of wooden components. Wooden structure deterioration results from the combined action of multiple factors and is essentially a coupling process between the inherent properties of wood and external environmental stresses. The species-specific characteristics of wood form the intrinsic basis for its decay resistance and structural performance. Factors influencing the natural durability of wood include anatomical structure, density, chemical composition, and their respective contents. Wood-decay fungi are the primary agents responsible for wood decay, and their growth and reproduction depend on sufficient nutrient sources, moisture, suitable temperature, oxygen, and appropriate pH conditions (Guo et al. 2010). On the one hand, the inherent chemical components of wood, namely cellulose, hemicellulose, and lignin, provide abundant nutrients for decay fungi. On the other hand, the optimal growth temperature for decay fungi ranges from 25 to 30 °C, and the optimal moisture content for reproduction is between 30% and 150% (Schwarze and Spycher 2005; Chen 2019). Within this range, wood moisture content is positively correlated with decay rate (Yang et al. 2025). When environmental conditions such as temperature and humidity meet the growth requirements of fungi, the components of wood cell walls are metabolized and decomposed by fungi. This process ultimately leads to wood decay, which manifests as performance degradation, shortened service life, and reduced use value (Goodell et al. 2020).

The assessment of the decay status of wooden components in ancient Chinese buildings mostly relies on non-destructive testing (NDT) techniques such as stress wave, resistograph, and ultrasonic testing (Chang et al. 2022; Mvolo et al. 2022; Zhang et al. 2022; Bai et al. 2023). Although these methods comply with the principle of minimum intervention in heritage conservation, they are limited to macroscopic defect identification. They cannot detect early-stage deterioration at the microscale, nor can they fully reflect the impact of decay on the internal structure of wood. Therefore, it is urgent to investigate the decay changes of wood at the micro level by focusing on its microstructure and chemical composition.

In the field of wood microstructural analysis, various analytical techniques offer distinct advantages. Polarized light microscopy characterizes cellulose degradation via the birefringence of crystalline cellulose regions (Cui et al. 2016), while fluorescence microscopy assesses lignin concentration based on its inherent autofluorescence (He et al. 1999; Balzano et al. 2022). Safranin O is a basic dye that stains lignin in wood tissues red, enabling qualitative assessment of lignin content in wood cells. Methylene blue, also a basic dye, is adsorbed by amorphous cellulose and hemicellulose in wood tissues; accordingly, methylene blue staining can be applied for the qualitative determination of amorphous cellulose and hemicellulose contents (Yu and Zhang 1989; Micco and Aronne 2007; Yang et al. 2026; Yang et al. 2020). In recent years, these micro-destructive testing techniques have been increasingly applied to assess the material condition of wooden components in historic buildings. Researchers have employed polarized light and fluorescence microscopy to quantify wood degradation (Gan et al. 2020; Yang et al. 2022; Li et al. 2019), and to establish decay grading criteria based on microstructural features (Cui et al. 2016; Yang et al. 2025).

This study examined wooden components from Traditional Dwelling No. 9, Dongmenli, Gulian Village using cross-validated polarized light microscopy, fluorescence microscopy, and histochemical staining to characterize chemical degradation across wood species, identify deterioration drivers, and propose tailored conservation and restoration strategies. The results are expected to provide theoretical references and technical solutions for the scientific protection of wooden cultural relic buildings in the Jinzhong region.

EXPERIMENTAL

Materials

Wood samples were collected from columns, beams, purlins, rafters, doors, and windows of Traditional Dwelling No. 9, East Gate Lane, Gulian Village, Qi County, Jinzhong City, Shanxi Province using an increment borer (Model 10-100-1027, Haglöf AB, Sweden) (Fig. 2).

Fig. 2. Sampling locations of wooden components

Sampling sites were prioritized on high-deterioration-risk sites: column bases, wall-embedded column sections, timber ends, eaves purlins, and eaves rafter tips. The samples included visibly decayed wood, insect-damaged wood, and sound wood with no apparent defects. These sampling sets can authentically reflect the material selection, service conditions, and diverse degradation patterns of the building’s wooden components. All samples were taxonomically identified by the authors, and the wooden components belong to six wood taxa: Larix sect. Multiseriales, Pinus tabulaeformis, Betula spp., Populus spp., Salix sect. Salix, and Ulmus sect. Madocarpus. Detailed sampling locations and quantities are presented in Table 1.

Control woods were sourced from healthy standing trees of Chinese larch, Chinese pine, birch, poplar, and elm, all aged over 30 years.

Table 1. Sample Information of Wooden Components

Staining Reagents

Safranin O was purchased from Phygene Biotechnology Co., Ltd., and methylene blue from Hunan BKMAM Holdings Co., Ltd. All staining reagents were of analytical grade and used without further purification.

