The Effect of Steaming Beech, Birch and Maple Woods on Qualitative Indicators of the Surface

: This work presents the effect of steaming beech, birch and maple woods on the resulting quality of the milled wood surface. The steaming process of the studied woods results in a targeted change in the color of the wood, which changes from the original light white-gray color to fine reddish-brown to dark brown color shades that are more or less saturated depending on the temperature of the saturated water steam. The color changes achieved during the modification process were identified using coordinates in the CIE L*a*b* color space. The achieved color changes were described through the total color difference of ∆ E* and defined through classification grades using a color scale. The technological process of wood steaming with saturated water steam for the purpose of a targeted change in the color of the wood and experimental measurements of the roughness of the milled wood surface proved that the wood steaming process has a positive effect on the roughness of the wood surface of the investigated trees, depending on the steaming temperature. The reduction of roughness in the process of the modification of beech wood compared with native wood was at the temperature of the saturated water steam as follows: t I = 105 ± 2.5 ◦ C by Ra = 12.3%, at t II = 125 ± 2.5 ◦ C by R a = 15.4%, at t III = 135 ± 2.5 ◦ C by Ra = 16.9%. By modifying birch wood at t III = 135 ± 2.5 ◦ C, the roughness decreased by Ra = 13.4%; the surface roughness decreased by Ra = 15.8% compared with native wood by modifying maple wood. The roughness of the milled surface of modified wood in individual treatment modes decreased compared with native wood, which means that the milled surface of modified wood is of a better quality, which is positive for its practical use.


Introduction
The quality of the machined surface is an important factor that is influenced by both the technical and technological parameters of machines and tools used for cutting and machining [1][2][3][4][5][6][7] as well as the properties of the processed material (type of wood, wood moisture, macro-, micro-or sub-micro-structure) [8], thereby last but not least affecting the marketability of the product [9,10].Authors from [11] define surface quality as a set of specific surface properties defined by ridges (peaks) and hollows.These features can also be named surface topography.The author from [1] characterizes the quality of the processed wood surface through the roughness and waviness of the surface as well as through possible damage to the wood surface by torn fibers and tool grooves, which are generally considered to comprise unevenness and to be the result of technological operations in which the integrity of the wood is violated due to processing.The standard from [12] evaluates the surface quality parameters of materials as the differences between surface unevenness, roughness (evaluated from the roughness profile) and waviness.According to the mentioned standard, we can define the real surface of the wood as the surface that borders the given body and separates it from the surrounding environment.In practice, The wood of the selected deciduous trees of Fagus sylvatica L., Betula pendula and Acer pseudoplatanus used in the research was from Štiavnické vrchy (Slovakia).Radial cross-sections with the dimensions of h = 40 mm × width w = 80 mm × length l = 600 mm were used for this research.The wood was manipulated from the sapwood of the given trees after undergoing sawing one meter above the ground to produce a total number of 120 pieces, a process which was undertaken separately for each tree species.The blanks of each wood were divided into four groups that separately represented 30 pieces of blanks for each type of wood.The blanks of the 1st group were untreated (native wood).The blanks of the 2nd group were modified through the steaming process in mode I., the blanks of the 3rd group were modified through the steaming process in mode II.and blanks of the 4th group were modified by the steaming process in mode III.

Steaming and Drying of Blanks
The technological process of the wood steaming of individual wood species was carried out in pressure autoclave APDZ 240 (Himmasch AD, Haskovo, Bulgaria) at a higher pressure of saturated water steam than the atmospheric pressure.Temperatures of saturated water steam in individual treatment modes are shown in Table 1.Temperatures t max and t min represent the temperature range in which saturated water steam is supplied to the autoclave for the implementation of the technological process.Temperature t 4 is the temperature of the saturated water steam in the autoclave after reducing the pressure of the water steam in the autoclave to atmospheric pressure, thereby enabling the safe opening of the pressure device and the selection of samples after the specified modification time.
The saturated water steam of unsteamed as well as steamed wood was dried to the final moisture content of w = 12 ± 0.5%.The drying technology was realized through the low-temperature drying mode of [27], with an emphasis on preserving the obtained color in the steaming process.

