Thermal Modification of Wood—A Review

Abstract

The thermal modification of wood has emerged as a sustainable and effective method for enhancing the physical, chemical, and mechanical properties of wood without the use of harmful chemicals. This review summarizes the current state-of-the-art in thermal wood modification, focusing on the mechanisms of wood degradation during treatment and the resulting changes in the properties of the material. The benefits of thermal modification of wood include improved dimensional stability, increased resistance to biological decay, and improved durability, while potential risks such as reduced mechanical strength, color change, and higher costs of wood under certain conditions are also discussed. The review highlights recent advances in process optimization and evaluates the trade-offs between improved performance and possible structural drawbacks. Finally, future perspectives are outlined for sustainable applications of thermally modified wood in various industries. Emerging trends and future research directions in the field are identified, aiming to improve the performance and sustainability of thermally modified wood products in construction, furniture, and other industries.

1. Introduction

In general, wood is considered a natural and renewable resource, which is readily available in large quantities, is non-toxic, and is also a cheap material obtained from biomass [1]. Regarding the chemical composition of wood, it is a natural composite composed mainly of three components (polymers): cellulose, hemicelluloses, and lignin. In addition to these, wood also contains small amounts of extractives. The chemical reactivity of woody biomass is directly influenced by the chemical structure of these components [2].

Based on its inherent properties, wood has been used since ancient times, with a certain type of wood or part of the tree being used to achieve the most favorable properties. The use of wood is limited due to its natural character, since wood comes from a variety of individual trees, and the material needs to be transformed or modified to be applied and to obtain the desired functionality, so the main objectives of wood modification depend on the end use of the material [1,2].

To overcome the unstable and unsatisfactory properties of wood biomass, various modification methods are used that affect the wood material and its weaknesses, which are mainly related to low dimensional stability, poor resistance to hardness and wear, high sensitivity to moisture, susceptibility to biological damage from fungi, termites, and the low resistance to UV radiation [1,2]. The process of wood modification is currently mainly used to improve the physical, mechanical, or aesthetic properties of wood and wood biomass. This process creates a material that can be removed at the end of its useful life without posing an environmental risk similar to that associated with the removal and disposal of unmodified wood [1].

The wood modification process and the associated wood modification industry are currently undergoing significant development due to increasing environmental concerns. The wood modification process essentially involves the use and application of various chemical, mechanical, physical, or biological methods to change the properties of a given wood material. In general, four main types of processes can be used to modify woody biomass:

(a)

chemical modification (active and passive modification);

(b)

thermally based processes (thermo treatment, thermo–hydrotreatment, thermo–mechanical treatment, thermo–hydro–mechanical treatment);

(c)

physical modification (electromagnetic radiation, plasma, and laser);

(d)

other processes (biological treatment, mineralization, novel media, biomimetics, etc.) [1,2].

Several fairly new technologies, such as thermal modification, acetylation, furfurylation, and different impregnation processes, have been successfully introduced to the market and demonstrate the potential of these modern technologies. In the field of wood preservation processes, thermal modification is an environmentally friendly process due to the absence of any chemical additives. Growing environmental concerns have led the scientific community to increase its interest in alternative wood modification processes (chemical and thermal modification) across the entire wood preservation sector [1,2,3].

Thermal Wood Modification in Scientific Publications

In the past decade, the number of scientific papers and documents covering the issue of thermal wood modification has increased. Figure 1 summarizes the number of scientific articles in the Scopus^® database of peer-reviewed literature using the keyword “thermal modification of wood”. Figure 1 shows that the number of publications published in this area from 2020 to 2025 was approximately 55. Compared to 2020, when only 5 papers were published related to thermal wood modification, this number increased to 15. The increased number of papers points to the development of thermal wood modification in general. Regarding the distribution of publications by document type, the largest representation of documents on “thermal modification of wood” falls under the category of articles, followed by conference papers, review papers, and other document types.

In the given search time frame specified (2020–2025), there were approximately 50 articles that contained the keyword “thermal modification of wood”. In contrast, keywords such as “wood products”, “heat treatment”, and “thermally modified wood” resulted in 10–20 articles for each of the keywords used.

The Scopus^® database applied to our analysis shows that the wood modification industry—specifically thermal wood modification, enjoys immense interest from the scientific community, with a large part of the interest driven by environmental concerns as well as the use of woody biomass in emerging and new applications. In this regard, it is reasonable that environmental and human safety assessments be applied to the development of new and unknown treatments, products, and materials based on woody biomass.

The purpose of this review is to summarize the thermal modification processes of woody biomass and, at the same time, present the dynamics of research interest in these wood modification processes along with an overview of documents and works related to this issue in the period 2020–2025. As part of this review, we also focused on the current progress achieved in the thermal modification of wood and the prospects for the use of thermally modified wood in the future.

2. Thermal Modification of Wood—Changes in the Chemical Structure

The most widely used method for wood modification is currently thermal treatment. The wood modification process has been described as a technique that improves certain properties of wood while posing no environmental risks. During the thermal treatment of woody biomass, the wood is heated to temperatures ranging from 160 to 240 °C, and depending on the temperature used, there can be major or minor changes in the wood components [1,2,3]. Hill et al. [3] stated in their work that thermal modification of wood occurs in the range from 150 °C to 240 °C, with the area below 150 °C defined as heat treatment or aging of wood, while the area above 240 °C is referred to as thermal degradation of wood, even though certain degradation processes occur across the entire temperature range. Candelier et al. [4] stated in their work that the thermal treatment process of wood involves low-temperature pyrolysis, which is applied in the temperature range of 180 to 250 °C in an atmosphere with a very low oxygen content. This process changes the physical properties of wood as well as its chemical properties, giving it new and, in some cases, enhanced characteristics [3,4]. Commercial thermal wood modification processes can use various media, such as superheated steam, nitrogen, oil, hydro thermolysis in an aqueous environment under pressure, or a vacuum as a medium [2,3]. The basic characteristics of the various thermal wood modification processes are shown in Figure 2 [1,2,3,4,5].

