Impact of Wood Structure Variability on the Surface Roughness of Chestnut Wood

Abstract

Wood constitutes a unique and valuable material that has been used from ancient times until nowadays in a wide variety of applications, in which the surface quality of wood often constitutes a critical factor. In this study, the influence of different wood areas and therefore, of different anatomical characteristic areas of chestnut wood (Castanea sativa Mill.) on the surface quality, was thoroughly studied, in terms of surface roughness. Five different chestnut tree trunks were harvested, from which five different disks were obtained corresponding to five different trunk heights. Surface roughness was measured on these disks on the transverse, radial, and tangential planes, on the areas of sapwood and heartwood, measuring the roughness in each point both vertically and in parallel to the wood grain. The results revealed that the examined roughness indexes (Ra, Rz, Rq) follow a parallel path to one another. In the case of all surfaces (transverse, radial, tangential) of the disks examined, when the measurement was implemented perpendicularly to the wood grain, a significantly higher roughness was recorded, compared to the wood grain measurements being implemented in parallel with the wood grain. Significant differences between heartwood and sapwood roughness were not demonstrated, although sapwood often appeared to exhibit a higher surface roughness than heartwood sites. Among the roughness values of the three different surfaces, the highest roughness in the vertical-to-wood-grain measurements was recorded by tangential surfaces, with slightly lower values on the transverse surfaces and the lowest roughness on radial surfaces. Meanwhile, for the measurements in parallel with the wood grain, the transverse surfaces presented significantly higher roughness values compared to the tangential and radial surfaces. Significant roughness differences were not detected among the surfaces at different trunk heights. Although, significant differences in roughness were recorded among different trees, it was observed that all the studied trees align with the identified and described within-tree trends.

1. Introduction

The quality of wood surfaces is highly crucial for the manufacturing of qualitative wood-based products and structures. More specifically, the surface roughness is a matter of great interest for numerous applications of wood in small- or bigger-dimension structures (furniture, floors, frames, paneling, table-tops, etc.), defining their appearance, texture, aesthetics, and user-generated sensation, among others [1,2]. Furthermore, low surface roughness values, with regard to several wood species and used adhesives, have demonstrated higher shear bonding strength results [3]. Wood surface quality depends mainly on the wood structure and the implementation of wood mechanical processing procedures, such as cutting, sanding, finishing, painting, curving, applying preservation methods, adhesives, coatings, other substance layers, etc. [4,5,6,7].

As a biological anisotropic material with a multidimensional surface area, wood texture is closely related to its structural and anatomical features (fibers, pores, tracheids, rays, etc.) and its formation and appearance are affected by various factors, either environmental or genetic. Different wood species, either softwood or hardwood, of different origin appear to show variability in structure and properties [8,9], while even in the same trunk, large differences in wood structure can be detected and, therefore, in wood surface quality as well. Characteristics, such as the growth ring width, their homogeneity and appearance, cell wall thickness, cell type composition and wood density, heartwood and sapwood ratio, earlywood and latewood ratio, wood moisture content, surface planes (tangential, radial, transverse), etc., undoubtedly influence the quality of a wood surface, forming numerous geometric peaks and valleys [10,11]. As reported by Sadoh and Nakato [12], the diffuse-porous wood species present lower surface roughness values than the ring-porous wood species. Örs and Gürleyen [13] reported that compared to the radial planes, the tangential planes presented higher surface quality. Lower surface roughness has been recorded in latewood than the earlywood [14], probably because of the thicker cell walls and the higher density of latewood. The factors of ring width, wood density, and ring angle also seem to highly influence the surface roughness [15].

Moreover, the mechanical processing using different cutting machines and tool influences also affect the wood surface roughness. The final quality of a wood surface is strongly dependent on cutting procedure kinematics, the feed speed, the rake angle [14], and other preparation processes such as sanding and finishing [16,17], with machining defects such as fuzzy, torn, or raised grain being correlated with high surface roughness. According to the literature, slow-feeding wood planes outperformed high-feeding wood planes in terms of surface roughness [18]. The level of maintenance of these machines and the cutting means they carry, the storage conditions of the wood, the moisture content fluctuation until the final use, and the subsequent dimensional stability of the wood, all constitute factors that usually provide a totally different wood substrate in terms of roughness [7,19].

