Comparative investigation of chemical and structural properties of charred fir wood samples by Raman and FTIR spectroscopy as well as X-ray-micro-CT technology

1 Introduction

The demand for renewable and recyclable materials has been heightened by the increasing focus on sustainable building practices. Wood is an excellent construction material for its sustainability, non-toxicity, relatively low cost, local availability and environmental friendliness for building components. Selecting wood products can contribute greatly to mitigating climate change. The prevalence of architectural designs that prioritise environmental sustainability is increasing, and researchers such as Čermak et al. (2019), Šeda et al. (2021), Machová et al. (2021), Kymäläinen et al. (2017, 2018, 2020, 2022, Ebner et al. (2021, 2022 have investigated the traditional technique of charring wood to improve its durability and its use in exterior applications. Consequently, adopting environmentally friendly techniques like surface charring shows to be highly advantageous for protection of wood for exterior, enabling the attainment of cost efficiency, contemporary design, ecological sustainability and cascading use.

Traditionally, the Yakisugi method involves tying three wooden boards together to form a triangular chimney (Ebner et al. 2022). As a starting energy, sawdust is wrapped in newspaper and set on fire. Due to the pulling effect of the chimney, the fire spreads quickly from the bottom to the top and the temperature rises rapidly (Ebner et al. 2021). Also, in addition to the Japanese Yakisugi process, other methods as for example: gas flame, an electric muffle furnace or a hot plate are employed to char wood surfaces (Kymäläinen et al. 2017, 2020). However, various processes take advantage of the principle that the cell wall components of wood change chemically at elevated temperatures (above 150 °C) (Sandermann and Augustin 1963). Based on the study by Runkel (1951), Fengel (1966) as well as Fengel and Przyklenk (1970), the effects of high temperatures on wood and its components were systematically investigated. Levoglucosan is considered the primary cleavage product in the thermal decomposition of cellulose (Sandermann and Augustin 1964). The degradation mechanisms of the polyoses are similar to those of the thermal degradation of cellulose (Sandermann and Augustin 1963). However, the polyoses exhibit lower thermal resistance compared to cellulose. The degradation reactions of lignin are based on cleavage of the lignin-polysaccharide bond and depolymerization of the molecules (Fengel and Przyklenk 1970). These effects can be observed in the region of the α- and β-aryl-ether bonds.

Shafizadeh (1984) divided the effects of temperature on wood into 2 phases, defining the threshold temperature of 300 °C. Below 300 °C, the displacement of water, the reduction of the degree of polymerization within the wood compounds, the formation of free radicals and carbonyl, carboxyl and hydroperoxide groups dominate, releasing carbon monoxide and carbon dioxide. Above a temperature of 300 °C, the decomposition mechanism includes the cleavage of the molecules, which yields sugar degradation products (e.g. furfural and levoglucosan) and volatile low molecular weight degradation products. Above 450 °C, volatile products no longer exist, and further bulk decomposition is due to the decomposition of the remaining char.

The major benefit of charring wood surfaces is generally linked to the reduced water absorption because charcoal is much more hydrophobic than untreated wood. However, less intense treatments leave amorphous polysaccharides intact, which interact with water. In contrast, excessive temperatures or time contributes to the formation of cracks on wood surfaces, which leads to increased water sorption (Kymäläinen et al. 2017).

Currently the focus of research primarily lies in the physical changes and the impact of treatment parameters on the moisture content of the wood or its water absorption (Čermák et al. 2019; Kymäläinen et al. 2017). Nevertheless, the chemical and structural properties of the charred wood surfaces remain unknown. The production process according to traditional Japanese methods is carried out manually, requiring experienced workers to perform the tasks (Ebner et al. 2021). On the other hand, the alternative method of achieving a gas-charred surface, more readily to industrial-scale applications, typically involves the use of a permanently installed machine. In this process, the boards are passed over a gas flame, and the charred layer thickness increases as the speed decreases. However, due to the high gas consumption involved in this process, the carbon layer is usually limited. On the central European market both variants are available and have a similar purchase price. The Yakisugi variant is labour intensive while the alternative requires high gas consumption. Therefore, in this study, the influence of two different charring processes on chemical compositions and structural differences is investigated by various complementary methods. The study employs advanced technologies such as UV Resonance Raman (UVRR), ATR-FTIR, micro-ATR and X-ray micro-CT to delve into the chemical composition and structural characteristics of the surface resulting from the combustion process. The primary objective is to ascertain whether a thicker layer of charcoal produced through the traditional Yakisugi method exhibits any notable distinctions compared to a thinner layer generated through an alternative, more industrial ready approach.