The types of thermal wood modification processes vary depending on the processing temperature and time, processing atmosphere, system (open, closed, wet, and dry), and type of wood (hardwood, softwood). Regarding the processing atmosphere, wood can be heated under a layer of inert gas, in a vacuum, under superheated steam, or in ambient air and oil-based environments. The thermal modification of wood under wet and dry conditions affects the cell wall, which retains moisture in wet conditions, while dry conditions mean the wood has been dried to almost zero moisture content. Hardwoods are more susceptible to thermodegradation than softwoods. Overall, the presence of either water or water vapor in the thermal modification of wood affects its chemical composition and heat transfer properties. Concerning the classification of systems as open or closed, in this context, a closed process represents the formation of a pressure system by the accumulation of degradation products and steam. Conversely, in an open system, a continuous flow allows degradation products and exhaust gases to exit into the atmosphere [3,4,5].

The technology used in these processes differs based on system type (open or closed), inert atmosphere (method of minimizing oxygen content in the thermal chamber), heat transfer method (conduction or convection), humidity and pressure conditions, and processing intensity (temperature–duration). The specific parameters used have a fundamental impact on performance, particularly on energy consumption, volatile compound formation, and their recovery [1,2,3,4].

The main reason for thermal modification of wood is to improve its properties, especially its durability and stability. As a building material, wood has several advantages, such as a high strength-to-weight ratio, effective thermal insulation, easy processing, and relative sustainability. However, it can also be vulnerable to biological attack, and its disadvantages include low dimensional stability and poor wear resistance. Natural wood is susceptible to damage from fungi, rot, and insects while also absorbing moisture, leading to deformation processes such as shrinkage, warping, or swelling [5,6].

As a widely studied and successful industrialized method, thermal modification is mainly used to improve the dimensional stability of solid wood or wood-based materials. During thermal modification, wood undergoes chemical transformations, whereby the structure of cell wall polymers is modified and the wood or wood-based material acquires new properties [3]. In addition, thermal modification improves the aesthetic and decorative appeal of wood by darkening its color [6].

Hemicelluloses, due to their amorphous structures, degrade first at relatively low temperatures (approximately 160 °C), and also exhibit a lower degree of polymerization and higher reactivity. The degradation of hemicelluloses produces volatile compounds such as water, formic acid, acetic acid, and furfural [2,3,4,5]. One negative effect during the thermal treatment of wood is the presence of oxygen, which accelerates the degradation processes in wood and therefore reduces the properties of the final material [2]. Amorphous cellulose also degrades at low temperatures similarly to hemicelluloses, but crystalline cellulose is less sensitive, and the ratio of amorphous to crystalline cellulose in wood changes with increasing processing temperature. Additionally, crystalline cellulose is only weakly affected by temperatures below 300 °C [2,3,4,5,6]. Regarding extractives, several authors have reported that these compounds evaporate during the first phase of thermal treatment, whereas new extractives formed through thermal degradation begin to appear from 160 °C onward. With increasing temperature and time, extractives may further degrade, resulting in by-products that evaporate or remain present in the wood. The authors also found that between temperatures of 100 and 200 °C, extractives migrated to the wood surface [2,7,8]. Regarding changes in the structure of lignin, due to the cleavage of C-O bonds in the side chain (especially β-O-4 ether bonds), lignin depolymerization occurs during heat treatment of wood. However, differences exist between lignins isolated from hardwood and softwood due to the proportion of syringyl units. Syringyl units provide the isolated lignin in hardwood with higher thermal stability. It is believed that thermal degradation of lignin begins at 120 °C, and the loss of aromatic C=O has been observed at temperatures of 160 °C and higher [6,9,10]. The lignin present in wood after thermal treatment of woody biomass has a higher syringyl/guaiacyl ratio (found in hardwood), higher molecular weight, fewer β-O-4 bonds, an increase in condensed C-C bonds, and a decrease in alcohol -OH groups. Due to the reduction of polysaccharide content and condensation reactions between lignin and the resulting degradation products, the lignin content is increased after the thermal modification of wood [9,10].

Kubovský et al., 2020 [11] focused their work on the investigation of structural changes in the main components of oak wood during thermal modification. Fourier transform infrared spectroscopy (FTIR) and size exclusion chromatography (SEC) were used to analyze chemical changes during thermal modification. The material used in this work consisted ofoak wood samples (Quercus robur L.) with dimensions of approximately 200 × 100 × 20 mm, and these samples were conditioned under the following conditions: relative humidity of approximately 65% and temperature of approximately 20 °C (up to an equilibrium moisture content of 12%). Thermal modification was carried out at atmospheric pressure at temperatures of 160, 180, and 210 °C for 2 to 3 h, with the final phase involving wood cooling until reaching a moisture content of 4 to 7%. All analyzed samples were disintegrated, ground, dried, and extracted. Subsequently, the cellulose, holocellulose, and isolated lignin samples were analyzed by FTIR (

Table 1. Changes in FTIR spectra of isolated lignin from heat-treated oak wood samples compared to untreated samples [11].

Wavenumber (cm−1)	Changes	Result and Consequence
3420	slight decrease in the amount of -OH groups	condensation reactions hydrolysis of acetyl groups from hemicelluloses
2940–2840	slight decreasing of band intensities	changes at the cellulose crystallinity level
1723	increase in absorbance band shift to lower wavenumbers	increase in carbonyl and carboxyl groups cleavage of β-O-4 linkages
1600–1500	slight increase and changes	lignin condensation carboxylation of polysaccharides
1420–1460	slight decrease	degradation of lignin cleavage of methoxyl groups
1219–1267	initial increase decrease slightly in all samples	C-O stretching of guaiacyl and syringyl ring
1190–950	slight decrease or permanent decrease	degradation of hydroxyl groups demethoxylation of lignin crosslinking

) and SEC (

Table 2. SEC results of the fraction of oak wood lignin and polysaccharides before and after heat treatment [11].

Sample	Temperature (°C)	Polydisperzity Index (PDI)	Degree of Polymerization
Lignin fraction	20	2.65	-
	160	2.84
	180	2.80
	210	2.79
Holocellulose fraction	20	4.17	1315
	160	4.20	1151
	180	5.33	905
	210	9.48	1700
Cellulose fraction	20	9.81	2382
	160	10.80	2376
	180	11.80	2197
	210	9.14	1894

).