Preparation and Histochemical Staining of Wooden Component Sections

The preparation procedure for wooden component sections was performed as described previously (Yang et al. 2024). (1) Sampling: Samples were cut into cubes of approximately 1 cm in size, ensuring that each transverse section contained at least one growth ring. (2) Softening: Wood samples were softened by boiling in water for 4 to 5 hours. (3) Sectioning: Treated specimens were sectioned using a Leica microtome (Model HistoCore AUTOCUT, Leica Biosystems, Germany) to obtain transverse, radial, and tangential sections with a thickness of 10 to 15 µm. (4) Staining: Sections were immersed in 3% safranin O aqueous solution and 0.1% methylene blue aqueous solution, respectively, for more than 2 hours. (5) Dehydration: Sections were dehydrated in a graded ethanol series (50%, 75%, 95%, and 100%) for 10 min at each concentration. (6) Clearing: Sections were cleared in xylene solution for 3 to 5 min. (7) Mounting: Sections were mounted on glass slides using neutral balsam.

Observation of Wooden Component Sections under Polarized Light, Fluorescence, Methylene blue, and Safranin O Staining

Microstructural observations of the prepared wood sections were performed using an upright fluorescence microscope (Model ECLIPSE Ni-U, Nikon Corporation, Japan). Micrographs were obtained under polarized light, fluorescence, and after staining with methylene blue and safranin O, respectively.

Polarized light microscopy was used to qualitatively assess the degradation state of crystalline cellulose regions in wood cell walls based on the Birefringence Brightness of Crystalline Cellulose (BBCC). Higher brightness indicates higher cellulose content, while reduced brightness reflects cellulose degradation (Kanbayashi and Miyafuji 2016; Cui et al. 2016; Yang et al. 2025). Fluorescence microscopy with a blue excitation filter block (515 to 560 nm) was used to qualitatively determine lignin degradation in wood cell walls based on the Green Fluorescence Brightness of Lignin (GFBL). Higher GFBL corresponds to higher lignin density and concentration (Ma et al. 2011; Liu et al. 2017; Kiyoto et al. 2018; Yang et al. 2020; Martin and Lopez 2023; Ji et al. 2024).

Based on the principle that amorphous cellulose and hemicellulose can adsorb methylene blue, the distribution and content of these components within cell walls can be qualitatively assessed. A deeper blue color indicates higher contents of cellulose and hemicellulose (Yang et al. 2026; Toy et al. 2025). Safranin O staining stains lignin red, enabling qualitative evaluation of lignin distribution and content in cell walls (Criswell et al. 2025). Deeper red coloration indicates higher lignin concentration (Vazquez-Cooz et al. 2002; Guillermo et al. 2004; Ding 2010).

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 degradation. If they decrease by 33% to 50% compared to the control, then it is defined as medium degradation. If they decrease by more than 50% compared to the control, then it is defined as serious degradation (Ji et al. 2024; Yang et al. 2025).

By comprehensively comparing the BBCC, the GFBL, and the staining results of methylene blue and safranin O, the degradation degree of chemical components and the overall deterioration state of wooden components can be systematically evaluated.

RESULTS AND DISCUSSION

Chemical Degradation Analysis of Elm Wooden Components

The degradation characteristics of elm wooden components under bright-field light, polarized light and fluorescence are presented respectively in Figs. 3 a-c. Under bright-field illumination (Fig. 3 a), the cell walls of vessels, ground tissue fibers, and wood rays in Sample No. 1 remained intact with no visible damage. Under polarized light (Fig. 3 b), Sample No. 1 exhibited distinct BBCC. Its signal intensity was 15 to 30% lower than that of the control wood, indicating that a large proportion of crystalline cellulose was retained in the sample. Under green fluorescence (Fig. 3 c), the GFBL of Sample No. 1 was weak, measuring only 40 to 50% of that of sound wood. This indicates partial degradation of the aromatic ring structure of lignin in the sample.

Fig. 3. Microscopic effects of elm wooden components under Bright-field light, Polarized light, Green fluorescence, Methylene blue, and Safranin O

Under methylene blue staining (Fig. 3 d), Sample No. 1 exhibits relatively distinct blue staining in the cell walls, indicating that partial hemicellulose and amorphous cellulose are retained in the sample. Under safranin O staining (Fig. 3 e), Sample No. 1 shows relatively pronounced red staining, suggesting a partial reduction in lignin content, which is broadly consistent with the fluorescence observations.