2 Materials and methods

3 Results and discussion

The different charring processes were carried out successfully for the fir samples. Figure 1 shows a comparison between the end-grain cutting of samples after the Yakisugi (Yak) and an alternative (Alt) process. On the visual inspection, the charring wood surfaces presented different appearances depending on the different method used, and it was clear that the charred wood layer from the Yak process is thicker compared to the alternative method (Alt).

Figure 1:

Visual inspection of treatment results on the end-grain cutting of fir boards: (a) one example of charred sample with the Yak method, (b) a sample after the Alt process.

Based on the geometric shape of the produced wood chimney of the Yakisugi method, the charred layer becomes always thicker from the edges of the boards to the centre position. The average of the charred layer was 3.0 mm with a standard deviation of 0.98 mm, whereas the coal layer from the alternative process showed an average of 0.4 mm with a standard deviation of 0.15 mm, and presented a more homogenic surface layer of the coal.

This is an interesting observation as the depth of the treatment has a great influence on the wood compositions and its structures and affects certain properties such as water absorption, cracking under weathering and thermal insulation (Sandberg et al. 2021). Hence, choosing the right treatment techniques could be a useful tool to fine-tune the properties of wood.

The results of the Fourier transform infrared (FTIR) spectroscopy showed that chemical changes can be observed due to the different charring processes (Figure 2). Moreover, the wavenumber, the material, the assigned functional groups and its corresponding reference are shown in Table 1 for the most important IR bands. The FTIR spectra obtained from the surfaces of the samples reveal characteristic alterations associated with fire or thermally treated wood. The pyrolysis layers of wood can be characterised by the functional groups of aromatic or aliphatic structures (Shafizadeh 1984).

Figure 2:

ATR FTIR spectra of untreated sample (fir (ref)) and two charred fir samples after using Yakisugi (fir (Yak)) and alternative method (fir (Alt)).

However, the impact of these two processes is quite similar. The range between 1700 cm⁻¹ and 800 cm⁻¹ displays significant changes in the bands related to carbohydrates and lignin. A new band around 1700 cm⁻¹ (C=O in phenolic esters) can be observed in the FTIR spectra (Bahng et al. 2011). The peak intensity of the absorbance bands at 1590 and 1270 cm⁻¹ of the charred wood samples were attributed to C=O and C–O groups, respectively (Chen et al. 2016). The signal at 1510 cm⁻¹ in the reference samples is no longer evident in both treated samples, which belongs to the stretching vibrations of the lignin aromatic ring (Faix 1991; Schnabel and Huber 2014). Instead, a distinct shift and broadening in the spectra shape can be observed between 1500 cm⁻¹ and 1300 cm⁻¹, indicating changes in the lignin structure associated with phenolic ring and phenolic OH groups.

Additionally, the modified samples exhibit overlapping and disappearance of signals at 1150 cm⁻¹ and 950 cm⁻¹, which correspond to strong signals in the reference samples and are related to C–C and C–O signals in carbohydrate components such as cellulose and hemicellulose (Kymäläinen et al. 2018). This observation supports the notion that during pyrolysis, carbohydrates undergo degradation.