Therefore, roughness reflects the combined effect of several different factors simultaneously interacting and perhaps that is one of the reasons why it has not been thoroughly comprehended so far, although it constitutes a property that significantly affects the utilization degree of wood in several applications. It has also been revealed that surface quality, and more specifically, surface roughness, is closely related to the duration and service life of timber and the respective structures it participates in [20]. Smoother surfaces prove to be more resistant to stress and wear [14]. Since high roughness corresponds to discontinuities in the wood tissue, it is expected and inevitable that this is also associated with the retention of a higher moisture content, the higher potential of wood biological damage, wood degradation by the action of microorganisms, a deterioration in wood substrate, etc. Therefore, the smoothness of wood surfaces has been associated with a longer service-life duration of wood and wood structures.

Currently, empirical procedures and models have been applied, using several surface roughness indexes of Ra, Rz, Rq, Rk and Rap, among others [21]. However, the most commonly used surface roughness indexes being recorded, in order to define wood surface roughness, are as follows: Ra, which corresponds to the average of the values of the roughness profile; Rz, the mean value of the roughness depths of different sampling lengths; and Rq, the largest roughness depth width. Determination methods can be applied either with or without contact with the wood surface. Contact methods employ a stylus tip, pneumatic methods, and tactile sensation [22]. A quite commonly used method is the use of a profilometer bearing a diamond stylus, which runs a path on the surface and records the surface roughness in different directions of wood grain [20,22,23].

The wood of chestnut is regarded as being classified among the most valuable timber species of Europe and presents a great range of uses and applications in the form of round timber, technical sawn wood, floors, furniture, high-value items, etc. [24,25]. It is a ring-porous hardwood species with earlywood vessels that are of significantly higher diameter than those of latewood, presenting a clear ring arrangement [25]. Especially when sapwood is being transformed into heartwood, the earlywood vessels are usually full of tyloses, while latewood vessels are polygonal and are found in groups generating a flame-like design. This species’ rays are more often uniseriate, rarely biseriate, and of different heights [8]. The parenchyma is mostly apotracheal and rarely paratracheal in places [8,25,26]. Chestnut wood is considered to be of medium density, approximately 0.57–0.63 g/cm³, and in general, density is strongly related to ring width. Especially for ring-porous hardwoods such as chestnut, growth rings of high width tend to demonstrate higher values of density, due to the fact that earlywood remains more or less stable, while the increase in growth ring width is attributed to the increase in the latewood part. However, density is also slightly affected by the cambium age and appears to decrease as the tree ages [27,28]. The outdoors exposure of wood, where intensive changes in environmental conditions take place, concerning relative moisture content, atmospheric precipitation, UV radiation, etc., causes chestnut wood to gradually deteriorate and discoloration of the surfaces occurs, regardless of whether the wood has been coated with mild hydrophobic solutions or not, which subsequently deteriorate the quality and roughness of wood surfaces [29].

According to the literature, only Sutcu and Karagoz [30] have dealt with surface quality of chestnut among other species. More specifically, they investigated how machining conditions (feed rate, spindle speed, step-over, axial depth, etc.) affected the roughness of chestnut, beech, and walnut specimens, concluding that the wood roughness was greatly influenced by the factors of cut depth, feed rate, and spindle speed. Therefore, a great lack of research measurements and data have been identified in the literature with regard to the surface roughness of chestnut wood and the factors influencing it, although chestnut constitutes such an important timber species, frequently used in applications where roughness is a crucial parameter. To the best of our knowledge, in the literature, there is no study providing surface roughness data of chestnut wood, concerning the potential surface roughness vertical variability, the variability among wood sections horizontally (sapwood, heartwood)/planes (transverse, radial, tangential), or findings/information about any potential correlation between structural characteristics and the roughness of chestnut wood.

Therefore, the aims of the current study are to thoroughly examine the surface quality of chestnut wood, in terms of surface roughness, examining the potential differences in the three wood surface planes (transverse, radial, and tangential), in the different areas of sapwood and heartwood, and to examine the potential variability “among different chestnut trees” and “among different trunk heights” of surface roughness. In addition, “between different direction measurements” (vertically/in parallel with the wood grain) will also be examined in order to conduct a thorough roughness characterization of chestnut wood. Potential correlations between wood structure variables and surface roughness are going to be examined. The implementation of the current research is anticipated to provide an insight into the scientific field of wood surface quality and roughness, as well as the impact of the anatomical characteristics of chestnut wood material on wood roughness and its rational utilization in various applications.

