PEER-REVIEW ARTICLE PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 656 The Impact of Drying Conditions on the Surface Color Changes of Pine Wood Aleksandra Suchta,a,* Jacek Barański,b,* Tatiana Vilkovská,c Ivan Klement,c and Peter Vilkovský c The article presents the impact of drying process and selected parameters on the color changes on the surface of pine wood (Pinus sylvestris L.). Three predetermined process conditions (modes), mild, normal, and intensive, were investigated. The experiments were conducted using a semi-industrial scale dryer designed at the Gdańsk University of Technology with a loading capacity of 0.55 m3. The drying process was applied to pine wood grown in the northern part of Pomeranian region in Poland. The specimens were selected taking into consideration the radial angle of the growth rings. During each drying mode, 25 pieces of wood with dimensions (thickness, width, length) of 0.03 m, 0.2 m, and 1.5 m, respectively, were dried. The total color change (ΔE) after drying process and color saturation (h*) before and after drying processes of the wood surface was determined using the normalized CIELAB methods. The obtained results indicated that the color change of the pine wood surface increased simultaneously with the intensification of the drying process. However, the normalized wood quality after drying under intensive drying process conditions remained within the standard limits. The application of intensive drying process conditions remarkably changes the surface color of the obtained material, while remarkably reducing the drying process duration. DOI: 10.15376/biores.19.1.656-669 Keywords: Wood drying; High temperature wood drying; Drying conditions; Pine wood; Surface color changes; Moisture content Contact information: a: Institute of Manufacturing and Materials Technology, Faculty of Mechanical Engineering and Ship Technology, Gdansk University of Technology, Poland; b: Institute of Energy, Faculty of Mechanical Engineering and Ship Technology, Gdansk University of Technology, Poland; c: Department of Wood Technology, Faculty of Wood Sciences and Technology, Technical University in Zvolen, Slovakia; *Corresponding authors: aleksandra.suchta@pg.edu.pl, jbaransk@pg.edu.pl INTRODUCTION The wood color changes that occur during the drying process are a well-known fact. The development of kiln drying schedules for the industrial drying of timber has resulted in a general increase of drying temperatures. It is also connected with advantages such as shorter drying time, fewer cracks, and less deformation. The main consequences of higher drying temperatures are color changes and darkening of wood, but also an increase of resin flow, especially around knots. The color of wood is one of the main criteria for assessing its quality, which impacts how wood products are perceived by customers, particularly in furniture, decorative products, decorative veneers, and flooring; thus, accurate matching of the color of different samples is required (Torres et al. 2010). Drying temperature, time, time in the capillary drying phase, and water content are parameters that influence the color PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 657 change (Sundqvist 2000). Sundqvist (2002) discovered that the coloration of the wood had the same intensity during drying when there was moisture content above the fiber saturation point (FSP) even when there was moisture content in the wood under the FSP. Another feature of the drying process is that the color change in pine wood increases markedly at drying temperatures above 70 °C, probably depending on the resin content (Tarvainen et al. 2001). According to Abrahão (2005) the color uniformity in wood is significantly important in its quality evaluation as it determines the final appearance of the wood product. The color expression often means a subjective experience dependent on the available light source, surface properties, and the individual viewer’s eye. To study color changes, color must be measured in an objective way. Objective color measurement is performed instrumentally in a way that corresponds to visual assessment, using