Micro-attenuated total reflection (micro-ATR) analysis was conducted to investigate the spectral changes from the uncharred region to the charred layer, aiming to understand their progression. Figure 3 provides a comparison of the specimens treated with Yak and Alt method. In Figure 3a–c, the samples and the measured points are depicted, accompanied by a colour scale representing the integration value of the carbohydrate and cellulose composition within the range of 1175–917 cm⁻¹. Representative spectra from different acquisition points are shown in Figure 3b and c. The spectra acquired from the uncharred area are displayed in green, the mixed area in blue, and the charred area in red for each sample. For the samples obtained by the alternative method, where the charred layer is relatively thin, the progression could be analysed to a limited extent.

Figure 3:

Micro-ATR analysis on specimens treated with alternative methods (a, b); specimens treated with Yakisugi method (c, d). Colour scale represents the intensity of the integration of 1175–917 cm⁻¹ spectral region in arbitrary units. In coloured square (a, c): red, charred layer; green, untreated wood; blue, intermediate layer.

However, it is important to note that in both samples, the characteristic spectra of fir, indicative of carbohydrates and cellulose, are almost absent due to their degradation during the charring processes.

In Figure 4a and b the UVRR spectra recorded at the cross section for each sample are shown. Going from wood to char, a decrease in the band at ∼1600 cm⁻¹ is apparent. This band in wood spectra relates to the vibrations of the lignin aromatic rings, which have a strong resonance in the UVRRS (Elmay et al. 2015; Pandey and Vuorinen 2008). The redshift of 15 cm⁻¹ and 11 cm⁻¹ for Alt and Yak samples, respectively, can be linked to thermal treatments inducing depolymerization and condensation processes which result in poly aromatic structures in chars (Nieto-Delgado et al. 2021).

Figure 4:

UVRR spectra recorded at the cross section for (a) Alt and (b) Yak samples; (c) and (d) the Gaussian deconvolution highlighting the components for charred woods spectra; (e) G and D band peak positions; (f) G and D bands FWHM; and (g) I_D/I_G values for Alt and Yak samples.

With pyrolysis, an overlapping of the two spectral components associated with hydrogen- and oxygen-rich amorphous carbon structures in chars can be observed (Guizani et al. 2017). As shown in the deconvoluted spectra in Figure 4c and d, both charred woods exhibit common features of carbons, commonly referred to as G (∼1600 cm⁻¹) and D (∼1400 cm⁻¹) peaks (Bormett et al. 1995; Schüpfer et al. 2021; Wang et al. 1990). The G peak derives from bond stretching of sp² atoms in aromatic rings and chains, while the D peak comes from breathing modes of sp² atoms in aromatic rings (Keown et al. 2007). UV radiation could excite a third peak called T around 1000 cm⁻¹ in chars (Ferrari and Robertson 2001), due to the C–C sp³ vibrations. Therefore, the absence of the T peak and the presence of the G and D peak in the UVRR spectra of both analysed samples suggest a high content of sp² hybridization of carbon atoms. Figure 4e–g present a summary of the spectral characteristics of the D and G bands, which enable a qualitative assessment of the charred wood. The G-band of the Yak sample appears slightly upshifted (∼1600 cm⁻¹) and narrow compared to the alternative process sample (∼1596 cm⁻¹). This suggests a first difference between the two chars related to the formation of larger, less distorted ring systems in the Yak sample (Ferrari and Robertson 2000). Hence, the D band is more downshifted and broader in the Yak than in the Alt sample (Figure 5e and f). This observation indicates the presence of more aromatic clusters, suggesting a higher temperature during the pyrolysis process (Smith et al. 2016). Finally, Figure 4g shows the intensity (band area) ratios between the D and G bands for both chars. The increased value for the Yak sample confirms the consuming and re-organization of more amorphous structures as the wood has been possibly exposed to higher temperatures (McDonald-Wharry et al. 2013).

Figure 5:

X-ray micro-CT slices (from 18 µm MinIP) virtually made from wood treated with two charring methods: (a) char layer of Yak; (b) char layer of Alt; (c) whole section with untreated and treated wood with Yak; (d) char layer of Yak; (e) untreated wood; (f) whole section with untreated and treated wood with Alt; (g) char layer of Alt; (h) untreated wood. The white scale bar corresponds to 1 mm.