Measuring the Color of Wood
Identification of the color of wood modified by steaming individual wood species as well as unsteamed wood in the CIE L*a*b* color space was determined using a color reader CR-10 colorimeter (Konica Minolta, Tokyo, Japan) using a D65 light source with an illuminated area of 8 mm.The measurement of the color of unsteamed and steamed wood was carried out on milled surfaces at a distance of 50 mm from the front of the blanks.The measured wood color values were evaluated based on changes in the CIE L*a*b* color space where the L* coordinate represents the lightness, the a* color coordinate represents the red color and the b* color coordinate represents the yellow color.
The value of the total color difference was calculated according to mathematical Equation (1): where L * 1 , a * 1 and b * 1 values represent the coordinates of the color space of the surface of dry-milled native wood, L * 2, a * 2 and b * 2 values represent the coordinates of the color space of the surface of dry-milled modified wood.

Milling of Blanks
Milling of unsteamed and steamed beech, birch and maple woods was carried out using a slotted spiral milling cutter type F193-16061 (IGM tools and machines, Tuchoměřice, Czech Republic) that was installed on a CNC 5-axis machining center SCM TECH Z5 from the manufacturer SCM Group, Rimini, Italy (Figure 1).Technical and technological parameters of CNC machining center are shown in Table 2.The orientation of the blanks during milling was always the same, with the length of the blank being on the x-axis, the width of the blank being in the direction of the y-axis and the thickness of the blank being on the z-axis.The feed rate was realized in the direction of the x-axis, and the material removal was in the direction of the y-axis.High-speed machining of wood by milling was carried out on the basis of the technological conditions listed in Table 3.

Measuring the Surface Roughness of Milled Wood
At the Department of Woodworking of the TU in Zvolen, in cooperation with the company KVANT s.r.o., the laser profilometer LPM-4 (KVANT s.r.o., Bratislava, Slovakia) was constructed on which the non-contact measurement of the surface roughness of milled, unsteamed and color-modified beech, birch and maple woods was performed.Figure 2 shows a roughness meter consisting of a solid aluminum structure that ensures stabilization during the measurement of the roughness of the wood surface.Height adjustment, head positioning or camera focus are realized using screws and grooves in the beam of the fixed structure.The principle of roughness measurement with this profilometer is based on triangulation laser profilometry measurement.The created profile along the cross-section of the measured object is photographed using a Marlin F131B digital camera (Allied Vision Technologies GmbH, Stadtroda, Germany) that captures the image using a laser line at a specified angle [28].
Coatings 2023, 13, x FOR PEER REVIEW 5 of 11 camera (Allied Vision Technologies GmbH, Stadtroda, Germany) that captures the image using a laser line at a specified angle [28].

Statistical Processing of Measured Data
From the measured data of the total color difference ∆E* of the wood of the studied trees and the roughness of the wood surface, the graphic dependences of ∆E* = f(t) and Ra = f(t) in the range of the temperatures were determined using the program STATISTICA 12 (Tibco, Palo Alto, CA, USA) where t = 105-135 • C and time τ = 9 h.Program processing of the measured results partially eliminated the influence of measurement errors caused by wood heterogeneity.The measured data were processed and the individual significance of the factors was evaluated using a multifactor analysis.

Results and Discussion
The process of steaming wood using the mentioned modes changes the color of the native wood from a light white-gray color with a yellowish tinge to the brown shades of the color achieved through mode I. and mode II.as well as to the dark brownish-gray color shades achieved through mode III.The saturation of wood coloring depends on the temperature of the saturated water steam in the technological process.The color of the wood in individual steaming modes is declared in Table 4, and the values on the coordinates of the CIE L*a*b color space are given in Table 5.

Statistical Processing of Measured Data
From the measured data of the total color difference ∆E* of the wood of the studied trees and the roughness of the wood surface, the graphic dependences of ΔE* = f(t) and Ra = f(t) in the range of the temperatures were determined using the program STATIS-TICA 12 (Tibco, Palo Alto, CA, USA) where t = 105-135 °C and time τ = 9 h.Program processing of the measured results partially eliminated the influence of measurement errors caused by wood heterogeneity.The measured data were processed and the individual significance of the factors was evaluated using a multifactor analysis.