spectrophotometers or colorimeters and is defined numerically based on the standardized colorimetric calculation developed by the International Commission on Illumination (CIE) (Barański et al. 2017, 2020). During processing, wood undergoes mechanical treatment, as well as drying or thermal modification, both of which substantially influence on changes in its natural color and chemical composition (Gonzalez de Cademartori et al. 2013; Gonçalez et al. 2014; Barcík et al. 2015). Significant changes in wood structure occur after exceeding a temperature of 180 °C, while the carbonization process begins at a temperature of 250 °C (Kačíková and Kačík 2011). Instrumental methods for determining colorimetric parameters, color differences are subject to ISO standards, which are also applicable in Poland. These standards include definitions of basic concepts, requirements for colorimetric systems, the fundamentals of colorimetric calculation, and information for correctly conducting instrumental color measurement (Kazimierska 2014). Colorimetric measurement is valuable in wood quality control and in evaluating the color of end wood products subjected to high-temperature drying process (Klement and Huráková 2015). The mechanism behind color change is a complex process influenced by various factors (Kudra et al. 2003; McCurdy et al. 2005; McDonald et al. 2010). The aim of the conducted experimental research was to determine the impact of selected parameters of drying process (modes), such as: relative air humidity, temperature etc. on the surface color changes (∆E), and color saturation (h*) on the pine wood (Pinus sylvestris L.), which was dried to the final moisture content about 8%. MATERIALS AND METHODS The drying modes were conducted in the experimental facility of semi-industrial kiln of 0.55 m3 load capacity, specially designed at the Gdańsk University of Technology, Poland (Figs. 1 and 2). Steam production was implemented using a steam generator. The provision of necessary heat was supplied to kiln, thanks to hot flue gas, by a heat exchanger placed inside the drying kiln. The heat exchanger was supplied by exhaust gas generated during natural gas combustion process in the combustion chamber. The heat exchanger and electric steam generator made it possible to keep a constant temperature and relative humidity, respectively, inside the kiln during experiments. PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 658 Fig. 1. Semi-industrial drying kiln - top view Fig. 2. Semi-industrial drying kiln - side view Steam circulation inside the drying kiln was forced via a circulating fan with drying medium’s velocity regulation up to 5.0 m/s. Fan and heat exchanger were located in the working area of kiln separated from the drying area by the wall. The two chimneys on the roof of the kiln controlled the drying conditions inside. The drying kiln was customized to dry all timber types and to obtain the final moisture content not exceeding 8%. The control system, which controls and monitors the drying process, was located outside the kiln. It contains four thermocouples to measure temperature inside the kiln and the wood in three chosen locations in dryer, respectively. The system also monitored the level of wood moisture content in eight different locations inside the stack of the drying material such as the midsection and outer layers, and the humidity of the environment inside the kiln, respectively. PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 659 The drying medium parameters inside the kiln were measured using E+E humidity and temperature sensor type EE31 with accuracy of ± 1.3% for humidity and 0.2 °C for temperature, respectively. The drying procedure was conducted using a two-stage process, overseen by the control system (Barański 2018; Klement et al. 2019). The moisture content of each dried wood sample was measured using a TANEL Co. (TANEL) resistance-based moisture content meter type WRD 100, that has a measurement accuracy of ± 2%. Prior to and following the drying procedure, samples were checked for the absolute moisture content through the gravimetric method (Eq. 1). This method relied on the