Colour Homogenisation and Photostability of Beech Wood (Fagus sylvatica L.) as Affected by Mild Steaming and Light-Induced Natural Ageing

Abstract

This study investigates the impact of mild steaming (105 °C and 120 °C for 12 h) on the colour characteristics and chemical stability of beech wood (Fagus sylvatica L.) during natural indoor ageing. Untreated and steamed samples of mature wood and false heartwood were analysed for CIELAB and CIELCh colour parameters (L*, a*, b*, C*, h°) and chemical changes using ATR-FTIR spectroscopy. Steaming resulted in a significant decrease in lightness (L*) and increased a*, b*, and C* values, producing darker and more saturated reddish-brown tones. It also reduced the visual differences between mature wood and false heartwood, enhancing colour uniformity. During the light-induced ageing period, steamed wood—particularly at 105 °C—exhibited improved colour stability, maintaining chroma and hue more effectively than untreated samples. Statistically significant interaction effects between treatment, time, and tissue type revealed that the ageing-related colour changes were jointly influenced by thermal modification and the anatomical characteristics of the wood. In the FTIR spectra, the most pronounced changes were observed in the absorption bands of the aromatic skeleton and carbonyl groups (1504 and 1732 cm⁻¹). These findings confirm that mild steaming alters the original aesthetic properties and colour of beech wood when exposed to an indoor environment.

1. Introduction

Wood colour plays a crucial role in the commercialisation of timber, particularly in interior applications such as flooring, veneers, and furniture. However, the colour variability between mature wood and false heartwood, along with the inherent differences between sapwood and heartwood, often restricts the market potential of beech wood. These variations can lead to inconsistent aesthetic appearances, which may diminish the commercial appeal of wood products, especially when uniformity is desired in high-end applications [1,2,3].

In addition to inherent colour differences, wood colour is not static—it undergoes significant changes when exposed to light, particularly ultraviolet (UV) radiation. This is particularly relevant in interior environments, where wood surfaces are regularly subjected to natural or artificial light [4,5,6]. Over time, these exposures can cause darkening or fading of the wood, further complicating the uniformity of wood colour, especially between mature wood and false heartwood. This dynamic process may have a substantial impact on the marketability and aesthetic longevity of wood products used indoors.

Recent advancements in colour determination techniques, particularly those based on the CIELAB and CIELCh systems, have enabled the quantification of wood surface colour through parameters such as lightness (L*), chromatic coordinates (a*, b*), chroma (C*), and hue angle (h°). These parameters have been used to explore correlations between wood colour and other physical, mechanical, or chemical properties [7,8]. While much research has focused on temperate species, beech (Fagus sylvatica L.) has attracted growing attention due to its commercial relevance in interior applications and its tendency to develop discolouration, particularly false heartwood [9,10].

Wood colour is influenced by a combination of intrinsic and extrinsic factors, including species-specific anatomical and chemical characteristics, genetic variability, silvicultural practices, environmental conditions during growth, drying regimes, and post-harvest treatments such as steaming or thermal modification [11,12,13,14]. A key contributor to colour and its stability is the presence of extractives—low-molecular-weight compounds that can include tannins, phenolics, flavonoids, and quinones. While the role of extractives has been extensively studied in tropical hardwoods, where their abundance leads to vivid and often durable colouration [15,16,17], similar compounds are also present in temperate species. In beech wood, for example, extractives such as hydrolysable tannins and polyphenols are implicated in both the initial colour and its transformation during ageing, particularly in the formation of false heartwood.

While various studies have highlighted the key drivers of colour variation, there is still a gap in understanding how light exposure affects colour dynamics, particularly in interior settings. The interplay between the natural colour of mature wood and false heartwood, and their reaction to light exposure, is vital for improving the predictability of colour changes over time. Such understanding is crucial for the sustainable use and commercialisation of beech wood in interior applications.