Integrated analysis of the microstructures of elm wooden components under bright-field illumination, polarized light, fluorescence, and histochemical staining (Fig. 3) reveals that Sample No. 1 undergoes slight cellulose degradation and moderate lignin degradation, with lignin degradation being more severe. It is concluded that the chemical components of Sample No. 11 are moderately degraded (Table S1).

Chemical Degradation Analysis of Poplar Wooden Components

The degradation characteristics of poplar wooden components under bright-field light, polarized light and fluorescence microscopy are shown respectively in Figs. 4 a-c. Under bright-field illumination (Fig. 4 a), the vessel cell walls of Sample No. 17 were severely damaged, and the ground tissue fibers and cell walls were slightly damaged.

Fig. 4. Microscopic effects of poplar wooden components under Bright-field light, Polarized light, Green fluorescence, Methylene blue, and Safranin O

The vessel cell walls of Samples Nos. 3 and 11 were slightly damaged, while their wood fiber cell walls remained largely intact. Under polarized light (Fig. 4 b), the BBCC of Sample No. 17 was very weak, with a reduction of 60 to 80% compared with the control wood, indicating severe reduction of crystalline cellulose content in Sample No. 17. The BBCC of Samples Nos. 3 and 11 was distinct but decreased by 45 to 50% relative to the control wood, suggesting partial degradation of crystalline cellulose in these two samples. Under fluorescence, the GFBL of Sample No. 17 was extremely weak, with a reduction of 70 to 90% compared with the control wood, indicating extensive degradation of the aromatic ring structure of lignin. Under green fluorescence (Fig. 4 c), the GFBL of Samples Nos. 3 and 11 were relatively distinct but decreased by 50 to 70% compared with the control wood, indicating moderate degradation of lignin. Under methylene blue staining (Fig. 4 d), cell walls of Samples Nos. 3, 11 and 17 all exhibit faint blue staining. However, Sample No. 17 shows deeper blue intensity than Samples Nos. 3 and 11. When combined with polarized light results, this indicates that crystalline cellulose in Sample No. 17 has undergone extensive degradation and been converted into a certain amount of amorphous cellulose. Under safranin O staining (Fig. 4 e), red staining is faint across all three samples, but the red intensity in cell walls of Samples Nos. 3 and 11 is overall stronger than that of Sample No. 17. This pattern is broadly consistent with the fluorescence observations.

Integrated analysis of microstructures of poplar wooden components under bright-field illumination, polarized light, fluorescence and histochemical staining (Fig. 4) reveals that both cellulose and lignin in Sample No. 17 were severely degraded, with lignin degradation being more severe than cellulose degradation. For Samples Nos. 3 and 11, cellulose had undergone moderate degradation while lignin was severely degraded. It is concluded that Sample No. 17 is severely degraded, and Samples Nos. 3 and 11 were moderately degraded (Table S1).

Chemical Degradation Analysis of Willow Wooden Components

Degradation characteristics of willow wooden components under bright-field light, polarized light and fluorescence microscopy are presented respectively in Figs. 5 a-c. Under bright-field illumination (Fig. 5 a), the vessel cell walls of Samples Nos. 15 and 16 were severely damaged, and their wood fiber cell walls were slightly damaged. The vessel cell walls of Sample No. 14 were slightly damaged, while its wood fiber cell walls remained largely intact. All cell walls of Samples Nos. 2 and 10 remained essentially intact. Under polarized light (Fig. 5 b), the BBCC of Samples Nos. 15 and 16 were extremely weak, with a reduction of more than 90% compared with the control wood, indicating almost complete degradation of crystalline cellulose in these samples. The BBCC of Sample No. 14 was relatively distinct but decreased by 50% to 70% relative to the control wood, suggesting moderate reduction of crystalline cellulose content. The BBCC of Samples Nos. 2 and 10 were distinct but decreased by 30% to 40% compared with the control wood, indicating that a large proportion of crystalline cellulose was retained. Under green fluorescence (Fig. 5 c), the GFBL of Samples Nos. 15 and 16 were extremely weak, with a reduction of 85% to 95% compared with the control wood, indicating severe degradation of the aromatic ring structure of lignin. The GFBL of Sample No. 14 was weak but decreased by 70% to 80% compared with the control wood, indicating relatively severe degradation of the aromatic ring structure of lignin. The GFBL of Samples Nos. 2 and 10 were distinct but decreased by 35% to 45% compared with the control wood, indicating partial degradation of the aromatic ring structure of lignin.

Under methylene blue staining (Fig. 5 d), the cell walls of Samples Nos. 14-16 show faint blue staining intensity, indicating a severe reduction in amorphous cellulose and hemicellulose contents. In contrast, Samples Nos. 2 and 10 exhibit distinct blue staining intensity, suggesting only a minor decrease in these components. Under safranin O staining (Fig. 5 e), the cell walls of Samples Nos. 14-16 display faint red staining intensity, while those of Samples Nos. 2 and 10 show relatively pronounced red staining. This pattern is broadly consistent with the fluorescence observations.