use of RADWAG Co. (RADWAG) precision balance type PS 510.R1, that has a measurement accuracy of 0.005 g. The achievement of absolute dryness for the samples occurred within a laboratory kiln operating at a controlled temperature of 103 ± 2 °C. The moisture content was computed using the Eq. 1, 𝑤 = 𝑚𝑤− 𝑚0 𝑚0 × 100, (1) where w is the moisture content (%), mw is the weight of the wet sample (g), and mo is the weight of the absolute dry specimen (g). To be able to dry the wood well and quickly, it is necessary to set the drying parameters correctly to set a lower equilibrium moisture content (EMC) than the moisture content (MC) of wood. This relation is called the drying gradient (U) and is calculated according to the Eq. 2: 𝑈 = 𝑀𝐶 𝐸𝑀𝐶 , (2) where U is the moisture gradient, EMC refers to equilibrium moisture content (%), and MC is moisture content (%). In general it can be said that a higher moisture gradient causes faster drying, but on the other hand more defects will be present in the wood as a result of faster drying. The drying gradient is decisive for both the quality of the dried wood and economy. Fig. 3. The explanatory figure of the wood sample used for measuring color changes Materials used for experiments were 75 pieces of pine samples (Pinus sylvestris L.) originating from the same location of northern part of Pomeranian region in Poland. The specimens were selected based on the radial angle of the growth rings. During each drying mode 25 pieces of samples were dried. The dimensions of the tested samples were as follows: 30 mm thickness, 200 mm width, and 1500 mm length. Numerical values of color coordinates for a specified light source were then expressed in three-dimensional (3D) color space according to the CIE standard. The color parameters (L, a*, b*) were measured PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 660 in three chosen locations before the drying and after the drying on each of tested samples, respectively shown in Fig. 3. The color was measured in distance from the edge of the sample was 15 cm and every 30 cm. All together, three measurements were done. The pine wood was dried under different drying modes with various conditions that are outlined in Tables 1, 2, and 3. The distance from the edge of the sample was 15 cm and every 30 cm were the measurement of color. Table 1. Parameters of the Drying Medium Used for the Mild Drying Process Table 2. Parameters of the Drying Medium for the Normal Drying Process Table 3. Parameters of the Drying Medium for the Intensive Drying Process In this study, the color change of dried wood was assessed using a three-axis system (Fig. 4), measuring the lightness (L*) and chromaticity coordinates (a*, b*), in accordance with ISO 11664-2 (2007) and ISO 11664-4 (2008) standards. The CIE-recommended CIELAB color system consists of two axes with parameters a* and b*, positioned at a right angle to each other, defining the color hue. The third axis represents lightness L*. It is perpendicular to the a* b* plane. Absolute Wood Moisture Content Air Temperature Equilibrium Moisture Content Drying Gradient Psychrometric Difference Relative Air Humidity MC ts EMC U ∆t φ (%) (°C) (%) (-) (°C) (%) > 40.0 to 26.0 (FSP) 50.0 11.3 3.5 to 2.3 6.3 70.0 25.9 to 8.0 60.0 9.1 2.3 to 1.1 9.0 62.0 Absolute Wood Moisture Content Air Temperature Equilibrium Moisture Content Drying Gradient Psychrometric Difference Relative Air Humidity MC ts EMC U ∆t φ (%) (°C) (%) (-) (°C) (%) > 40.0 to 26.0 (FSP) 70.0 9.0 4.4 to 2.9 9.0 65.0 25.9 to 8.0 80.0 7.5 2.9 to 1.3 11.7 60.0 Absolute Wood Moisture Content Air Temperature Equilibrium Moisture Content Drying Gradient Psychrometric Difference Relative Air Humidity MC ts EMC U ∆t φ (%) (°C) (%) (-) (°C) (%) > 40.0 to 26.0 (FSP) 90.0 14.1 2.8 to 2.6 2.8 90.0 25.9 to 18.0 90.0 10.0 2.6 to 1.8 7.3 75.0 17.9 to 8.0 120.0 4.0 3.5 20.0 3.0 PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 661 Fig. 4. Schematic of three-axis color change measurement system (Barański et al. 2020) To measure color changes CIELAB Color spaces were determined with the use of the Color Reader CR-10, Konica Minolta Company. The parameter h* defines