This study aims to investigate the light-induced colour variations in steamed (hydrothermally treated) beech woods in interior conditions, focusing on the differential effects on mature wood and false heartwood. Additionally, Fourier-transform infrared (FTIR) spectroscopy will be employed to analyse the chemical changes that occur in these wood types under various treatments and light exposure. By addressing these issues, this study seeks to provide insights into enhancing the marketability of beech wood for interior applications, where aesthetic consistency and colour stability are paramount.

2. Materials and Methods

2.1. Materials

The wood material used in this study was European beech (Fagus sylvatica L.) with false heartwood, harvested from a managed forest stand in the Kremnické vrchy region (Slovakia). The centre lumber with a thickness of h = 50 mm was produced by cutting along the edge, which was subsequently divided into two asymmetrical parts by longitudinal sawing. By transverse shortening of these parts, 2 m long blanks were produced and randomly sorted into 3 groups. The wood was treated at either 105 °C or 120 °C for 12 h in a saturated steam environment, followed by slow cooling within the chamber, as described in [18]. Steaming of wood was carried out in a pressure autoclave AZ 240 (Himmasch AD, Haskovo, Bulgaria) installed at Sundermann s.r.o. Banská Štiavnica (Slovakia). Following the treatment, the samples were conditioned at a temperature of 20 °C and a relative humidity of 55% until they reached an equilibrium moisture content of 8–10%. Small samples with dimensions of 150 × 80 × 15 mm were cut from a single piece of treated board. The samples were then sequentially sanded using sandpaper with grit sizes P100, P120, and P150, thoroughly cleaned of dust, and finally conditioned in a climatic chamber.

2.2. Ageing Exposures

Each sample was partially covered with aluminium foil (30 microns thick), leaving one half exposed and the other half protected to enable direct comparison of surface changes. The samples were then placed behind a glass window consisting of thermal-insulation double glazing (U-factor 1.1 W·m⁻²·K⁻¹), oriented to the west, allowing exposure to natural light while limiting external environmental effects. During the 35-day exposure period, the interior temperature was maintained in a relatively stable range of 20 to 25 °C, and the relative humidity fluctuated slightly between 50% and 55%, simulating standard indoor conditions. The exposure location was Technical University in Zvolen. Specific information of the location is as follows: GPS coordinates 48.572024, 19.118499, altitude 283 m above sea level, and the cumulative amount of sunshine duration of 35-day exposure was 140 h, with the sun shining for approximately 9 h 38 min each day. The percentage of sunny days during this period was 48%. The UV index at Technical University in Zvolen reached a maximum of 5, indicating a level of moderate exposure.

2.3. Evaluation of Colour Change

The colour changes on sample surfaces were visualised and quantified using the CIELAB colour space and its cylindrical representation, CIELCh. The CIELAB system is a three-dimensional model consisting of three axes: the L*-axis represents lightness (L* = 100 corresponds to white, L* = 0 to black), while chromaticity is described in the ab plane—with a* ranging from green (−) to red (+) and b* from blue (−) to yellow (+).

In addition to L*, a*, and b*, the colourimeter directly provided values for chroma (C*) and hue angle (h°), which are cylindrical transformations of the a* and b* coordinates. Hue refers to the basic colour tone, expressed in degrees (0° = red, 90° = yellow, 180° = green, and 270° = blue), and chroma describes the colour’s saturation. Colours with high chroma appear vivid, while those with low chroma appear dull or pastel.

The measurements were taken at 10 spots on each sample. Colour measurements were carried out with a colourimeter Colour Reader CR-10 (Konica Minolta, Osaka, Japan). Colour coordinates were calculated based on the D65 illuminant and 10° standard observer with a test window diameter of 8 mm.

The ΔL*, Δa*, Δb*, ΔC*, and Δh° were calculated based on the averaged values of the data set using the following equations:

∆L* = L*₂ − L*₁

(1)

∆a* = a*₂ − a*₁

(2)

∆b* = b*₂ − b*₁

(3)

∆C* = C*₂ − C*₁

(4)

∆h° = h°₂ − h°₁

(5)

where index ‘1’ represents the colour coordinate value of the referential sample before steaming (resp. ageing), and index ‘2’ is the value of the same coordinate after steaming (resp. ageing).