Regarding the isolated holocellulose fraction from the thermally modified samples, the FTIR spectrum reveals prominent bands at 2897 cm⁻¹ and 3338 cm⁻¹, which increase with thermal modification, due to the oxidation and hydrolysis of certain groups from hemicelluloses. A constant decrease in signal intensity (band at 1732 cm⁻¹) was observed in all analyzed samples with increasing processing temperature, indicating the cleavage of acetyl chains in hemicelluloses. The band at 1633 cm⁻¹ showed a continuous but slight decrease as temperature rose. This trend may result from the cleavage of α–alkyl–aryl ether bonds. The authors observed that the absorption band at 1371 cm⁻¹ slightly increased in all samples (the most significant increase in the sample treated at 210 °C). The band at 1241 cm⁻¹ indicated a continuous decrease, confirming the existence of more condensed structures. The bands at 1030 cm⁻¹ and 897 cm⁻¹ gradually intensified with increasing treatment temperature, confirming that gradual changes in the cellulose structure occurred during thermal modification [11].

The results of FTIR analysis show that during the thermal treatment of wood, specifically oak wood in this case, thermal modification leads to an increase in the number of carbonyl groups in lignin, while the cleavage of aliphatic chains and demethoxylation occur. Higher wood processing temperatures lead to condensation reactions of lignin and an increase in its molecular weight. The degradation of hemicelluloses was more pronounced at 180 °C, while there was a decrease in molecular weight and an increase in polydispersity [11].

In a study by Lourenço et al., 2020 [12], structural changes in lignins of various wood species, specifically eucalyptus, were characterized. All wood species were subjected to a mild thermal modification, which took place at a temperature of 160 °C, followed by an increase in the modification temperature to 230 °C for 3 h. The changes in lignin structure were analyzed using different methods such as nuclear magnetic resonance (¹³C NMR, 1D and 2D NMR), SEC, and analytical pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC/MS). Eight species of eucalyptus wood were analyzed, cut into boards with dimensions of 60 × 7.5 × 2 cm. Thermal modification was carried out in an open reactor with gradual heating. At the end of the process, the boards were cooled in air and stored at a constant relative humidity of 60%. Afterward, the treated boards were crushed, ground, and subjected to further analyses. Chemical analysis of thermally treated and untreated wood showed a weight loss of approximately 10 to 12% (mainly due to the release of hemicelluloses). Cellulose was found to be relatively thermally stable; however, the lignin content in all wood species was significantly reduced, and there was an increase in extractives, which can be attributed to products formed during the degradation processes of hemicelluloses and lignin [12]. NMR analysis revealed that thermal modification led to partial demethylation of lignin and its condensation, while the amount of aryl ether bonds was significantly reduced (cleavage of β-O-4 and α-O-4 bonds) (

Table 3. Chemical and structural analysis of lignins (E. globulus and E. propinqua) isolated from untreated and thermally modified wood, as revealed by Py-GC-MS [12].

Sample	Total Lignin (%)	Syringyl Units (S) Guaiacyl Units (G) p-Hydroxyphenyl Units (H)	Total Carbohydrates (%)	S/G Ratio H:G:S
Lignin fraction (E. globulus; untreated)	91.5	72.4 16.1 0.5	1.5	4.5 1:18:81
Lignin fraction (E. globulus; thermally treated)	92.7	69.3 17.8 2.3	2.0	3.9 2:20:78
Lignin fraction (E. propinqua; untreated)	90.4	47.2 32.7 4.9	1.2	1.4 6:39:55
Lignin fraction (E. propinqua; thermally treated)	94.6	52.9 32.5 4.4	2.5	1.6 5:37:58

).

The results show that lignin undergoes demethylation during heat treatment with cleavage of aryl ether bonds, leading to its depolymerization. However, the decrease in molecular weight was relatively moderate. The increased condensation and polyphenol content may enhance resistance to microorganisms and hydrophobicity.

Kačíková et al., 2020 [13] analyzed the changes in the structure of lignins from teak and iroko wood resulting from thermal modification. Lignin is one of the main components of wood. Structural changes significantly affect the properties of the final products and materials derived from wood, as well as the recycling of thermally treated wood. Two types of tropical wood species were used as input material, as mentioned above. The wood samples were prepared at dimensions of 20 × 20 × 300 mm and modified according to the ThermoWood process outlined in

Table 4. Conditions of thermal modification and results of structural changes in wood lignins [13].

Sample	Conditions	KL	DL	S/G Ratio	PDI
Teak (Tectona grandis)	160, 180, and 210 °C open system chamber with steam, 3 h	35.4 *	8.6 *	0.9 *	3.7 *
		39.3	11.4	0.9	3.5
		39.5	15.3	0.8	3.9
		40.5	18.3	0.6	4.0
Iroko (Milicia excelsa)	160, 180, and 210 °C open system chamber with steam, 3 h	29.0 *	4.1 *	0.8 *	2.0 *
		29.0	4.5	1.0	2.1
		29.9	4.7	1.0	2.0
		36.9	7.2	0.9	2.1

* control samples.

. After 3 h, the chamber temperature was reduced, and the samples were moistened to achieve a moisture content of 5 to 7%. Thermally modified samples and untreated samples were ground and extracted. Subsequently, lignin (Klason lignin; KL) was isolated using acidolysis and dioxane (DL), followed by alkaline nitrobenzene oxidation. The isolated lignins were then analyzed by FTIR and SEC (Table 4).

FTIR analysis revealed that several degradation and condensation reactions were observed during the heat modification of wood. During the heat treatment, the cleavage of methoxyl groups and side chains, as well as the cleavage of β-O-4 ether bond, and oxidation were observed. Changes in the S/G ratio and cleavage of alkyl-aryl bonds differed between the two types of wood.

Some authors have focused on the effect of heat treatment on chemical structure changes in different types of wood [14,15,16]. The conditions of heat treatment for various wood types, along with the structural changes before and after thermal modification are given in

Table 5. Chemical composition of different types of untreated and thermally treated wood [14,15,16].