the hue of the wood color and is one of the key properties contributing to wood color analysis, defined (in the CIECAM02 model) as “the degree to which a stimulus can be described as similar or dissimilar to stimuli described as red, green, blue, and yellow”. Hue can also be quantified (Eq. 3), using a single number, often corresponding to an angular position around the central or neutral point or axis in the color space coordinate system (Fig. 5), ℎ∗ = tan−1 𝑏∗ 𝑎∗ (3) where ℎ∗ is the parameter determining the hue of the wood where 0o is red, 90o is yellow, 180o is green, and 270o is blue, 𝑏∗ is the parameter determining the color of wood in the yellow-blue direction (−]), and 𝑎∗ is the parameter determining the color of wood in the red-green direction (−]). Fig. 5. Description of color change in the hue, saturation, and brightness space (Canon Inc.) PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 662 The change in wood color before and after drying (expressed in the CIELAB color space) can be compared using the total color difference ∆E, which represents the distance between these colors in the three-dimensional CIELAB space (Fig. 6). Fig. 6. Schematic of total wood color change evaluation described by the parameter ∆E* (Barcík et al. 2015) Color differences in the CIE space can be straightforwardly determined by calculating their spatial distance, equal to the square root of the sum of the squares of differences for each of the three coordinates of the two compared colors, according to the relationship (in accordance with ISO 11664-4 (2008) and ISO 11664-6 (2022) standards, ∆𝐸∗ = √∆𝐿∗2 + ∆𝑎∗2 + ∆𝑏∗2 (4) where ∆𝐸∗ is the parameter determining the total wood color change after drying (%), ∆𝐿∗ is the difference of the parameter determining the color change after drying in the white- black direction, ∆𝑎∗ is the difference of the parameter determining the color change after drying in the red-green direction, ∆𝑏∗ is the difference of the parameter determining the color change after drying in the yellow-blue direction. The total color change ∆E* calculated using Eq. 4 was determined according to the criteria described (Cividini et al. 2007), concerning the distribution of color changes (Table 4). Table 4. Criteria for Distribution of Color Change (Cividini et al. 2007) ∆E* Value (-) Color Change Description (-) ∆E* < 0.2 Undetectable difference 0.2 < ∆E* < 2.0 Slight difference 2.0 < ∆E* < 3.0 Color change visible with a high-quality filter 3.0 < ∆E* < 6.0 Color change visible using a medium-quality filter 6.0 < ∆E* < 12.0 Significant color changes ∆E* > 12.0 Different color PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 663 To illustrate the research methodology, a statistical analysis (STATISTICA, file version 12.0.1133.15, StatSoft CR s.r.o., Australia) was conducted for each of the samples, determining mean, maximum, and minimum values as well as standard deviation. Subsequently, a statistical analysis was performed using the Duncan test and box plots. RESULTS AND DISCUSSION The measurement data presented in Tables 5, 6, and 7 provide a summary of the results of the total color change for the tested samples after drying using three drying programs. Table 5. Results of Total Color Change Measurement for Mild Drying Value Measured Parameters Before Drying Measured Parameters After Drying Total Color Change L* a* b* L* a* b* ΔE* Mean 70.6 9.1 29.9 74.6 6.7 26.5 6.4 Maximum 76.1 12.1 35.6 78.5 9.1 29.9 9.9 Minimum 67.6 7.1 26.1 70.2 4.1 23.5 3.9 Standard Deviation 2.6 1.5 3.3 2.5 1.5 1.7 2.0 Table 6. Results of Total Color Change Measurement for Normal Drying Value Measured Parameters Before Drying Measured Parameters After Drying Total Color Change L* a* b* L* a* b* ΔE* Mean 77.0 6.7 25.6 74.3 8.0 29.2 4.8 Maximum 77.9 7.8 28.5 76.0 9.3 31.9 6.9 Minimum 76.1 5.1 23.9 72.1 7.0 26.5 3.1 Standard Deviation 0.5 0.8 1.7 1.3 0.7 1.4 1.1 Table 7. Results of Total Color Change Measurement for Intensive Drying Value Measured Parameters Before Drying Measured Parameters After Drying Total Color Change L* a* b* L* a* b* ΔE* Mean 74.2 6.6 24.9 65.2 12.0 33.3 13.9 Maximum 76.5 7.5 27.6 69.5 16.5 45.9 