Results and Discussion
The process of steaming wood using the mentioned modes changes the color of the native wood from a light white-gray color with a yellowish tinge to the brown shades of the color achieved through mode I. and mode II.as well as to the dark brownish-gray color shades achieved through mode III.The saturation of wood coloring depends on the temperature of the saturated water steam in the technological process.The color of the wood in individual steaming modes is declared in Table 4, and the values on the coordinates of the CIE L*a*b color space are given in Table 5.
Table 4.The color of the wood of the studied trees in the process of steaming.

Wood Native Wood Wood Color Modification Modes
Operating principle of the LPM-4 as follows: 1-camera, 2-laser, 3-sample and 4-distance between the LPM-4 and the measured object.

Statistical Processing of Measured Data
From the measured data of the total color difference ∆E* of the wood of the studied trees and the roughness of the wood surface, the graphic dependences of ΔE* = f(t) and Ra = f(t) in the range of the temperatures were determined using the program STATIS-TICA 12 (Tibco, Palo Alto, CA, USA) where t = 105-135 °C and time τ = 9 h.Program processing of the measured results partially eliminated the influence of measurement errors caused by wood heterogeneity.The measured data were processed and the individual significance of the factors was evaluated using a multifactor analysis.

Results and Discussion
The process of steaming wood using the mentioned modes changes the color of the native wood from a light white-gray color with a yellowish tinge to the brown shades of the color achieved through mode I. and mode II.as well as to the dark brownish-gray color shades achieved through mode III.The saturation of wood coloring depends on the temperature of the saturated water steam in the technological process.The color of the wood in individual steaming modes is declared in Table 4, and the values on the coordinates of the CIE L*a*b color space are given in Table 5.
Table 4.The color of the wood of the studied trees in the process of steaming.

Wood Native Wood Wood Color Modification Modes
Operating principle of the LPM-4 as follows: 1-camera, 2-laser, 3-sample and 4-distance between the LPM-4 and the measured object.

Statistical Processing of Measured Data
From the measured data of the total color difference ∆E* of the wood of the studied trees and the roughness of the wood surface, the graphic dependences of ΔE* = f(t) and Ra = f(t) in the range of the temperatures were determined using the program STATIS-TICA 12 (Tibco, Palo Alto, CA, USA) where t = 105-135 °C and time τ = 9 h.Program processing of the measured results partially eliminated the influence of measurement errors caused by wood heterogeneity.The measured data were processed and the individual significance of the factors was evaluated using a multifactor analysis.

Results and Discussion
The process of steaming wood using the mentioned modes changes the color of the native wood from a light white-gray color with a yellowish tinge to the brown shades of the color achieved through mode I. and mode II.as well as to the dark brownish-gray color shades achieved through mode III.The saturation of wood coloring depends on the temperature of the saturated water steam in the technological process.The color of the wood in individual steaming modes is declared in Table 4, and the values on the coordinates of the CIE L*a*b color space are given in Table 5.
Table 4.The color of the wood of the studied trees in the process of steaming.

Wood Native Wood Wood Color Modification Modes
Operating principle of the LPM-4 as follows: 1-camera, 2-laser, 3-sample and 4-distance between the LPM-4 and the measured object.

Statistical Processing of Measured Data
From the measured data of the total color difference ∆E* of the wood of the studied trees and the roughness of the wood surface, the graphic dependences of ΔE* = f(t) and Ra = f(t) in the range of the temperatures were determined using the program STATIS-TICA 12 (Tibco, Palo Alto, CA, USA) where t = 105-135 °C and time τ = 9 h.Program processing of the measured results partially eliminated the influence of measurement errors caused by wood heterogeneity.The measured data were processed and the individual significance of the factors was evaluated using a multifactor analysis.

Results and Discussion
The process of steaming wood using the mentioned modes changes the color of the native wood from a light white-gray color with a yellowish tinge to the brown shades of the color achieved through mode I. and mode II.as well as to the dark brownish-gray color shades achieved through mode III.The saturation of wood coloring depends on the temperature of the saturated water steam in the technological process.The color of the wood in individual steaming modes is declared in Table 4, and the values on the coordinates of the CIE L*a*b color space are given in Table 5.
Table 4.The color of the wood of the studied trees in the process of steaming.

Wood Native Wood Wood Color Modification Modes
Operating principle of the LPM-4 as follows: 1-camera, 2-laser, 3-sample and 4-distance between the LPM-4 and the measured object.