The interpretation of ∆L*, ∆a*, ∆b*, ∆C*, and ∆h° are provided below: ∆L*: a positive value is lighter than the reference, and a negative value is darker than the reference; ∆a*: a positive value is redder than the reference, and a negative value is less red than the reference, ∆b*: a positive value is more yellow than the reference, and a negative value is less yellow than the reference; ∆C*: chroma or saturation difference; a positive value is clearer and brighter than the reference, and a negative sample is duller than the reference; ∆H°: hue or shade difference.

The total colour difference (∆E*_ab) was determined using the following equation:

$$∆\mathrm{E}{*}_{\mathrm{ab}}=\sqrt{{∆\mathrm{L}*}^2+{∆\mathrm{a}*}^2+{∆\mathrm{b}*}^2}$$

(6)

2.4. ATR-FTIR Spectroscopy

The surface of the experimental wood samples with dimensions of 150 × 80 × 15 mm was measured using a Nicolet iS10 FTIR spectrometer equipped with a Smart iTR attenuated total reflectance (ATR) sampling accessory with a diamond crystal (Thermo Fisher Scientific, Madison, WI, USA). The resolution was set at 4 cm⁻¹ for 32 scans. The wavenumber range varied from 4000 to 650 cm⁻¹. Each investigated area on the surface of the samples was measured four times, and the obtained spectra were subsequently averaged. OMNIC 9.0 software (Thermo Fisher Scientific, Madison, WI, USA) was used to evaluate the spectra. Absorption bands were identified using the literature [19,20]. The spectra were normalised at 1372 cm⁻¹. The values of changes in absorbances at selected wavenumbers (1504 and 1732 cm⁻¹) during natural ageing (NA) were calculated according to the following relationship:

∆A_NA = (A_NA − A_REF) ∗ 100/A_REF (%)

(7)

2.5. Statistical Analyses

All statistical analyses were performed using Statistica 14 and Excel. Descriptive statistics, including arithmetic means, standard deviations (SDs), and minimum and maximum values, were calculated for each colour parameter (L*, a*, b*, C*, and h°) to characterise the variability within treatment groups and wood tissue types.

To evaluate the significance of the effects of steaming (Treatment), wood tissue type (Part), and natural ageing time (Time), as well as their interactions, a three-way analysis of variance (ANOVA) was conducted. ANOVA tested the main effects and all two-way and three-way interaction terms (Treatment × Time, Treatment × Part, Time × Part, and Treatment × Time × Part).

3. Results

3.1. Colour Changes Due to Steaming

Steaming beech wood at 105 °C and 120 °C induced a marked darkening, evidenced by a significant reduction in lightness (L*) across both mature wood and false heartwood (

Table 1. Colour parameters for unsteamed and steamed beech wood.

Wood Colour Parameters	Unsteamed		Steamed at 105 °C		Steamed at 120 °C
	Mature Wood	False Heartwood	Mature Wood	False Heartwood	Mature Wood	False Heartwood
L*	81.57 (0.88) [79.90–82.70]	66.71 (1.76) [63.70–68.90]	69.46 (2.53) [59.20–71.60]	66.32 (2.40) [62.80–71.50]	58.09 (1.68) [56.90–65.00]	54.65 (0.97) [52.20–56.10]
a*	5.74 (0.50) [4.90–6.60]	10.88 (0.48) [10.30–11.70]	10.81 (0.44) [10.00–11.60]	10.03 (0.80) [8.40–11.70]	10.85 (0.93) [7.10–11.70]	10.87 (0.62) [10.00–11.90]
b*	15.20 (0.39) [14.70–15.80]	18.30 (0.79) [17.40–19.70]	17.82 (0.38) [17.00–18.40]	16.73 (0.55) [15.50–17.60]	17.72 (0.54) [16.90–18.70]	15.67 (0.38) [14.90–16.30]
C*	16.27 (0.50) [15.70–17.10]	21.28 (0.90) [20.20–22.70]	20.81 (0.53) [19.70–21.70]	19.52 (0.84) [17.90–21.10]	20.75 (0.65) [19.50–21.80]	19.07 (0.57) [18.00–20.00]
h°	69.31 (1.29) [67.30–72.10]	59.24 (0.52) [58.30–60.10]	58.81 (0.67) [57.70–59.90]	59.13 (1.45) [56.30–62.00]	58.52 (2.45) [57.10–68.60]	55.29 (1.26) [53.00–56.70]