Sample	Conditions	Extractives (%)	Lignin (%)	Hemicelluloses/Cellulose (%)	Ref.
African padauk	160, 180, 210 °C	11.62 *	33.77 *	25.72/40.50 *	Corleto et al. [14]
	ThermoWood	10.63	34.88	24.50/41.04
	Process,	10.49	35.55	21.73/40.49
	3 h	9.47	39.69	9.77/44.38
Merbau (Intsia spp.)	160, 180, 210 °C	12.30 *	34.10 *	17.10/41.40 *	Ditommaso et al. [15]
	ThermoWood	9.86	33.21	15.80/42.80
	Process,	7.75	35.54	9.90/47.20
	3 h	11.89	44.67	1.70/50.00
Black poplar (Populus nigra L.)	Superheated steam 160, 190, 220 °C, 2 h	1.80 *	24.12 *	29.96/52.15 *	Kozakiewicz et al. [16]
		2.40	23.45	28.52/53.20
		3.90	22.41	22.41/54.42
		6.40	30.76	3.58/60.12

* control samples.

.

In the scientific literature, one can find numerous reviews or articles on thermal modification, thermally modified wood, and last but not least, on the physical or chemical properties of such modified wood.

Table 6. A summary of publications and documents dealing with the thermal modification of wood [17,18,19,20,21,22,23,24,25,26,27,28].

Main Topics Discussed	Ref.
Effect of delignification on thermal degradation reactivities of hemicellulose and cellulose in wood cell walls	Wang et al. [17]
Properties of thermally modified teakwood	Lengowski et al. [18]
Chemical and mechanical characterization of thermally modified Gmelina arborea wood	Minkah et al. [19]
The effect of heat flux to the fire-technical and chemical properties of spruce wood (Picea abies L.)	Zachar et al. [20]
Effect of oxidizing thermal modification on the chemical properties and thermal conductivity of Norway spruce (Picea abies L.) wood	Čabalová et al. [21]
Degradation of chemical components of thermally modified Robinia pseudoacacia L. wood and its effect on the change in mechanical properties	Sikora et al. [22]
Changes in the chemical structure of thermally modified spruce wood due to decaying fungi	Vidholdová et al. [23]
Colour and chemical changes of black locust wood during heat treatment	Kačík et al. [24]
Kinetic studies on photo-degradation of thermally-treated spruce wood during natural weathering: Surface performance, lignin and cellulose crystallinity	Mastouri et al. [25]
Impact of thermal modification on color and chemical changes of African padauk, merbau, mahogany, and iroko wood species	Gaff et al. [26]
Analysis of chemical properties of thermally treated Pinus roxburghii sargent wood.	Gupta et al. [27]
Chemical composition and color of short-rotation teak wood thermally modified in closed and open systems	Gomes et al. [28]

presents the literature on the thermal degradation of the polysaccharide and lignin components of the cell wall, as well as the role of extractives. It summarizes several articles and reviews dealing with the above-mentioned issues and lists their publication dates from the years 2021–2023.

In this study by Liu et al. [29], the authors investigated the chemical structure of both unmodified wood and thermally modified wood in different environments (vacuum, nitrogen, steam and air). For the experiment, wood samples, namely beech wood (Fagus Silvatica L.), were used with dimensions of 20 × 20 × 20 mm, dried to a constant weight (103 °C). The heat treatment processes were carried out in different ovens, with a preheating phase in which the temperature was gradually increased to 103 °C and maintained for 1 h, followed by further heating to either 180 °C or 210 °C (depending on the conditions), with a retention period of 4 h, and then cooling before transferring the samples (100 °C). Beech wood powder was used for chemical composition analysis. The results showed that, in terms of weight loss, the severity of degradation at 210 °C increased in the following order: vacuum (3.51%) < nitrogen (7.16%) < steam (8.88%) < air (13.27%). This indicates that a higher processing temperature led to a greater weight loss. Regarding the chemical composition before and after heat treatment, the results indicated its composition changed more significantly at higher processing temperature, with the hemicellulose content decreasing from 29.3% (untreated sample) to 23.8% in vacuum contidions (210 °C), 19.4% in nitrogen (210 °C), 17.5% in steam (210 °C) and 14.7% in air (210 °C). In contrast, cellulose was chemically more stable at higher temperatures, while the lignin content increased from 27.4% (untreated sample) to 32.0% in vacuum (210 °C), 37.7% in nitrogen (210 °C), 39.8% in steam (210 °C) and 41.8% in air (210 °C). According to the FTIR results, it can be said that the temperature of 180 °C did not have a significant effect on the chemical composition of the wood, but increasing the temperature to 210 °C changed the chemical structure more fundamentally and clearly (

Table 7. The number average molecular weight (M_n), weight average molecular weight (M_w), polydispersity index of lignin (PDI), and index of crystallinity (CrI) of unmodified wood and thermally modified wood [29].

Conditions	Mw (g/mol)	Mn (g/mol)	PDI	CrI (%)
Untreated sample (Fagus Silvatica L.)	3319	2413	1.38	47.7
Vacuum, 210 °C	3769	2262	1.67	~48
Nitrogen, 210 °C	3206	1961	1.64	~48
Steam, 210 °C	3347	1923	1.74	50.1
Air, 210 °C	3968	2095	1.89	51.8

).