Fig. 5. Microscopic effects of willow wooden components under Bright-field light, Polarized light, Green fluorescence, Methylene blue, and Safranin O

Integrated analysis of the microstructures of willow wooden components under bright-field illumination, polarized light, fluorescence, and histochemical staining (Fig. 5) reveals that both cellulose and lignin are severely degraded in Samples Nos. 14 to 16, with Sample No. 14 exhibiting more severe lignin degradation than cellulose degradation. Samples Nos. 2 and 10 undergo moderate degradation of both components. It can be concluded that Samples Nos. 14 to 16 are severely degraded, while Samples Nos. 2 and 10 are moderately degraded (Table S1).

Chemical Degradation Analysis of Birch Wooden Components

Degradation characteristics of birch wooden components under bright-field light, polarized light and fluorescence microscopy are shown respectively in Figs. 6 a-c.

Fig. 6. Microscopic effects of birch wooden components under Bright-field light, Polarized light, Green fluorescence, Methylene blue, and Safranin O

Under bright-field illumination (Fig. 6 a), the vessel cell walls of Samples Nos. 4, 5, and 13 were slightly damaged, while the ground tissue fibers and cell walls remained relatively intact. Under polarized light (Fig. 6 b), the BBCC of Samples Nos. 4, 5, and 13 were distinct, with a reduction of 35% to 50% compared with the control wood, indicating a certain degree of reduction in crystalline cellulose content. Under green fluorescence (Fig. 6 c), the GFBL of Samples Nos. 4, 5, and 13 were relatively distinct, with a reduction of 40% to 50% compared with the control wood, indicating partial degradation of the aromatic ring structure of lignin.

Under methylene blue staining (Fig. 6 d), Samples Nos. 4, 5 and 13 show relatively distinct blue staining intensity in cell walls, indicating that a certain amount of amorphous cellulose and hemicellulose remains in the wooden components. Under safranin O staining (Fig. 6 e), the corresponding red staining confirms residual lignin in the components, which is broadly consistent with the fluorescence observations.

Integrated analysis of the microstructures of birch wooden components under bright-field illumination, polarized light, fluorescence and histochemical staining (Fig. 6) reveals that both cellulose and lignin in Samples Nos. 4, 5 and 13 undergo moderate degradation. It can be concluded that Samples Nos. 4, 5 and 13 are moderately degraded (Tab. 2).

Comprehensive analysis of the microstructural images of birch wooden components under bright field, polarized light, fluorescence, methylene blue staining, and safranin O staining (Fig. 6) leads to the conclusion that Samples Nos. 4, 5, and 13 suffered moderate degradation .

Chemical Degradation Analysis of Chinese Larch Wooden Components

Degradation characteristics of Chinese larch wooden components under bright-field light, polarized light and fluorescence microscopy are presented respectively in Fig. 7 a-c. Under bright-field illumination (Fig. 7 a), a small portion of latewood cell walls in Samples Nos. 6 and 7 were fractured, while their vessel cell walls remained intact. Both latewood and vessel cell walls in Sample No. 9 remained intact. Under polarized light (Fig. 7 b), the BBCC of Samples Nos. 6, 7, and 9 were distinct, with a reduction of 35% to 50% compared with the control wood, indicating a certain degree of reduction in crystalline cellulose content. Among them, Sample No. 9 exhibits higher polarized light intensity than Samples Nos. 6 and 7. Under green fluorescence (Fig. 7 c), the GFBL of Samples Nos. 6, 7, and 9 were relatively distinct, with a reduction of 30% to 40% compared with the control wood, indicating partial degradation of the aromatic ring structure of lignin.

Under methylene blue staining (Fig. 7 d), Samples Nos. 6 and 7 exhibit stronger blue staining intensity in cell walls than Sample No. 9, indicating extensive degradation of crystalline cellulose into amorphous cellulose in these samples. This is consistent with the polarized light observations. Under safranin O staining (Fig. 7 e), Samples Nos. 6, 7 and 9 show relatively pronounced red staining in cell walls, which is broadly consistent with the fluorescence observations.

Integrated analysis of the microstructures of Chinese larch wooden components under bright-field illumination, polarized light, fluorescence, and histochemical staining (Fig. 7) reveals that both cellulose and lignin in Samples Nos. 6, 7 and 9 undergo moderate degradation. It can be concluded that Samples Nos. 6, 7 and 9 are moderately degraded (Table S1).