Note: Data are presented as mean (standard deviation) with range [min–max].

). Notably, redness (a*) increased in mature wood, signalling a shift towards warmer tones, while in false heartwood, this parameter remained largely stable. Yellowness (b*) exhibited divergent trends, rising in mature wood but decreasing in false heartwood, highlighting tissue-specific colour responses to steaming.

Chroma (C*) rose appreciably in mature wood, intensifying colour saturation and vividness, whereas in false heartwood, it showed a slight, minimal decrease. This subtle contrast contributed to a more balanced and homogeneous visual texture between wood tissues. Hue angle (h°) shifted moderately in mature wood, reflecting a nuanced transition from yellow-red hues in untreated wood towards deeper reddish tones post-steaming; in false heartwood, this change was less pronounced.

Together, steaming homogenised the overall colour, reducing the natural contrast typically observed between darker false heartwood and lighter mature or sapwood. These effects intensified at 120 °C, confirming the crucial role of temperature in modulating wood colour transformations during steaming.

3.2. Colour Changes After Ageing

The absolute values of colour parameters L*, a*, b*, C* and h°, measured for covered and exposed surfaces after ageing, are presented in

Table 2. Colour parameters of untreated and steamed beech wood after ageing.

Wood Colour Parameters	Unsteamed		Steamed at 105 °C		Steamed at 120 °C
	Mature Wood	False Heartwood	Mature Wood	False Heartwood	Mature Wood	False Heartwood
Covered
L*	80.08 (0.83) [78.80–81.00]	66.98 (1.40) [65.40–68.90]	69.84 (0.87) [68.80–71.00]	65.06 (2.20) [63.80–69.20]	57.62 (0.56) [56.90–58.50]	55.54 (1.52) [52.70–56.60]
a*	5.72 (0.31) [5.30–6.20]	9.80 (0.51) [9.30–10.70]	11.10 (0.45) [10.50–11.60]	11.02 (0.34) [10.50–11.50]	10.38 (0.25) [10.0–10.60]	9.74 (0.21) [9.50–9.90]
b*	14.58 (0.28) [14.10–14.90]	17.14 (0.27) [16.90–17.50]	17.94 (0.46) [17.20–18.30]	17.10 (0.25) [16.70–17.40]	16.48 (1.14) [15.30–18.50]	14.94 (0.32) [14.40–15.30]
C*	15.68 (0.22) [15.40–16.00]	19.72 (0.41) [19.40–20.40]	21.08 (0.66) [20.10–21.70]	19.52 (0.84) [17.90–21.10]	19.50 (1.06) [18.30–21.30]	17.84 (0.32) [17.30–18.20]
h°	68.66 (1.38) [66.20–70.10]	60.22 (1.14) [58.40–61.20]	58.26 (0.50) [57.50–58.90]	57.30 (0.81) [56.50–58.70]	57.70 (1.36) [56.70–60.20]	56.90 (0.55) [56.50–57.80]
Exposed
L*	75.72 (1.21) [74.30–77.60]	68.64 (1.81) [66.50–70.20]	69.80 (0.49) [68.90–70.20]	65.70 (1.94) [63.10–68.50]	60.84 (0.30) [60.40–61.30]	59.08 (1.21) [57.00–60.10]
a*	8.26 (0.38) [7.70–8.70]	9.32 (0.52) [8.60–10.10]	10.08 (0.23) [9.80–10.40]	9.90 (0.33) [9.40–10.30]	8.94 (0.22) [8.70–9.20]	8.30 (0.15) [8.10–8.50]
b*	16.54 (0.91) [15.10–17.30]	18.50 (0.24) [18.20–18.80]	19.38 (0.18) [19.10–19.60]	17.82 (0.32) [17.30–18.10]	16.76 (0.20) [16.50–17.00]	15.46 (0.53) [14.60–16.00]
C*	18.46 (0.97) [17.20–19.40]	20.76 (0.45) [20.20–21.40]	21.70 (0.18) [21.50–21.90]	20.36 (0.36) [19.90–20.80]	18.96 (0.25) [18.70–19.30]	17.58 (0.54) [16.70–18.10]
h°	63.84 (1.50) [61.90–66.20]	63.32 (0.99) [61.90–64.80]	62.10 (0.16) [61.80–62.20]	60.96 (0.83) [60.00–62.30]	61.88 (0.29) [61.60–62.20]	61.78 (0.50) [61.00–62.30]