2. Materials and Methods

2.1. Sample Preparation

For the purposes of this study, five chestnut (Castanea sativa Mill.) trees were harvested from a coppice forest in Sithonia Peninsula (Chalkidiki, Greece). The trees were as straight as possible, without any apparent defects. The trees were aged 25–27 years and their diameter ranged from 19.1 cm to 24 cm. Tree trunk disks of 3 cm thickness were obtained for approximately every 1 m of height from the tree trunk base (starting from the height of 1 m) to the top, taking a total of 5 disks per trunk (25 disks in total).

The disks were transferred to laboratory infrastructure and conditioned in a closed chamber under stable conditions (60% relative humidity, 20 ± 3 °C) until a constant weight was achieved. The moisture content of the disk wood was recorded according to the ISO 13061-1 [31] standard to be 7.6–8.5%. All the measurements were performed on the different surface planes of these disks (Figure 1).

First of all, the disks were further cut using a band saw (Figure 2A) in order to form samples of smaller dimensions bearing clear transverse, radial, and tangential surfaces concerning the heartwood and sapwood areas of wood (Figure 2B). Afterwards, the samples were code marked, sanded using 80-grit sandpaper on a sanding machine under the same processing conditions (TC-US400, Einhell, Germany) (Figure 2C), and a polishing technique was applied to all the samples to ensure the comparability and reliability of the results. In general, if high-grit sanding is applied during surface preparation, roughness would be only affected by wood structure and ingenuine properties. However, it is difficult to relate wood anatomy to roughness, since sanding is a procedure that may be conducted in different conditions and with different means. For instance, when high-speed cutting is performed, softwood species are strongly affected [4,32]. Therefore, in the current study, only one and the same person/operator implemented all the sample sanding processes, applying the same sanding process on the same device, spending the same time on each wood sample (approximately 2 min/surface). All the specimens were visually and empirically examined and determined to have been appropriately sanded.

2.2. Physical and Chemical Property Assessment

Prior to the roughness measurement, the dry density and maximum moisture content of the samples (maximum moisture that can be absorbed/retained) were determined according to processes described by Tsoumis [8], applying the equation of R₀ = M₀/V₀, where R₀ corresponds to the dry density (g/cm³), M₀ is the dry mass (g), and V₀ is the dry volume (cm³). Specifically, samples were formed in stripes without bark (beginning from the diameter line of the trunk cross-section) and then, each stripe was split into pieces of approximately 2–5 annual rings. Dry volume of the pieces was estimated with the application of the water displacement method based on Archimede’s principle (which states that a body immersed in a fluid is subjected to an upwards force equal to the weight of the displaced fluid. This is a first condition of equilibrium) and was the dry weight after heating for 24 h in an oven at 103 ± 2 °C. Maximum moisture content was determined by applying weight measurements before and after the kiln drying for 24 h and until the stabilization of the values. In most cases, measurements were conducted before and after the extraction of the samples with hot water, so as to evaluate the relation between the extractives presence and maximum moisture content and dry density and therefore, also with wood surface roughness.

The wood extractive content (of those extracted with boiling water, referring mainly to tannins, gum, sugars, coloring substances) of chestnut wood was also measured in the current research, applying the common lab process (100 °C, 3 h, in a thermal jacket and water vapor cooler system) [9].

2.3. Roughness Measurements

The measurements were conducted using a profilometer “Mitutoyo Surftest SJ-301” fine stylus, based on ISO 21920-2:2021 [33] methodology (Figure 3). The measurements were implemented on the prepared disks, at selected wood surface areas without any defect, both in parallel with and perpendicular to the grain, on transverse, radial, and tangential sections/planes of heartwood and sapwood areas. The applied methodology [33], as well as the instructions of the profilometer manufacturer and previous published studies [5,34,35] were followed. The parameters of roughness that were recorded were Ra (mean arithmetic profile deviation), Rz (mean peak to valley height), and Rq (maximum roughness), based on previous relevant studies dealing with wood surface quality. Approximately 15 measurements were obtained from each studied case (heartwood/sapwood, transverse/radial/tangential section, parallel/perpendicular to the grain).