27.7 Minimum 72.4 5.7 22.0 59.2 8.5 27.4 7.0 Standard Deviation 1.3 0.5 1.5 3.2 2.3 5.1 5.6 PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 664 From the results, it was observed that the color change of pine wood after drying, as indicated by the ΔE* value, increased when the process was performed at temperatures above 100 ℃, where the air and superheated steam mixture were used as drying medium. It was found that the temperature of the drying environment had the most remarkable effect on the color change of wood. Temperatures up to 80 °C were used for both mild and normal drying, while the difference in those drying modes was not great. For this reason, a smaller E difference could be measured in normal mode than in mild mode. The wood itself also had a certain influence on the measured values in terms of its natural color and remarkable annual rings. In the intensive drying mode, the temperature of the drying environment from 90 to 120 °C was used, which also caused the greatest change, as indicated by E. This relationship was also observed by other researchers (Unsal et al. 2003; Esteves et al. 2008; Gonzalez de Cademartori et al. 2013), who subjected eucalyptus and pine wood to thermal modification, as well as to the rectification processes (Moura and Brito 2011). These authors also noted that the average values of the a* and b* parameters decreased with increasing drying temperature, which was confirmed by this study. The other way around, the results of (Sundqvist 2002) showed that colorization was most intense at the wood surface, and color homogeneity was not affected by raised temperatures during drying. Figure 7 presents a bar chart illustrating the color change of pine wood after drying under three conditions mild, normal, and intensive, defined by the ΔE parameter. The color change of pine wood was the smallest for the normal drying process (mean ΔE* value of 4.81(-), with a standard deviation of 1.1(-)). The most significant color change was obtained during the intensive drying process (mean ΔE* value of 13.9 (-), with a standard deviation of 5.6(-)). Fig. 7. Bar chart depicting the color change of wood after drying under three programs: mild, normal, and intensive, defined by the parameter ΔE* P a ra m e te r D e te rm in in g T o ta l C o lo r C h a n g e A ft e r D ry in g ∆ E ( % ) PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 665 Table 8 presents a quantitative summary of the dried pine wood boards showing specific quality criteria considering the total color change resulting from the application of three different drying conditions. All types of drying contributed to the total color change of the wood surface, which is visible using a medium-quality or higher-grade filter. Table 8. Distribution Percentages of the Dried Pine Wood Batch Based on the Total Color Change for the Utilized Drying Methods Mild Normal Intensive ΔE* < 3.0 Invisible or small or visible with a high- quality filter 0.0% 0.0% 0.0% 3.0 < ΔE* < 6.0 Visible with a medium quality filter 56.7% 76.7% 0.0% 6.0 < ΔE* < 12.0 Noticeable color change 43.3% 23.3% 56.7% ΔE* > 12.0 Different color 0.0% 0.0% 43.3% Based on the obtained results, an increase in the intensity of the drying process led to a more pronounced color change in the dried material. The obtained results indicate a strong statistical dependence (p ≈ 0). Duncan’s test (Table 9) confirmed a significant statistical difference in the total color change of wood after drying using intensive drying conditions compared to the other conditions (mild and normal). The drying program involving a mixture of air and superheated steam at temperatures above 100 ℃ requires an increased relative air humidity in the initial stage of the process and its significant reduction as the drying medium temperature exceeds 100 ℃. This can lead to an increase in the ΔE* parameter value, which translates to a more pronounced color change. Table 9. Duncan’s Test for the Total Color Change of Pine Wood Depending on the Used Drying Program Mild Normal Intensive ΔE1* ΔE2* ΔE3* Mild ΔE1* - 0.083619 0.000114 Normal ΔE2* 0.083619 - 0.000053 Intensive ΔE3* 0.000114 0.000053 - Figures 