Note: Data are presented as mean (standard deviation) with range [min–max].

along with their means, standard deviations, and respective minimum and maximum values. These data provide a reference framework for evaluating the extent of colour change as influenced by both steaming intensity (105 °C and 120 °C) and wood tissue type (mature wood vs. false heartwood).

Minimal changes were observed on covered surfaces, indicating a high degree of colour stability under protected conditions; this is also illustrated in Figure 1 and Figure 2a. In contrast, exposed surfaces displayed notable changes in colour parameters, reflecting the combined effect of thermal treatment, tissue type, and ageing (Table 2 and Figure 1 and Figure 2b). These effects are examined in more detail in the following section.

Statistical analysis (

Table 3. Statistical significance of colour parameters in untreated and steamed beech wood after interior ageing based on three factors of ANOVA analysis.

	L*	a*	b*	C*	h°
Absolute	●●●	●●●	●●●	●●●	●●●
Treatment	●●●	●●●	●●●	●●●	●●●
Time	●●	●●●	●●●	—	●●●
Part	●●●	●●●	—	●	●●●
Treatment × Time	●●●	●●●	●●●	●●●	●●●
Treatment × Part	●●●	●●●	●●●	●●●	●●●
Time × Part	●●●	●●●	—	●●●	●●●
Treatment × Time × Part	●●●	●●●	●●	●●●	●●●

Note: ●●● statistically hight significant change >99.9%, ●● statistically significant change >99%, ● low significant change >95%, — unsignificant change <95%.

) revealed that interior ageing significantly affects the colour characteristics of the surface of beech wood, with the features of these changes depending on the prior thermal treatment (steamed vs. unsteamed wood), the type of wood tissue (mature wood vs. false heartwood), and their mutual interactions.

Considering the main effects, the impact of Time (ageing) was highly significant for parameters a*, b*, and h°, indicating notable chromatic changes, particularly shifts towards warmer or more saturated hues. Moderate changes were also observed in L* (lightness), whereas C* (chroma) remained relatively stable during ageing, suggesting that colour intensity is less affected than tone.

Thermal treatment by steaming (Treatment) had a highly significant effect on all colour parameters, confirming that steaming not only alters the initial colour properties of the wood but also influences how colour changes during ageing. The difference between mature wood and false heartwood (Part) was most pronounced in L*, a*, and h°, indicating tissue-specific behaviour in the ageing process, likely due to anatomical and chemical variations.

The interaction effects were also statistically significant. The Treatment × Time interaction indicates that steamed and unsteamed wood undergo ageing-related colour changes differently. Likewise, the Treatment × Part interaction highlights that the effect of steaming varies between wood tissues. The Time × Part interaction demonstrates that the ageing process is not uniform across tissue types.

The most complex response was found in the three-way interaction (Treatment × Time × Part), which was statistically significant for all parameters except b*, where the change was less pronounced. This interaction clearly shows that colour changes during ageing result from a combined influence of treatment, time, and tissue type and cannot be interpreted independently.