To understand how the two charring processes modify the internal structure of wood, X-ray computed micro-tomography was performed on both samples. To increase the contrast of low-density elements, minimum intensity projections (MinIP) were applied on 3 consecutive slices, each 6 μm thick, obtaining the selected section (total thickness 18 μm) reported in Figure 5.

Analysing the wood anatomy in the uncharred part in Figure 5, early wood is characterized by large cells and thin walls, while late wood exhibits a dense structure with small, compact cells. The higher cellular density of late wood imparts a brighter appearance in X-ray micro-CT slices.

In general, both charring processes result in cell wall rupture. However, the sample subjected to the alternative method exhibits the complete collapse of the internal structure of the wood. On the contrary, the Yakisugi process (Yak) provokes two outcomes: the development of substantial cavities in late wood and elongated voids in early wood (Figure 5a).

From Figure 5a and b the charred part of sample Alt is visibly reduced (in thickness and volume) compared to that of the sample Yak. On the other hand, the structure of sample treated with Yak method appears intact and less damaged, than sample treated with Alt method in terms preservation and visibility of wood macrostructure. In addition, the charred layer of Yakisugi method is significantly higher than the after alternative process.

The enhanced structural integrity observed in the Yakisugi method may be attributed to two key factors: the deeper penetration of heat facilitated by the chimney-like constructions, and the restricted availability of oxygen during the burning process. In contrast, when using a gas burner on a single surface, an unlimited supply of oxygen is accessible for combustion (Kymäläinen et al. 2017). This finding suggests that not only flame temperature and surface temperature play a crucial role, but also the specific temperature reaching the core of the board. This temperature distribution effectively maintains the macrostructure of the wood. However, it also promotes the degradation of polysaccharides and hemicellulose within the wood matrix, which are actively involved in water exchange reactions, while simultaneously preserving the wood skeleton. Consequently, it is imperative to carefully regulate and maintain the desired temperature while limiting the amount of oxygen present. This controlled approach promotes the thermal degradation of wood components instead of complete combustion and extensive degradation of the wood structure.

4 Conclusions

Various analytical methods were employed to explore and compare the chemical structure and structural characterization differences resulting from two potential methods of surface charring on fir wood and the differences between the traditional handcrafted technique and a modern method using a gas burner were evaluated.

From the charred samples, using ATR-FTIR analysis it was possible to observe three spectral contributions which most probably derived from the residual lignin. The spectral range of 1700 cm⁻¹ to 800 cm⁻¹ shows significant changes in carbohydrate and lignin bands in comparison to the reference samples. The modified samples also show overlapping and disappearing signals at 1150 cm⁻¹ and 950 cm⁻¹, suggesting the degradation of carbohydrates during pyrolysis. Overall, the FTIR spectra of the two charred techniques showed minimal differences.

The UVRR study has helped to pin point chemical differences in the samples treated by the two distinctive methods. In particular, the possible lower temperature of the alternative production method may result in more disordered carbonaceous materials than those obtained by the Yakisugi method. The Yak samples demonstrate a higher presence of aromatic clusters, supporting the findings of the X-ray micro-CT study that the chimney allows for higher temperatures to be obtained within the board.

The investigation with X-ray micro-CT helped to understand how the two charring processes affect the morphological structure of the wood. The Yakisugi method enhanced structural integrity. The higher thickness may be attributed to deeper heat penetration facilitated by chimney-like constructions and restricted oxygen availability during burning, which the alternative method lacks. This controlled temperature distribution helps to maintain wood macrostructure and promotes thermal degradation of wood components while limiting combustion and extensive structure degradation. Although the Yak method shows higher thickness overall, the Alt method achieves a greater level of homogeneity throughout the sample.

The observed variations in the layers of charcoal strongly depend on the employed treatment process. Future investigations are necessary to explore the weather protection properties associated with the alternative method. Additionally, investigating the temperatures reached during the carbonization process and oxygen access may be crucial to determine their impact on the resulting carbonaceous materials and their suitability for practical applications.