8, 9, and 10 provide a visual interpretation of the change in hue and color saturation of wood in a 2-dimensional space resulting from the drying process. Mild and normal wood dryings resulted in a slight change in hue (h*) and color saturation (Cab *) of pine wood compared to intensive drying (Table 10). The chosen parameters for gentle and normal drying led to a change in the hue of the tested pine wood towards the red spectrum (Figs. 8 and 9). For intensive drying of pine wood, both the hue and color saturation underwent a more substantial change towards the yellow spectrum (Fig. 10). Gonçalez et al. (2014), showed that drying of tropical wood species appeared to inversely reduce wood lightness. PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 666 Fig. 8. The average change in hue (h*) and color saturation of pine wood (Cab *) after mild drying Fig. 9. The average change in hue (h*) and color saturation of pine wood (Cab *) after normal drying Fig. 10. Average change in hue (h*) and color saturation of pine wood (Cab*) after intensive drying Table 10. Average Values of the Coefficients Describing Hue (h*), Color Saturation (C* ab), and Lightness of Saturation (Sab) of Pine Wood Before and After Drying Average Values of the Parameters Before Drying Average Values of the Parameters After Drying h* C*ab Sab (%) h* C*ab Sab (%) Mild 73.0 31.3 40.4 76.0 27.4 34.5 Normal 75.3 26.5 32.5 74.7 30.3 37.3 Intensive 75.0 25.7 32.8 70.1 35.4 47.6 PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 667 CONCLUSIONS 1. Results indicated a much stronger negative effect of intensive drying on the lightness (L) value. It seems important that lower intensities of drying had much less effect on L value. 2. The obtained results showed a significant difference in the total color change (E) of wood after drying using intensive drying conditions comparing to other drying modes (mild and normal). 3. The drying schedule involving an air and superheated steam mixture at temperatures above 100 ℃ require a higher relative air humidity value in the initial stage of the drying process and its significant reduction as the drying medium temperature exceeds 100 ℃. That leads to an increase in the ΔE* value. 4. Mild and normal drying modes of wood resulted in a slight change in the hue (h*) and color saturation (Cab*) of pine wood (towards red color) in relation to intensive drying. After the process, the wood had changed its hue (h*) and saturation (Cab*) towards a more yellow color. ACKNOWLEDGEMENTS This work was supported by the Slovak Research and Development Agency under contract no. APVV-21-0049. This work was also supported by the Scientific Grant Agency of the Ministry of Education, Science, Research, and Sport of the Slovak Republic and the Slovak Academy of Sciences project VEGA no. 1/0063/22. REFERENCES CITED Abrahão, C. P. (2005). Estimation for Some Properties of the Wood of Eucalyptus urophylla by Spectrometry, Ph.D. Dissertation, Federal University of Viçosa, Viçosa, Brazil. Barański, J., Klement, I., Vilkovská, T., and Konopka, A. (2017). “High temperature drying process of beech wood (Fagus sylvatica L.) with different zones of sapwood and red false heartwood,” BioResources 12(1), 1861-1870. DOI: 10.15376/biores.12.1.1861-1870 Barański, J. (2018). “Moisture content during and after high and normal temperature drying processes of wood,” Drying Technology 36(6), 751-761. DOI: 10.1080/07373937.2017.1355319 Barański, J., Konopka, A., Vilkovská, T., Klement, I., and Vilkovský, P. (2020). “Deformation and surface color changes of beech and oak wood lamellas resulting from the drying process,” BioResources 15(4), 8965-8980. DOI: 10.15376/biores.15.4.8965-8980 Barcík, Š., Gašparík, M., and Razumov, E. Y. (2015). “Effect of temperature on the color changes of wood during thermal modification,” Cellulose Chemistry and Technology 49(9-10), 789-798. PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 668 Cividini, R., Travan, L., and Allegretti, O. (2007). “White beech: A tricky problem in the drying process,” in: ISCHP, Québec City, Canada, pp. 135-140. Esteves, B., Velez Marques, A., Domingos, I., and