The change in lightness (ΔL*) due to ageing decreased after ageing, indicating progressive darkening of the wood surfaces (Figure 2). This effect was most pronounced in untreated wood, particularly in the mature wood, where the colour difference between mature wood and false heartwood increased over time. In steamed wood, both mature and false heartwood showed a general tendency towards increased lightness. In the case of wood steamed at 105 °C, the change in ΔL* was minimal. However, in wood steamed at 120 °C, a clear tendency towards increased lightness was observed.

In untreated mature wood, ageing resulted in an increase in Δa* values, indicating a shift towards redder tones, while Δb* values also increased, reflecting a yellowing effect typically associated with natural photo-oxidation. In false heartwood, Δa* decreased, accompanied by only a minimal increase in Δb*. Ageing of the steamed wood surface led to a negative change in Δa*, indicating that the surface became less red. After ageing of the surface steamed at 120 °C, this reduction was 2.5 times greater in mature wood and 20 times greater in false heartwood.

With ageing, the surface of both mature wood and false heartwood steamed at 105 °C became more yellow. However, the surface of both tissues steamed at 120 °C became less yellow. Changes in colour saturation (∆C*) due to ageing were expressed as positive values in mature wood, where the surface appeared clearer and brighter, and as negative values in false heartwood, which appeared duller. After ageing, the surface of both mature wood and false heartwood steamed at 105 °C became more saturated. In contrast, surfaces steamed at 120 °C appeared duller.

Ageing also influenced hue angle (Δh°), where a positive shift indicates movement on the colour wheel from red towards yellow. The smallest hue change occurred in false heartwood steamed at 105 °C, while the largest was observed in false heartwood steamed at 120 °C. An exception was untreated mature wood, where Δh° was negative, indicating a shift from yellow towards red.

The total colour difference (ΔE*_ab), based on changes in ∆L*, ∆a*, and ∆b*, is presented in Figure 2. In unsteamed wood after ageing, the value was 6.51 for mature wood and 2.49 for false heartwood, indicating a 2.6-fold greater colour change in mature wood. In steamed wood treated at 105 °C, the ΔE*_ab values were 1.76 for mature wood and 1.26 for false heartwood, a 1.4-fold difference in favour of mature wood. For wood steamed at 120 °C, the values were 3.48 for mature wood and 5.13 for false heartwood, indicating a 1.5-fold greater change in false heartwood. According to one study [21], ΔE*_ab values at this level correspond to an observable colour difference that is barely perceptible to the human eye.

These results indicate that steaming not only enhances the initial visual appeal but also contributes to colour integrity under indoor exposure. Moreover, steaming effectively reduces the colour contrast between mature wood and false heartwood, both immediately after treatment and following ageing, promoting a more uniform and aesthetically stable surface. Hence, steaming not only improves the initial appearance of beech wood but also supports its colour retention under interior conditions.

3.3. Chemical Changes Detected by ATR-FTIR

The fingerprint region of the ATR-FTIR spectra for beech wood before and after natural ageing is presented in Figure 3 (false heartwood) and Figure 4 (mature wood).

Both mature wood and false heartwood exhibited a marked reduction in the intensity of key absorption bands, namely, at 1504 cm⁻¹ (aromatic skeletal vibrations), 1461 cm⁻¹ (asymmetric C–H deformations in –CH₃ and –CH₂– groups in lignin and xylan), 1423 cm⁻¹ (aromatic skeletal vibrations in lignin and C–H in-plane deformation in cellulose), 1326 cm⁻¹ (C–O vibrations in syringyl and guaiacyl units, OH in plane bending in cellulose, and CH₂ wagging), and 1235 cm⁻¹ (syringyl ring and C–O stretch in lignin and xylan). These spectral changes reflect the degradation of the principal structural components of wood, especially lignin and hemicelluloses, as a result of natural ageing under light exposure over 35 days.