Grinins et al. [30] also monitored the chemical composition of wood after heat treatment, which occurred in a closed process under nitrogen pressure. As input samples, silver birch (Betula pendula) and Scots pine (Pinus sylvestris) with dimensions of 1000 × 100 × 25 mm were selected. During the experiment, different conditions were chosen: maximum temperature of 150–180 °C, modification time ranging from 30–180 min, and initial nitrogen pressure of 3–6 bar. The most significant changes in wood (Betula pendula) were observed in the content of acetone extracts, xylan, and acetyl groups, while glucan and lignin showed less pronounced changes. The thermal modification of Scots pine in a nitrogen environment increased the amount of acetone extracts, lignin, glucan, and mannan, but led to a reduction in xylan and acetyl groups. The most significant changes in the chemical structure of silver birch wood were found at 160 and 170 °C. Increasing the initial pressure led to a decrease in the content of xylan and acetyl groups. The most significant changes in the chemical structure of Scots pine wood occurred after treatments at 170 °C and 180 °C. Wood of Scots beech (Fagus Silvatica L.) was thermally modified at three temperatures: 180, 200, and 220 °C in a preheated steam environment (temperature maintenance for 3 h). Weight loss was 1.4 ± 0.1% (180 °C), 2.7 ± 0.2% (200 °C), and 8.6 ± 0.6% (220 °C). The effect of thermal modification was characterized based on FTIR, extract content, and pH changes. The explanation of these changes was additionally supported by FTIR analysis of the main isolated wood components (cellulose, holocellulose, and lignin) from modified wood. The results of these analyses showed that the decrease in the intensity of the bands attributed to C=O in holocellulose is caused by deacetylation and degradation of hemicelluloses. The intensity at 1216 cm⁻¹ may be related to the modification temperature, which suggests the formation of new C–O or C=O bonds through reactions that occur during modification, mainly oxidative reactions. The greatest influence on the intensity at 1736 cm⁻¹ is due to deacetylation and hemicellulose degradation [31].

Spruce wood (Picea abies, Krast) was thermally modified according to the conditions of the ThermoWood process (heat treatment at 160, 180, and 210 °C). During the treatment, the samples were protected by steam. Using chemical analysis of wood before and after thermal modification, the authors found that thermally modified wood generally had higher levels of extractives, apparent lignin, and cellulose, but lower hemicellulose content, suggesting that thermal modification of spruce wood caused a decrease in polysaccharide content, mainly through hemicellulose degradation. The hemicellulose content of thermally modified wood at 210 °C was reduced by 72.39%. FTIR analysis revealed that thermal modification caused a decrease in infrared absorbance belonging to lignin and carbonyl groups [32].

3. Benefits of Thermal Modification of Wood

Thermally modified wood is used in various applications, including construction, wood products, and furniture, due to its environmentally friendly properties and associated sustainability benefits, including enhanced dimensional stability, improved aesthetics, reduced weight, and increased biological resistance [32] (Figure 3). Regarding the biological resistance of wood, thermal modification alters the chemical composition of the wood, making it less vulnerable to insects, rot, and fungi, thus prolonging the lifespan of thermally modified wood [32,33].

In the work by Esteves et al. [33], the biological properties of untreated and thermally treated Cryptomeria japonica wood were investigated. The heat treatment was carried out at a temperature of 212 °C for 2 h. The work included an evaluation of termite durability to determine whether thermal modification of the wood enhances or reduces the resistance of the wood to termites. The results of this evaluation showed that both treated and untreated samples exhibit a high susceptibility to subterranean termites; however, over time, a higher mortality rate was observed in the treated wood. According to the literature, wood heat treatment must be carried out above 215 °C (with weight loss exceeding 20%) to enhance biological resistance to termites [33,34].

In another study, ash and acacia wood were heat-treated and tested under various environmental conditions. The study included an assessment of wood’s biological resistance to termites [35]. The study concluded that all wood samples experienced similar levels of termite attack, with acacia wood exhibiting slightly greater resistance to termites and lower survival rates among wood samples. The conclusions suggest that heat modification did not significantly improve termite resistance, but may have led to the production of substances more toxic to termites (that heat modification could produce substances more toxic to termites). The research confirmed that termites primarily degrade cellulose (as evidenced by reduction of levoglucosan and the S/G ratio in lignin), but cellulose degradation was lower in modified wood.

As repeatedly emphasized by FTIR analysis, thermal modification of wood significantly affects its properties, especially in the hemicellulose-rich region (1730–1740 cm⁻¹) due to the fact that hemicelluloses are the least thermally stable component of wood and undergo significant degradation during thermal modification. Within the spectral regions, attention should be paid to the zones related to the C=O stretching in hemicelluloses and aromatic skeleton vibrations in lignin. These regions mainly characterize the reactions between degradation processes and structural changes in wood during thermal modification. An increase in aromaticity and a reduction in polar groups in FTIR spectra contribute to moisture absorption and increased resistance of wood to biological agents [36].

Various wood samples (Gmenila arborea) were heat-treated using a process similar to ThermoWood, where the samples were modified at 180, 200, and 220 °C (3 h). In the work by Minkah et al. [37], the effects of thermal modification on the resistance to decay by decaying (wood-destroying) fungi as well as attack by subterranean, arboreal, and drywood termites were determined. The wood samples were cut into boards with dimensions of 20 × 50 × 650 mm (wood moisture content below 20%). A 65 L laboratory reactor was used for heat modification. The results show that the wood boards after thermal modification achieved a weight loss of 5.44%, 10.08% and 15.13% at 180 °C, 200 °C, and 220 °C. Regarding the effectiveness of this modification, it positively influenced the resistance of Gmenila arborea wood to biodeterioration by white rot Trametes versicolor and tree termites (Nasutitermes corniger) as the process temperature increased. However, thermally modified Gmelina arborea wood was highly susceptible to Macrotermes rot under field conditions.

Environmental technologies such as thermal modification can improve the durability and dimensional stability of woody biomass. In this paper by Paes et al. [38], the effect of heat treatment on improving the resistance of two wood species (Corymbia citriodora and Pinus taeda) to white and brown rot fungi was evaluated. Treatment temperatures were set at 160, 180, 200, 220, and 240 °C in an electric furnace under a nitrogen atmosphere. Seven boards measuring 6 × 16 × 56 cm were used for each temperature. The results showed that modification temperatures of 160 and 180 °C reduced the biological resistance of Corymbia citriodora wood. Treatment temperatures of 200, 220, and 240 °C showed a satisfactory increase in the resistance to decay for both species. Rhodonia placenta was the most degrading fungus at temperatures below 200 °C.

The moisture content of wood and its dimensional stability are other properties that have an impact on the intended use of wood. Since wood is generally classified as a hygroscopic material, this means that due to the absorption of water, wood can change its dimensions and its dimensional stability changes with changes in moisture content [36,39].

Table 8. Measurement of the dimensional stability of untreated and thermally modified wood [36,40,41,42,43].