Pereira, H. (2008). “Heat induced colour changes of pine (Pinus pinaster) and eucalyptus (Eucalyptus globulus) wood,” Wood Science and Technology 42(5), 369-384. DOI: 10.1007/s00226-007-0157-2 Gonçalez, J. C., Bezerra Marques, M. H., Sousa Karas, M. C., Janin, G., and Gomes Ribeiro, P. (2014). “Effect of drying process on Marupá wood color,” Maderas. Ciencia y Tecnología 16(3), 337-342. DOI: 10.4067/S0718-221X2014005000026 Gonzalez de Cademartori, P. H., Schneid, E., Gatto, D. A., Stangerlin, D. M., and Beltrame, R. (2013). “Thermal modification of Eucalyptus grandis wood: Variation of colorimetric parameters,” Maderas. Ciencia y Tecnología 15(1), 57-64. DOI: 10.4067/S0718-221X2013005000005 ISO 11664-2 (2007). “Colorimetry - Part 2: CIE standard illuminants,” International Organization for Standardization, Geneva, Switzerland. ISO 11664-4 (2008). “Colorimetry - Part 4: CIE 1976 L*a*b* Colour space,” International Organization for Standardization, Geneva, Switzerland. ISO 11664-6 (2022). “Colorimetry - Part 6: CIEDE2000 colour-difference formula,” International Organization for Standardization, Geneva, Switzerland. Kacíková, D., and Kacík, F. (2011). Chemical and Mechanical Changes During Thermal Treatment of Wood, Technical University in Zvolen, Zvolen, Slovakia. Kazimierska, M. (2014). “Obiektywna ocena barwy wyrobów użytkowych [Objective assessment of the color of consumer products],” Technologia i Jakość Wyrobów 59, 44-47. (In Polish) Klement, I., and Huráková, T. (2015). “High temperature drying of beech wood with content of red heartwood,” in: Selected Processes at the Wood Processing, Hokovce, Slovakia. Klement, I., Vilkovská, T., Barański, J., and Konopka, A. (2019). “The impact of drying and steaming processes on surface color changes of tension and normal beech wood,” Drying Technology 37(12), 1490-1497. DOI: 10.1080/07373937.2018.1509219 Kudra, V. S., Vitter, R. M., and Gaida, Y. I. (2003). “Effect of false heart on the quality of beech wood,” Lesnoe Khozyaistvo 5, 23-24. McCurdy, M. C., Pang, S., and Keey, R. B. (2005). “Measurement of colour development in Pinus radiata sapwood boards during drying at various schedules,” Maderas. Ciencia y Technologia 7(2), 79-85. DOI: 10.4067/S0718-221X2005000200002 McDonald, A. G., Fernandez, M., Kreber, B., and Laytner, F. (2010). “The chemical nature of kiln brown stain in Radiata pine,” Holzforschung 54, 12-22. Moura, L. F., and Brito, J. O. (2011). “Effect of thermal rectification on colorimetric properties of Eucalyptus grandis and Pinus caribaea var. hondurensis woods,” Scientia Forestalis 39(89), 69-76 . Radwag (2023). “PS 510.R1 precision balance technical data,” RADWAG Balances and Scales, (http://radwag.com/en/ps-510-r1-precision-balance,w1,E34,101-102-111#1), Accessed 01 Oct 2023. Sandoval-Torres, S., Wahbi, J., Francoise, M., and Puiggali, J.-R. (2010). “Causes of color changes in wood during drying,” Forestry Studies in China 12, 167-175. DOI: 10.1007/s11632-010-0404-8 Sundqvist, B. (2000). Wood Color Related to Kiln Drying, Ph.D. Thesis, Luleå Tekniska Universitet, Luleå, Sweden. PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Suchta et al. (2024). “Pine drying & surface color,” BioResources 19(1), 656-669. 669 Sundqvist, B. (2002). “Wood color control during kiln-drying,” Forest Products Journal 52, 30-37. Tanel (2023). “WRD-100 Wood Moisture Meter Technical Data,” TANEL Electronics & IT General Partnership, (http://www.tanel.com.pl/moisture_meter.php? kod_produktu=wrd100), Accessed 01 Oct 2023. Tarvainen, V., Saranpää, P., and Repola, J. (2001). “Discoloration of Norway spruce and Scots pine timber during drying,” in: Proceedings of the 7th International IUFRO Wood Drying Conference, Tsukuba, Japan, pp. 294-299. Unsal, O., Korkut, S., and Atik, C. (2003). “The effect of heat treatment on some properties and colour in eucalyptus (Eucalyptus camaldulensis Dehn.) wood,” Maderas. Ciencia y Tecnología 5(2), 145-152. DOI: 10.4067/S0718- 221X2003000200006 Article submitted: September 28, 2023; Peer review completed: November 11, 2023; Revised version received and accepted: November 15, 2023; Published: December 1, 2023. DOI: 10.15376/biores.19.1.656-669