In contrast, increased absorbance was observed at 1732 cm⁻¹, corresponding to unconjugated C=O stretching in acetyl, carbonyl, and carboxyl groups, and at 1650 cm⁻¹, attributed to conjugated C=O stretching vibrations (Figure 3, Figure 4 and Figure 5), indicating the formation of oxidation products.

The most pronounced chemical change was observed at 1504 cm⁻¹, a band associated with aromatic skeletal vibrations of the lignin macromolecule. A significant reduction in intensity at this wavenumber confirms the cleavage of lignin’s aromatic skeleton due to photodegradation (Figure 3, Figure 4 and Figure 5). Quantitatively, in unsteamed wood, after 35 days of light-induced natural ageing, the intensity at this band decreased by 79.83% in false heartwood and 61.00% in mature wood, confirming a higher susceptibility of false heartwood to photochemical degradation. In steamed wood, lignin degradation became more pronounced with increasing steaming temperatures, with intensity drops reaching 87.46% in false heartwood and 85.78% in mature wood at 120 °C, indicating that thermal treatment enhances the susceptibility of both tissues to subsequent photodegradation. The differences between false heartwood and mature wood become less distinct, suggesting a homogenising effect of the thermal treatment.

Simultaneously, an increase in absorbance at 1732 cm⁻¹ was recorded, indicating oxidation and the formation of carbonyl-containing structures. In false heartwood, absorbance at this band increased by 54.17% in unsteamed samples and up to 85.78% after steaming at 120 °C. In mature wood, the increase ranged more moderately, from 57.02% to 61.29%, regardless of steaming.

4. Discussion

The results confirm that steaming alters the visual and chemical characteristics of beech wood in ways that contribute positively to its aesthetic and functional performance. Immediately after steaming, the wood becomes darker and more saturated, as reflected by the decrease in lightness (L*) and increase in a* and b* values. These findings align with previous observations of steamed or thermally treated wood, such as ThermoWood^®, where elevated temperatures induce darkening due to the formation of coloured degradation products of hemicelluloses and lignin [14,22,23,24,25].

In addition to colour modification, steaming led to a clear homogenisation of wood appearance, reducing the contrast between false heartwood and mature wood. This colour equalisation effect has important practical implications for decorative applications, especially in species like beech, where the presence of false heartwood often lowers visual quality and market value. Similar trends were reported by the authors of [6,26,27] for other temperate hardwoods, where steaming reduced intra-species colour variation and improved the aesthetic appeal of wood surfaces.

During the light-induced natural ageing period, steamed samples exhibited better colour stability compared to untreated wood. This was particularly evident in the total colour difference (ΔE*_ab), where steamed wood showed lower values across both tissue types, especially at lower steaming temperatures. Ageing significantly affected parameters a*, b*, and h°, indicating a gradual shift towards warmer and more saturated hues, while lightness (L*) also changed moderately over time. The distinction between mature wood and false heartwood was most apparent in L*, a*, and h°, suggesting tissue-specific ageing behaviour, likely due to anatomical and chemical differences. Moreover, the statistically significant interaction effects between treatment, time, and tissue type underscore the complex and interdependent nature of the colour change process, highlighting the need to consider the long-term dynamics of ageing rather than isolated measurements.

The decrease in the intensity of the aromatic skeletal vibration at 1504 cm⁻¹, particularly in unsteamed false heartwood, indicates that lignin is especially vulnerable to photodegradation. In our FTIR analysis, this band decreased by 79.83% in unsteamed false heartwood and by 61.00% in mature wood, confirming the greater susceptibility of false heartwood to UV-induced degradation. However, the differences between mature wood and false heartwood diminished with increasing steaming temperature (with intensity drops reaching 87.46% and 85.78%, respectively, at 120 °C), suggesting a homogenising effect of thermal treatment (Figure 3, Figure 4 and Figure 5).