Statistical Processing of Measured Data
From the measured data of the total color difference ∆E* of the wood of the studied trees and the roughness of the wood surface, the graphic dependences of ΔE* = f(t) and Ra = f(t) in the range of the temperatures were determined using the program STATIS-TICA 12 (Tibco, Palo Alto, CA, USA) where t = 105-135 °C and time τ = 9 h.Program processing of the measured results partially eliminated the influence of measurement errors caused by wood heterogeneity.The measured data were processed and the individual significance of the factors was evaluated using a multifactor analysis.

Results and Discussion
The process of steaming wood using the mentioned modes changes the color of the native wood from a light white-gray color with a yellowish tinge to the brown shades of the color achieved through mode I. and mode II.as well as to the dark brownish-gray color shades achieved through mode III.The saturation of wood coloring depends on the temperature of the saturated water steam in the technological process.The color of the wood in individual steaming modes is declared in Table 4, and the values on the coordinates of the CIE L*a*b color space are given in Table 5.
Table 4.The color of the wood of the studied trees in the process of steaming.

Wood Native Wood Wood Color Modification Modes
Operating principle of the LPM-4 as follows: 1-camera, 2-laser, 3-sample and 4-distance between the LPM-4 and the measured object.

Statistical Processing of Measured Data
From the measured data of the total color difference ∆E* of the wood of the studied trees and the roughness of the wood surface, the graphic dependences of ΔE* = f(t) and Ra = f(t) in the range of the temperatures were determined using the program STATIS-TICA 12 (Tibco, Palo Alto, CA, USA) where t = 105-135 °C and time τ = 9 h.Program processing of the measured results partially eliminated the influence of measurement errors caused by wood heterogeneity.The measured data were processed and the individual significance of the factors was evaluated using a multifactor analysis.

Results and Discussion
The process of steaming wood using the mentioned modes changes the color of the native wood from a light white-gray color with a yellowish tinge to the brown shades of the color achieved through mode I. and mode II.as well as to the dark brownish-gray color shades achieved through mode III.The saturation of wood coloring depends on the temperature of the saturated water steam in the technological process.The color of the wood in individual steaming modes is declared in Table 4, and the values on the coordinates of the CIE L*a*b color space are given in Table 5.
Table 4.The color of the wood of the studied trees in the process of steaming.

Wood Native Wood Wood Color Modification Modes
Operating principle of the LPM-4 as follows: 1-camera, 2-laser, 3-sample and 4-distance between the LPM-4 and the measured object.

Statistical Processing of Measured Data
From the measured data of the total color difference ∆E* of the wood of the studied trees and the roughness of the wood surface, the graphic dependences of ΔE* = f(t) and Ra = f(t) in the range of the temperatures were determined using the program STATIS-TICA 12 (Tibco, Palo Alto, CA, USA) where t = 105-135 °C and time τ = 9 h.Program processing of the measured results partially eliminated the influence of measurement errors caused by wood heterogeneity.The measured data were processed and the individual significance of the factors was evaluated using a multifactor analysis.

Results and Discussion
The process of steaming wood using the mentioned modes changes the color of the native wood from a light white-gray color with a yellowish tinge to the brown shades of the color achieved through mode I. and mode II.as well as to the dark brownish-gray color shades achieved through mode III.The saturation of wood coloring depends on the temperature of the saturated water steam in the technological process.The color of the wood in individual steaming modes is declared in Table 4, and the values on the coordinates of the CIE L*a*b color space are given in Table 5.
Table 4.The color of the wood of the studied trees in the process of steaming.

Wood Native Wood Wood Color Modification Modes
Operating principle of the LPM-4 as follows: 1-camera, 2-laser, 3-sample and 4-distance between the LPM-4 and the measured object.

Statistical Processing of Measured Data
From the measured data of the total color difference ∆E* of the wood of the studied trees and the roughness of the wood surface, the graphic dependences of ΔE* = f(t) and Ra = f(t) in the range of the temperatures were determined using the program STATIS-TICA 12 (Tibco, Palo Alto, CA, USA) where t = 105-135 °C and time τ = 9 h.Program processing of the measured results partially eliminated the influence of measurement errors caused by wood heterogeneity.The measured data were processed and the individual significance of the factors was evaluated using a multifactor analysis.