Wood Sample	Conditions	Results	Ref.
Iroko (Milicia excelsa)	ThermoWood process 220 °C, 3 h	swelled more quickly insufficient dimensional stability	[36]
Ayous (Triplochiton scleroxylon)	ThermoWood process 190 °C, 3 h	swelled more slowly (~ 44%) good dimensional stability	[36]
Chinese parasol tree (Firmiana simplex)	tin alloy thermal modification bath 150 and 210 °C 2 and 8 h	significantly improves the dimensional stability of wood (reduction in swelling, water absorption, and density)	[40]
Black poplar (Populus nigra L.)	chamber nitrogen atmosphere 160, 190, and 220° 2 h and 6 h	weight loss, lower density lower equilibrium moisture content lower swelling anisotropy	[41]
Ash wood (Fraxinus exlecsior L.)	a metal autoclave a steam environment under atmospheric pressure 185 °C, 3 h	improvement of absorbency and swelling values eduction in hydrophilicity by 28.9% more dimensionally stable (21.1%), reduced water affinity	[42]
Hornbeam wood	pyrolysis of wood heating 200 °C, 1 to 6 h	reduction of water absorption the environment for development of fungi decreases	[43]

summarizes articles and documents from various authors who have investigated and characterized the dimensional stability of wood before and after thermal modification.

Thermal modification depends on several factors that affect the effectiveness of this process, such as the type of wood, the temperature and heating rate, and the processing atmosphere used. In addition to chemical transformations during heat treatment, this process can improve the aesthetic and decorative appeal of wood. The heat treatment process changes the color of the wood to a darker and richer shade that is attractive to many [26,27,28,29,30]. Gaff et al. [26], in their work, evaluated the color homogeneity, or, in other words, the color change during heat treatment of four tropical wood species that were modified by the Thermowood process. The results showed that with increasing temperature, a decrease in lightness (L*) and a simultaneous decrease in chromaticity values (a*, b*) were observed, which indicates a darkening and browning of the wood surface. A significant correlation was analyzed between the lightness difference value (ΔL*) and the hemicellulose content (r = 0.88–0.96) for all studied wood types. Using FTIR analysis, the authors found that the breakdown of bonds (C=O and C=C) in hemicelluloses and lignin is responsible for the formation of chromophoric structures, which cause the color change in wood.

Borůvka et al. [44], in their publication, investigated the physical properties (thermal and aesthetic) of thermally modified birch wood. The authors used five stages of heat treatment, ranging from 160 °C to 200 °C in increments of 10 °C (max. 3 h). The results show that with an increase in temperature to 200 °C, conductivity decreased by approximately 20%, diffusivity by 6%, volumetric heat capacity by 15% and effusivity by approximately 18%. Similarly, as processing temperature increased, whiteness, total color difference, and gloss decreased (44%, 38.4%, and 18.2%, respectively).

Kačík et al. [32] also reported that increasing the modification temperature of spruce wood results in a decrease in the whiteness, and the wood becomes darker, acquiring a dark brown hue at the highest temperature. The total color difference ΔE also increased several times.

The viscoelastic properties and water vapor sorption of thermally modified poplar wood (Populus tomentosa Carr.) were evaluated based on color changes to obtain information about changes in dimensional stability and hygroscopicity. The heat treatment was carried out at temperatures of 180, 200, and 220 °C for 2, 4, 6, 8, and/or 10 h in an air atmosphere. The results showed that poplar wood darkened, and its L* value decreased from 61.42 to 32.23 as temperature and duration decreased. Furthermore, during the heat treatment, equilibrium moisture content decreased by 37%, hygroscopicity of the wood also decreased, as did water diffusion, meaning that cell wall swelling was limited, leading to a decrease in hydrophilicity and dimensional stability [45].

Herrera-Builes et al. [46] evaluated some physical and mechanical characteristics of Pinus oocarpa wood that was heat-modified at 170 and 190 °C for 2.5 h in saturated steam. The results indicated that during the heat treatment process, a weight loss of 2.4% (170 °C) and 3.3% (190 °C) occurred. The heat treatments used increased the dimensional stability of the treated Pinus oocarpa wood. Similarly, heat modification changed the color parameters of untreated wood, increasing saturation, darkening the color, redness, and yellowing. In summary, it can be said that thermal modification at a temperature of 170 °C improved the wood’s qualities (higher density, durability, darker colors, and better dimensional stability) (Figure 4).

Some authors further stated in their works that thermal modification not only affected the wood’s density but also its thermal conductivity. Čabalová et al. [21] found that thermal modification of spruce wood reduced its density, and consequently, thermal conductivity also decreased. The results of their study showed a significant correlation between hemicellulose degradation and density (as well as thermal conductivity). Similar results were confirmed by other researchers [47,48].

4. Risks of Thermal Modification of Wood

Wood modification by thermal treatment is one of the preferred methods for improving the properties of wood (i.e., dimensional stability, biological resistance, reduced shrinkage and swelling). Nowadays, it is gaining increasing recognition due to its ecological approach to wood modification. The process is non-toxic because no chemicals are used. Thermal modification is a process that does not involve chemicals, but only utilizes heat and steam. This makes it a more environmentally friendly alternative to chemical impregnation [40,41,42,43,44,45]. On the other hand, as with any process, thermal modification of wood can present certain disadvantages or risks that affect the resulting properties of the wood in undesirable ways (Figure 5).

Thermal modification of wood generally affects its properties, e.g., hygroscopicity, dimensional stability, resistance to biological agents, certain mechanical properties and properties, as well as color and odor. As mentioned above, most of these properties, similar to the properties of the initial biomass or raw material, are affected by the intensity of thermal modification (temperature and process duration). In addition to the properties that are desirable for thermally modified wood and advantageous for some applications, thermal processing of wood can also result in undesirable properties (reduced strength, increased brittleness, high cost, characteristic smell, etc.) (Figure 5).

Sosins et al. [48] investigated the effect of thermal modification of aspen wood in a closed process under nitrogen pressure on its mechanical properties. The maximum temperature ranged from 160 to 170 °C for 60 to 180 min with a nitrogen pressure of 4–5 bar. The wood was characterized after thermal modification in terms of modulus of elasticity, modulus of rupture, and Brinell hardness. The result of this analysis showed that the modulus of elasticity decreased by 5 to 31% for all heat-modified samples. On the other hand, the modulus of elasticity values increased after most treatments. Regarding the Brinell hardness analysis, the hardness values decreased with each treatment.