Furthermore, the increase in absorbance at 1732 cm⁻¹, associated with unconjugated carbonyl groups (C=O), suggests that light-induced oxidation reactions occurred during ageing. This observation is consistent with the mechanism proposed by the authors of [28,29,30,31], in which photodegradation of lignin generates phenoxy radicals that react with oxygen to form quinones and other carbonyl compounds. Furthermore, reducing lignin on the wood surface improves cellulose access to UV radiation and related oxidation reactions. Specifically, absorbance at 1732 cm⁻¹ increased by 54.17% in unsteamed false heartwood and by 57.02% in mature wood, with values rising up to 85.78% and 61.29%, respectively, after steaming at 120 °C.

Steaming enhanced carbonyl formation in false heartwood more than in mature wood, indicating that thermal pretreatment may increase the availability of reactive sites for oxidation in this tissue. The highest degree of oxidation was actually observed in false heartwood steamed at 120 °C, confirming that steaming at higher temperatures does not uniformly suppress oxidation; its effects differ depending on the type of wood tissue.

In mature wood, we observed a moderate increase, smaller than in other samples. This suggests that oxidation reactions were still occurring, contributing to an increase in the 1732 cm⁻¹ band. However, this effect was partially offset by concurrent degradation of hemicelluloses, which is known to reduce the intensity of this band. These two opposing processes—oxidation increasing the carbonyl content and hemicellulose degradation reducing it—appear to act in parallel. Their relative influence likely depends on the thermal treatment and anatomical characteristics of the wood, as also noted by the authors of [23].

Taken together, these findings underscore that steaming improves not only the initial visual appearance of beech wood but also modifies its chemical response to light-induced ageing. In false heartwood, steamed samples exhibited significantly higher oxidation and lignin degradation following steaming, suggesting a different reactivity of this tissue type to thermal pretreatment and subsequent light exposure. The observed differences can be explained by the different chemical compositions of false heartwood and mature wood. While false heartwood contains a slightly higher proportion of lignin, mature wood is richer in polyphenolic extractives [32,33,34]. Polyphenols act as UV absorbers, and antioxidants scavenge free radicals formed by UV radiation, slowing down oxidative degradation of wood. Some groups of polyphenols are being explored as bio-based UV stabilisers in wood coatings and composites [35,36].

By minimising colour variability, and altering the chemical degradation pathways, steaming offers a promising strategy for valorising lower-grade timber. This approach can help extend the service life and enhance the aesthetic appeal of wood products intended for interior use.

To further improve the performance of steamed wood—especially in light-exposed interior environments—surface treatments such as transparent UV-resistant coatings, oil–wax systems, or finishes enriched with natural antioxidants (e.g., tannins or lignin stabilisers) are recommended. These treatments can mitigate ongoing oxidative changes, enhance dimensional stability, and preserve the visual qualities of both mature wood and false heartwood over time. The choice of surface finish should be tailored to the anatomical and chemical characteristics of the wood tissue, as well as to the expected exposure conditions.

5. Conclusions

Steaming of beech wood at 105 °C and 120 °C improved colour uniformity between mature wood and false heartwood and enhanced overall colour stability during natural light-induced ageing. This was confirmed by reduced total colour difference (ΔE*_ab) values and more consistent changes in chromatic parameters (a*, b*, and h°), particularly in steamed samples. While chroma (C*) remained relatively stable, significant shifts in hue and lightness indicated that ageing primarily affects tone rather than intensity.

The influence of steaming extended beyond initial colour properties, as statistically significant interaction effects (Treatment × Time × Part) revealed that the course of colour ageing is jointly determined by thermal treatment, duration, and tissue type. The ΔE*_ab values differed by treatment and tissue type, with unsteamed mature wood showing a 2.6-fold greater change than false heartwood, steaming at 105 °C reducing this to 1.4-fold, and steaming at 120 °C reversing the trend to a 1.5-fold greater change in false heartwood.

Spectral changes detected by ATR-FTIR showed that steaming affected both degradation and oxidation processes in the wood. The results suggest that the different structures of false heartwood and mature wood also affect the course of these processes.

Steaming thus represents a sustainable pretreatment for enhancing the visual stability of beech wood, especially for interior use. For further protection, complementary surface treatments such as UV-resistant or antioxidative coatings are recommended.