The measurement points were chosen randomly on the surface of the samples, in order to ensure that both areas of earlywood or latewood in the three directions would be involved and the whole surface area of each sample would be covered. More specifically, a 2-dimensional rectangular sampling grid with points spaced 10 mm apart was placed over each of these created areas of the wood disk specimens. Points found at each intersection of the grid were marked on the surface and included in the measurements. Particular attention was paid to ensure the representativeness of the samples.

Additionally, prior to the process of roughness measuring, a calibration of the instrument preceded, and temperature conditions were approximately 20 ± 3 °C [7,34,36].

2.4. Statistical Analysis

The statistical analysis of the results was implemented, initially applying a one-way ANOVA, using the statistical package of SPSS (Statistics PASW), in order to identify which of the surface roughness values of these categories differed from one another in a statistically significant manner. Secondly, a two-way ANOVA was applied examining each of the variable pairs, in which significant differences were recorded. In this way, potential variable correlations were investigated and the impact of each of the different factors (independent variables) on the dependent variable, level of surface roughness (Ra), were assessed. All possible pairings and correlations among the variables were examined using the “enter” approach. Non-statistically significant variables were excluded from the analysis through the “stepwise” approach. The “stepwise” method, known for its strictness, permits only statistically significant variables with a meaningful effect on the dependent variable to be included in the multiple linear regression analysis. To detect potential autocorrelation in the residuals of the regression analysis, the Durbin Watson statistic was calculated. All statistical analyses were conducted with a significance level set at p-value = 0.05.

3. Results and Discussion

3.1. Physical and Chemical Properties Assessment

According to the results of this research, the average wood dry density was approximately 0.60 g/cm³ and ranged from 0.57 g/cm³ to 0.63 g/cm³ (

Table 1. Density (dry and basic), maximum moisture content values (%), and extractive content (%) of the chestnut wood studied.

Tree		Dry Density g/cm3	Basic Density g/cm3	Maximum MC %	Extractives %
1	x	0.565	0.526	110.236	13.208
	s±	0.022	0.022	7.683	3.179
	n	20	20	20	4
2	x	0.595	0.599	102.235	11.385
	s±	0.033	0.030	7.063	2.326
	n	23	23	23	4
3	x	0.599	0.561	100.696	14.133
	s±	0.019	0.0202	4.736	2.6441
	n	21	21	21	4
4	x	0.607	0.564	96.651	15.084
	s±	0.025	0.026	5.958	3.2797
	n	20	20	20	4
5	x	0.633	0.583	92.354	10.637
	s±	0.037	0.030	6.951	1.6802
	n	23	23	23	4
Mean	x	0.600	0.599	100.434	12.889

x: mean value, s±: standard deviation value, n: number of examined samples.

). The mean maximum moisture content of chestnut wood was measured to be 100.43%.

Extractive content, as well as other factors such as the presence of tyloses, especially in the heartwood area, are considered to be strongly related to increased wood density especially in the heartwood area, and as a result, to a decrease in surface roughness. The roughness and, in general, the surface quality of wood, is closely related to its density, as well as its anatomical characteristics and chemical composition [9]. More specifically, the higher the density of the wood, the smoother its surface, most of the time.

Therefore, as can be seen in Table 1, chestnut is a species with a high content of extractives and this is in line with medium to high values of its density, especially in the heartwood area [8,27].

3.2. Surface Roughness

The three examined roughness indexes, Ra, Rz, and Rq, follow a parallel path to one another, a trend that is also highlighted in the provided summary/clustering diagrams of the five trees (Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8). Based on this and the available literature, the Ra index was used in the statistical analyses of the current study to detect correlations among different wood structure factors.