Results and Discussion
The process of steaming wood using the mentioned modes changes the color of the native wood from a light white-gray color with a yellowish tinge to the brown shades of the color achieved through mode I. and mode II.as well as to the dark brownish-gray color shades achieved through mode III.The saturation of wood coloring depends on the temperature of the saturated water steam in the technological process.The color of the wood in individual steaming modes is declared in Table 4, and the values on the coordinates of the CIE L*a*b color space are given in Table 5.
Table 4.The color of the wood of the studied trees in the process of steaming.From the measured values of the wood color of individual trees on the lightness coordinate of L* and the color coordinates of a* and b*, the total color difference of ∆E* of the wood color change in the steaming process was calculated using mathematical Equation (1).Graphical dependences of the total color difference of ∆E* on the temperature of the saturated water steam of individual modes are shown in Figure 3.  and steamed beech, birch and maple woods determined by steaming modes in a pressure autoclave.From the measured values of the wood color of individual trees on the lightness coordinate of L* and the color coordinates of a* and b*, the total color difference of ΔE* of the wood color change in the steaming process was calculated using mathematical equation (1).Graphical dependences of the total color difference of ΔE* on the temperature of the saturated water steam of individual modes are shown in Figure 3.The light white-gray color of native beech, birch and maple woods with a yellowish tinge was identified in the CIE L*a*b* color space with the values listed in Table 5.The measured values on the native wood color coordinates of the investigated trees are comparable to the values reported by the authors [20,23,[29][30].