In another work by Nhacil et al. [49], the authors analyzed the effect of thermal treatment on the mechanical properties of wood (Brachystegia spiciformis and Julbernadia globiflora). In this study, heat modification was applied to the wood of both species at three different temperatures (215 °C; 230 °C; 245 °C) for 2 h. For Brachystegia spiciformis wood, the modulus of elasticity decreased by 10.2%, the modulus of elasticity by 50.8%, the compressive strength parallel to the grain by 29.2%, and the Brinell hardness by 23.5%. Julbernadia globiflora wood showed a similar trend in terms of mechanical properties, with a 6.9% decrease in the modulus of elasticity, a 53.2% decrease in the modulus of rupture, and a 21.9% decrease in the compressive strength parallel to the grain.

Bytnerová et al. [50] reported that the mechanical properties of black poplar wood change as a result of heat modification in a nitrogen atmosphere. The authors analyzed the effect of heat modification on the compressive strength parallel to the grain, the modulus of elasticity, and the modulus of elasticity. The result was an increase in compressive strength for all modifications; the largest 16% increase was achieved at a temperature of 160 °C and for 2 h. At the same time, an increase in the modulus of elasticity was found, while a decrease occurred when the temperature was increased to 200 to 220 °C on the other hand, in all modification processes there was a decrease in the modulus of elasticity with an increase in temperature and treatment time.

As mentioned, thermally treated wood has, in addition to advantages, certain disadvantages associated with a decrease in tensile and bending strength, loss of toughness, instability of color when exposed to external influences, and surface cracking. High temperatures can reduce the bending and tensile strength of wood, as well as its impact resistance. Thermally modified wood is more brittle and prone to cracking [49,50,51].

Although the darker color is an advantage for many, it may not be suitable for all applications, and it can fade over time due to UV radiation. Regular application of UV protective coatings is necessary to maintain the color.

Grinins et al. [30] reported that thermally modified wood species (applied with three commercial coatings) exposed to weathering for 3 months showed poor color stability and resistance to fungal growth. The main reason for applying the coating was to preserve the appearance of the wood while protecting it from UV radiation and moisture [31,52].

Horbachova et al. [53] found that the color fastness of thermally modified ash wood samples was higher compared to unmodified wood. There was a decrease in the L coordinate from 68.4 to 33.6, indicating a decrease in lightness, i.e., a darkening of the wood.

As previously mentioned, thermal modification can improve the aesthetic properties of wood, ultimately increasing its marketability and recyclability [32]. In some decorative applications, a deep dark color is desirable, which, together with enhanced attractiveness of the wood, provides UV protection, thereby extending the life of the final product. The color uniformity achieved by thermal treatment can be a significant selling point for both manufacturers and consumers [32,54].

Thermal processing of wood has entered the market with the aim of improving certain properties of wood and achieving the form and functionality required by architects, engineers, and designers. Its favorable performance, low cost, and lightweight nature create a significant market potential for thermally treated wood products that can replace energy-intensive materials and construction methods [54,55].

Recent advances in the processing of wood materials should mainly promote their recycling, reuse, end-of-life use, and low-carbon economy (mitigating climate change, promoting sustainability, reducing energy consumption, pollution, and emissions, as well as increasing process efficiency) [55].

After heat treatment, wood becomes more brittle and has increased fragility, which results in the production of finer dust particles. The quality of thermally modified wood can depend on the accuracy and control of the process. Improperly performed modification can lead to degraded properties [56,57]. Nakagawa et al. [57] found in their work that thermal modification results in a decrease in transverse bending strength. On the other hand, fracture and Janka hardness tests revealed greater brittleness of thermally modified samples. The result of this study indicates that heat treatment of wood (western hemlock) has an important effect on its bending strength, fracture properties, and hardness. The modulus of elasticity in the cross-section decreased by 28%. Due to high temperatures, the wood loses moisture and its structure changes, which leads to increased brittleness.

In addition, another disadvantage of thermally modified wood is its price, which can increase significantly. The thermal modification process is more costly than conventional wood drying, which results in a higher price for the final product [56,57,58,59]. Similarly, the processing of thermally modified wood generates finer dust particles, which can pose a health risk to workers. In general, all types of wood processing operations produce various fine wood fractions (fine chips, powder, and sanding dust) [58,59].

Zhang et al. [60] found that higher temperatures during the thermal modification of wood led to a decrease in wood strength (reduction in cutting force) as processing temperature increased. Conversely, higher processing temperatures increased wood brittleness, which caused the appearance of more burrs and fragments of wood tissue (increased surface roughness) (

Table 9. A summary of articles dealing with the disadvantages of the thermally modified wood [61,62,63,64,65,66].

Main Topics Discussed	Ref.
Study on the morphological characteristics of thermally modified bamboo milling dust	Cui et al. [61]
Influence of degradation products from thermal wood modification on wood-water interactions	De Ligne et al. [62]
Characterization of the odorous constituents and chemical structure of thermally modified rubberwood	Li et al. [63]
Exploratory thermal modification of some West Coast Canadian wood species	Liu et al. [64]
Surface quality of planed tangential and radial sections of thermally modified Silver fir wood	Ajdinaj et al. [65]
Surface roughness of thermally modified and unmodified selected wood species after sanding	Adamčík et al. [66]

).

5. Prospects of Thermal Modification of Wood

Thermal modification of wood at high temperatures is becoming one of the important technologies with the highest achieved conversion rate on the market and a broad perspective in the future. A very favorable feature of this whole process is the fact that no chemicals and reagents are added to the production of thermally modified wood [67].

In addition to the favorable properties of wood achieved during thermal modification, such as better dimensional stability, biological durability, wood color and environmental friendliness, there are certain disadvantages in thermal processing of wood, such as a decrease in mechanical properties and surface wettability of wood, high energy consumption in production and high emissions. However, thermal processing is still considered a relatively ecological modification method; it is simpler and relatively free of pollution compared to chemical modification [65,66,67,68,69].