Chestnut wood, characterized by its medium value of density (approximately 0.57 g/cm³) and relatively medium to high hardness values (approximately 41.07 N/mm² tangentially and 40.29 N/mm² radially), was expected to show medium surface roughness values [37,38,39]. In general, density and porosity are inversely correlated with each other; therefore, both highly affect smoothness, since high-density samples show lower roughness, though sometimes, also a higher resistance during the sanding process [38,39]. Fortino et al. [40] found a correlation of surface roughness with the hardness and scratch resistance of the wood, which applied in the presence of different moisture contents. They concluded that the effect of wood structures such as earlywood and latewood, and sapwood and heartwood were crucial factors. For instance, the sapwood zones in softwood species show a higher scratch resistance (compared to heartwood), with higher scratch resistance values recorded in the tangential direction. Finally, the scratch resistance is lower in earlywood locations compared to latewood.

In the current study, statistically significant differences in roughness were not detected among the values of the different heights of the tree (among the different tree disks, 1–5, of each tree) when comparing the roughness values in the same variables (measurement direction, specimen’s surface planes, areas of sapwood/heartwood). More specifically, according to the statistical analysis, the surface roughness (Ra) was affected by only 4.5% by the height of the specimen location in the trunk (different disks), presenting higher roughness in the higher tree heights. Statistically significant differences were recorded only between the lowest disks (1–2) and the highest disk (disk 5), proving that there is a slight difference in the surface roughness of the wood depending on the location of the wood specimen in the trunk longitudinally (trunk height). This could be probably attributed to the fact that in the lower tree heights, near the base of the tree, the wood is more mature and the density is higher due to the higher proportion of heartwood, among other factors.

The surface roughness, in general, was affected by 72.9% by the factors “Measurement Direction” (vertically/parallel), “Planes” (transverse, radial, tangential surfaces), and the factor “Area” (sapwood/heartwood) in combination. The interaction of the factor trunk “Height” with the abovementioned factors of “Direction”, “Planes”, and “Area” affected the variability of Ra only by 8%, providing more evidence that the impact of trunk height can be considered to be of lower significance.

Nevertheless, significant differences appeared among different trees (comparing the values of measurements of the respective variables of direction, plane types, disks, etc.), with “Tree 3” revealing the statistically significant and lowest roughness compared to the rest of the trees, while the described within-tree trends that were detected apply to each of the tree cases.

The results of the current study also revealed that in all surface planes (transverse, radial, tangential) of the examined disks, when the measurement was implemented vertically to the wood grain, a statistically significant higher roughness value was recorded, compared to the measurement implemented in parallel with the wood grain, which corresponds to statistically significant differences in all the studied cases. It is characteristic that 67.6% of the variability of roughness is being influenced by the factor of measurement “Direction” (orientation of vertically/parallel to wood grain). This tendency could be easily explained, taking into account that vertical to the wood grain, higher height differences are encountered due to earlywood–latewood transition zone areas, different growth rings, etc. Chestnut, as a ring-porous hardwood species, also demonstrates differences in cell wall thickness between the earlywood and latewood areas and as is widely accepted [41], the surface roughness is strongly associated with cell wall thickness. More specifically, latewood fibers present thicker cell walls than those of earlywood [8]. The presence of earlywood vessels that are characterized by a much higher cell diameter compared to latewood results in density differences in each of the growth rings [8].

In this study, statistically significant differences between heartwood and sapwood roughness values were not demonstrated, although sapwood appeared in some cases to exhibit slightly a higher surface roughness than the corresponding heartwood sites (referring to the same direction of roughness measurement). The wood tissue found in the heartwood part of the trunk consists of cells that have ceased to serve as part of the tree’s conduit system and the cells have been filled with storage/healing substances, extracts, etc., presenting a slightly higher density.

The density of wood, in combination with its structural characteristics and chemical composition, are all strongly correlated with its roughness [9], with the higher density corresponding most of the time to smoother surfaces.