Wood Native Wood Wood Color Modification Modes
During the steaming process of wet wood, depending on the temperature and time, the formation of acetic acid and formic acid occurs as well as the degradation of polysaccharides in the form of the oxidation of carbohydrates and pectin and dehydration The light white-gray color of native beech, birch and maple woods with a yellowish tinge was identified in the CIE L*a*b* color space with the values listed in Table 5.The measured values on the native wood color coordinates of the investigated trees are comparable to the values reported by the authors [20,23,29,30].
During the steaming process of wet wood, depending on the temperature and time, the formation of acetic acid and formic acid occurs as well as the degradation of polysaccharides in the form of the oxidation of carbohydrates and pectin and dehydration of pentoses turning into 2-furaldehyde; free radicals and phenolic hydroxyl groups also begin to form in lignin, resulting in the formation of new chromophoric groups that cause a change in wood color [31][32][33][34].
From the measured values of the lightness coordinate L*, it follows that the lightness (L 0 *) of native wood has a decreasing tendency depending on the temperature of saturated water steam where at the temperature of saturated water steam t III = 135 ± 2. with the increase in the temperature of the saturated steam is not directly proportional.At higher temperatures of the steaming process, the decrease in lightness is higher and the darkening of the wood is more pronounced.
The decrease in the values of the lightness coordinate L* is in accordance with knowledge about the darkening of wood in thermal and hydrothermal technological processes such as the steaming of wood declared in the works of [19][20][21][35][36][37], drying using warm moist air or using superheated water steam [38,39].
Changes in the chromatic coordinate of red color a* have an increasing tendency.The value of the red color of native beech wood a 0 * = 6.8 increases at the temperature of saturated water steam t III = 135 ± 2.5 • C to the value of a 3 * = 12.7.A similar increase in the red color coordinate of a* was also measured for birch and maple woods, as Table 5 declares.Significantly smaller magnitudes of changes were recorded for red color coordinate a* compared with the changes for lightness coordinate L*.The most pronounced red-brown color shade was recorded when wood was steamed at the temperature of saturated water steam t II = 125 ± 2.5 • C, which is visible from the values of ∆a*.
Changes in the chromatic coordinate of the yellow color b* are neither significant nor contradictory.In mode I., at the temperature of saturated water steam t I = 105 ± 2.5 • C, a slight increase in values was recorded for all woody species, and a slight decrease or oscillation was subsequently recorded at higher temperatures.These changes indicate the formation of less stable compounds with the absorption of the spectrum of electromagnetic radiation with a yellow color wavelength of 560 nm.The said compounds react with water steam or extraction products to form other thermal decomposition products with lower or zero absorption of the yellow wavelength electromagnetic radiation spectrum.
Changes in the color of the wood of the studied trees during the steaming process, in addition to changes in the individual coordinates of the CIE L*a*b color space, were also analyzed through the overall color difference of ∆E*.The total color differences of wood color change ∆E*, which were caused by the technological process of steaming using saturated steam at a temperature in the interval between 105 • C and 135 • C, lie in the range of values of ∆E* = 8.9-28.8.Changes in the color difference are determined mainly by changes in the lightness coordinate of L* of steamed wood and, to a lesser extent, by changes in the chromatic coordinates of red color a* and yellow color b*.
From the course of changes in the color of the wood of the examined wood species in the steaming process and the total color difference of ∆E*, the following points can be made: (a) A slight darkening and acquisition of a brown color shade of the wood in the interval of values ∆E* ≈ 8-10 occurs under the conditions of steaming beech, birch and maple woods at a temperature of saturated water steam of t I = 105 • C (Mode I); (b) Beech, birch and maple woods at a temperature of saturated water steam t II = 125 • C (Mode II) and also beech wood at a temperature of saturated water steam t III = 135 • C (Mode III) acquire a significant darkening of the wood to a brown-red color in the interval of values ∆E* ≈ 16-21; (c) Under the conditions of steaming birch and maple wood at a temperature of saturated water steam t III = 135 • C (Mode III), the wood acquires a dark brown-red color.The total color difference has a value of ∆E* ≈ 23-29.
The achieved color changes in the process of steaming the wood of the studied trees were classified according to the achieved color shades and color changes according to the classified classification degrees of ∆E* given in the work of Dzurenda [40].
The process of steaming wood through saturated water steam changes the properties of the surface of the milled material (roughness) depending on the steaming temperature and the type of steamed wood, as declared in Figure 4.
Alongside the dependence of the average roughness of the milled surface of beech wood on the color change of the steamed wood, the roughness of the milled surface decreases as the value of the total color difference of ∆E* increases.
If the average roughness of the milled surface of unsteamed beech wood is Ra = 6.5 µm, then the roughness of the milled surface of red-brown beech wood acquired through steaming mode I. at the temperature of saturated water steam t I = 105 ± 2.5 • C is reduced.This reduction consists of the Ra = 12.3% roughness of the brown beech wood that was obtained through steaming mode II. at t II = 125 ± 2.5 • C. and decreases by Ra = 15.4%.Moreover, the roughness of steamed beech wood of a dark brownish-gray color acquired during steaming through mode III. with a temperature of t III = 135 ± 2.5 • C decreased by Ra = 16.9% compared with the roughness of native wood.The achieved color changes in the process of steaming the wood of the studied trees were classified according to the achieved color shades and color changes according to the classified classification degrees of ΔE* given in the work of Dzurenda [40].
The process of steaming wood through saturated water steam changes the properties of the surface of the milled material (roughness) depending on the steaming temperature and the type of steamed wood, as declared in Figure 4. Alongside the dependence of the average roughness of the milled surface of beech wood on the color change of the steamed wood, the roughness of the milled surface decreases as the value of the total color difference of ∆E* increases.
If the average roughness of the milled surface of unsteamed beech wood is Ra = 6.5 µm, then the roughness of the milled surface of red-brown beech wood acquired through steaming mode I. at the temperature of saturated water steam tI = 105 ± 2.5 °C is reduced.This reduction consists of the Ra = 12.3% roughness of the brown beech wood that was obtained through steaming mode II. at tII = 125 ± 2.5 °C.and decreases by Ra = 15.4%.Moreover, the roughness of steamed beech wood of a dark brownish-gray color acquired during steaming through mode III. with a temperature of tIII = 135 ± 2.5 °C decreased by Ra = 16.9% compared with the roughness of native wood.
Similarly, decreases in the surface roughness of steamed birch and maple woods are shown in Figure 4.When steaming birch wood, the roughness of the milled surface in individual modes decreased by RaI = 12.9%, RaII = 12.8% and RaIII = 13.4%.The roughness of the milled surface of steamed maple wood represented a decrease in individual modes at the levels of RaI≈11.9%,RaII≈10.8% and RaIII≈15.8%compared with the roughness of the surface of native wood.
The decrease in the roughness of the steamed wood can be attributed to the increased brittleness of the wood due to the loss of amorphous polysaccharides [41].The fragility of steamed wood causes the protrusions to break during the cycloidal movement of the tool in the machining process rather than because of the tearing off of the fibers.This fact results in the creation of a milled surface with less roughness.Authors of the study [42] state that the roughness of thermally treated wood, where an Ra value was employed, up to the thermal treatment temperature of t = 160 °C is lower than the roughness of native wood.From a thermal treatment temperature of above 160 °C, the roughness increases, and it is even higher than the roughness of native wood at a temperature of 210 °C.Similarly, decreases in the surface roughness of steamed birch and maple woods are shown in Figure 4.When steaming birch wood, the roughness of the milled surface in individual modes decreased by Ra I = 12.9%, Ra II = 12.8% and Ra III = 13.4%.The roughness of the milled surface of steamed maple wood represented a decrease in individual modes at the levels of Ra I ≈ 11.9%, Ra II ≈ 10.8% and Ra III ≈ 15.8% compared with the roughness of the surface of native wood.
The decrease in the roughness of the steamed wood can be attributed to the increased brittleness of the wood due to the loss of amorphous polysaccharides [41].The fragility of steamed wood causes the protrusions to break during the cycloidal movement of the tool in the machining process rather than because of the tearing off of the fibers.This fact results in the creation of a milled surface with less roughness.Authors of the study [42] state that the roughness of thermally treated wood, where an Ra value was employed, up to the thermal treatment temperature of t = 160 • C is lower than the roughness of native wood.From a thermal treatment temperature of above 160 • C, the roughness increases, and it is even higher than the roughness of native wood at a temperature of 210 • C.
From the comparison of the measured values of the roughness of beech, birch and maple woods in relation to the wood density of individual wood species, both for native and steamed wood, it follows that the roughness of the wood surface increases with increasing density.In our case, the average roughness of the surface of unsteamed beech wood with a value of Ra = 6.5 µm is 4.6% higher compared with the roughness of the surface of birch wood with an average value of Ra = 6.2 µm as well as compared with the roughness of the surface of maple wood with an average value of Ra = 5.0 µm, which is 23.1% higher.
Similar findings result from a comparison of the surface roughness of steamed beech, birch and maple woods to changes in density during the steaming process.The process of