The prospects for thermal modification of wood are very promising. With the increasing emphasis on sustainability and green building materials, the demand for thermally modified wood continues to grow. Its excellent properties in terms of durability and stability make it suitable for a wide range of applications. It is expected that with further technological developments, the thermal modification process will become more efficient and affordable, thus expanding its use [65,66,67,68,69]. Thermal modification generally represents an attractive alternative to conventional wood preservation methods that rely on biocides. Over the past decade, thermal treatment of wood has been successfully developed, leading to the commercialization of thermally treated products in several European countries. Furthermore, the use of biocide-treated wood is increasingly restricted due to environmental concerns. Climate change and sustainable development are two of the most important factors shaping the development of the wood industry today [68,69,70].

Thermal modification of wood is rapidly expanding due to its sustainability, improved wood properties, and compliance with environmental regulations. It is becoming a key technology for modern wood use, supporting a greener construction and furniture industry [67]. The prospects for thermal modification of wood are quite promising, driven by growing demand for sustainable, durable, and eco-friendly materials in construction, furniture, and other wood-related industries. Here’s an overview of the key future trends and opportunities:

(a)

Increased demand for sustainable building materials

Thermal modification is a chemical-free, environmentally friendly treatment that aligns well with the increasing focus on green building standards and certifications (e.g., LEED, BREEAM). As the construction industry seeks alternatives to tropical hardwoods and toxic preservatives, thermally modified wood offers a renewable, durable option.

(b)

Expansion into new markets and applications

Flooring, decking, outdoor furniture, cladding, and interior joinery are already common uses, but future growth may extend to structural components, musical instruments, and even marine applications due to improved durability and stability. Lightweight construction and modular building trends may benefit from thermally modified wood’s improved mechanical properties.

(c)

Technological improvement

Advances in thermal modification technology, such as more precise control over treatment parameters, could further enhance wood properties and reduce production costs. Integration with other treatments (e.g., impregnation with natural oils or eco-friendly preservatives) could improve performance and expand functionality.

(d)

Cost reduction and increased accessibility

As the technology matures and economies of scale improve, costs are expected to decrease, making thermally modified wood more competitive with traditionally treated or exotic woods. Its broader adoption worldwide is expected, especially in regions with abundant softwoods but limited access to tropical hardwoods.

(e)

Environmental and regulatory support

Stricter regulations on chemical wood preservatives and deforestation push industries toward safer alternatives. Efforts to reduce the carbon footprint of wood products with longer lifespans and lower chemical usage, enhancing the appeal of thermal modification.

(f)

Research and innovation

Ongoing research is exploring hybrid modification methods, their effects on different wood species, and long-term performance under diverse climates. Innovations may address current limitations such as reduced mechanical strength and brittleness [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80].

The Use of Thermally Modified Wood

Thermally modified wood is utilized in a variety of applications where enhanced durability, stability, and aesthetic appeal are important (Figure 6). Thermally modified wood is widely applied in various fields due to its improved properties, including increased resistance to moisture, decay, and pests, better dimensional stability, and attractive appearance [70,71,72,73,74].

Outdoors, it is mainly used for terraces, garden flooring, cladding, fences, pergolas, and gazebos, where it endures weather conditions for a long time without the need for chemical treatment. Indoors, it is popular for flooring, wall and ceiling paneling, doors, windows, and furniture, where its stability, reduced water absorption, and rich dark color are valued [77,78,79]. Additionally, thermally modified wood has specialized applications, such as in saunas due to its resistance to high temperatures and humidity, in musical instruments for enhanced acoustic properties, and in small boats and sports equipment exposed to water. In construction, it is primarily used where high mechanical strength is not critical, such as in structural frames, cladding, or prefabricated components, with its lightness and stability offering advantages for modular buildings [75,76,77,78,79,80,81,82].

Thermally modified wood has a wide range of applications, especially where high resistance to moisture, rot, and deformation is required [4,44,71,72,73,74,75,76,77,78,79,80,81,82]:

a

Exterior decking: Due to its stability and resistance to weathering, it is ideal for decks.

b

Cladding and siding: Provides a durable and aesthetically pleasing exterior covering for buildings.

c

Balconies and railings: Resists moisture and biological attack.

d

Windows and doors: Improved stability reduces problems with deformations.

e

Garden furniture: Withstands weathering and rot.

f

Saunas: Low water absorption and resistance to high temperatures make it a suitable material for saunas.

g

Flooring: Both indoors and outdoors, where resistance to moisture is needed.

h

Structural elements: In some less stressed structures where stability and durability are important.

i

Musical instruments: Its stability and acoustic properties are also utilized in the music industry.

j

Interior cladding and panels: For aesthetic appearance and stability.

6. Conclusions

The thermal modification of wood is an innovative and environmentally friendly technology that significantly improves the physical, chemical, and mechanical properties of wood, requiring the use of chemicals. The current state of research and development in 2025 confirms that thermally modified wood exhibits better dimensional stability, increased resistance to biological pests and rot, as well as an attractive appearance due to natural darkening. Despite certain limitations, such as a partial reduction in mechanical strength and greater brittleness, the process is continually being improved through optimization of treatment parameters and integration with other environmentally friendly technologies. Therefore, thermal modification represents a promising alternative to traditional chemical wood treatments, promoting the sustainable use of wood raw materials in construction, furniture, and other industries. Future developments aim to expand applications, reduce costs, and overcome existing technical challenges, which will drive the greater adoption of this technology in the global market.

The thermal modification of wood causes significant changes in its chemical structure, which serve as the foundation for improving its properties. High temperatures during treatment cause the decomposition of hemicelluloses, the alteration of lignin, and the reduction of the hydroxyl group content, leading to a reduced ability of wood to bind moisture. These chemical transformations improve dimensional stability and resistance to biological agents such as fungi and insects, but at the same time, may lead to a weakening of some mechanical properties of wood, such as elasticity. Research from 2025 emphasizes the importance of optimizing process parameters such as temperature, time, and treatment atmosphere to achieve a balanced improvement in properties while minimizing negative effects on the wood structure. Ongoing research aimed at a detailed understanding of chemical changes and their impact on the mechanical and physical properties of wood will help further improve the technology.