Among the roughness values of the three different surfaces examined on the disks, it was observed that the highest roughness values (Ra) were detected, in most disk cases, on the tangential surface of the disks, then on the transverse disks, and finally, the lowest Ra values were recorded on the radial surfaces (concerning the vertical-to-the-grain measurements). This finding could be attributed to the presence of radii on the radial surfaces that probably make the wood surface smooth, as well as to the fact that the tangential surfaces in the samples taken corresponded mostly to the sapwood part of the tree, and therefore contained a higher proportion of sapwood than heartwood, which probably contributed to an increase in the surface roughness. Additionally, concerning the vertical measurements of all the categories, statistically significant differences were not recorded among the different categories. Meanwhile,, regarding the parallel-to-the-grain measurements of all the categories, statistically significant differences were found, with the transverse surfaces of the examined wood specimens to reveal the highest roughness values (Ra) (Figure 9). Therefore, when the roughness measurements were conducted parallel to the wood grain, the highest roughness was observed in transverse surfaces, which corresponded to a statistically significant difference between transverse and the other two surface planes (tangential and radial), and these two did not differ significantly from one another. This much higher roughness recorded in transverse surfaces (when measured parallel to the wood grain) could be explained by the fact that chestnut is a ring-porous wood species and when the measurement of roughness is conducted in parallel orientation to the wood grain, there is a high potential for the measurement to be implemented alongside the earlywood area, which consists of cross-sectional cut vessels (of large diameter) [8] that would definitely increase the roughness of the surface.

After the examination of all possible pairs of factors in terms of combination and interaction, we decided to apply a “decomposition” of the statistical analysis, abstracting the factor “Area” (heartwood–sapwood) due to the fact that it did not demonstrate a statistically significant impact. Moreover, the results of the remainder of the variable combinations revealed that the 67.6% of the variability of surface roughness (Ra) is being influenced by the factor of measurement “Direction” (vertically/parallel) and only 7.8% by the factor “Planes” (transverse, radial, tangential), proving that the “Planes” variable, although inducing statistically significant differences in roughness values, has quite a low impact on the surface roughness (Figure 10).

Additionally, regarding the transverse plane measurements, the Ra variability was found to be influenced by the factor measurement “Direction” (vertically/parallel) by 20.9% (

Table 2. Analysis of variance output—Univariate tests of Ra (dependent variable).

Surface Planes		Sum of Squares	df	Mean Square	F	Sig.	Partial Eta Squared
Transverse	Contrast	1383.516	1	1383.516	395.408	<0.001	0.209
	Error	5227.442	1494	3.499
Radial	Contrast	4972.470	1	4972.470	1421.129	<0.001	0.488
	Error	5227.442	1494	3.499
Tangential	Contrast	5373.363	1	5373.363	1535.704	<0.001	0.507
	Error	5227.442	1494	3.499

df: degrees of freedom, F: F-value is the ratio of between-group and within-group variation, Sig.: significance.

). In the radial plane measurements, the impact of the measurement “Direction” factor on Ra was found to be 48%, while in tangential measurements, 50.7% of the Ra variability was affected by the “Direction” factor. The effect of the “Planes” factor (transverse, radial, tangential) on the Ra values with regard to the vertical measurements was very low (1.1%), while for the parallel measurements, it was 18.5%.

4. Conclusions

The results of the current study revealed that the three roughness indexes, Ra, Rz, and Rq, that were studied had a parallel progression with one another. When the measurement was carried out vertically to the wood grain on any of the disks’ surfaces (transverse, radial, or tangential), the resulting roughness was noticeably higher compared to when the measurements were implemented in parallel to the wood grain. Although sapwood seems to more frequently exhibit higher surface roughness than the comparable heartwood areas, no discernible differences in roughness between the two types of wood were found. When the roughness measurements were conducted vertically to the wood-grain, the tangential surfaces demonstrated slightly higher roughness values among the three surfaces (transverse, radial, and tangential), with the transverse surfaces showing slightly lower roughness values, and the radial surfaces showing the lowest values. Nevertheless, when the measurements were in parallel to the wood grain, the transverse surfaces had significantly higher roughness values compared to the tangential and radial surfaces. There were no significant variations in surface roughness among the different disks or trunk heights. Conversely, notable variations in roughness were seen amongst the various trees, with Tree No. 3 exhibiting the lowest roughness (and the most significant statistical difference among the trees studied), when compared to the other trees. But, as it happened, each of the studied chestnut trees was found to fit the recognized and described within-tree trends in detail.

The findings of this research are expected to contribute to the rational and thorough utilization of chestnut wood, which constitutes a particularly significant, commercially valuable species of hardwood, as well as to the integration and advancement of fundamental scientific knowledge, which is crucial for guaranteeing the generation of wooden structures characterized by superior surface quality and a longer service life.