Figure 2 .
Figure 2. Operating principle of the LPM-4 as follows: 1-camera, 2-laser, 3-sample and 4-distance between the LPM-4 and the measured object.2.6.Statistical Processing of Measured Data From the measured data of the total color difference ∆E* of the wood of the studied trees and the roughness of the wood surface, the graphic dependences of ΔE* = f(t) and Ra = f(t) in the range of the temperatures were determined using the program STATIS-TICA 12 (Tibco, Palo Alto, CA, USA) where t = 105-135 °C and time τ = 9 h.Program
, 13, x FOR PEER REVIEW 6 of 11 Maple Table 4 lists the values on the coordinates of the CIE L*a*b* color space of unsteamed and steamed beech, birch and maple woods determined by steaming modes in a pressure autoclave.

Figure 3 .
Figure 3. Graphical dependence of the total color difference ΔE* on the steaming mode.

Figure 3 .
Figure 3. Graphical dependence of the total color difference ∆E* on the steaming mode.

Figure 4 .
Figure 4. Graphical dependence of the average roughness of wood on the steaming mode.

Figure 4 .
Figure 4. Graphical dependence of the average roughness of wood on the steaming mode.

Table 1 .
Modes of modification of blanks through saturated water steam.

Table 2 .
Technical and technological parameters of CNC machining center SCM Tech Z5 (ACM Group, 2017).

Table 2 .
Technical and technological parameters of CNC machining center SCM Tech Z5 (ACM Group, 2017).

Table 3 .
Parameters of the wood milling process.

Table 4 .
The color of the wood of the studied trees in the process of steaming.

Table 5 .
Measured values of the wood color of the examined trees in the CIE L*a*b* color space.

Table 5 .
Measured values of the wood color of the examined trees in the CIE L*a*b* color space.

Table 5 .
Measured values of the wood color of the examined trees in the CIE L*a*b* color space.

Table 5 .
Measured values of the wood color of the examined trees in the CIE L*a*b* color space.

Table 4
lists the values on the coordinates of the CIE L*a*b* color space of unsteamed and steamed beech, birch and maple woods determined by steaming modes in a pressure autoclave.

Table 5 .
Measured values of the wood color of the examined trees in the CIE L*a*b* color space.

Table 5 .
Measured values of the wood color of the examined trees in the CIE L*a*b* color space.