## 2 Materials and Methods
On the veneers, 10-12 measurements of roughness parameters were randomly implemented on the surface of typical wood structure areas and, respectively, another 10-12 measurements of roughness were conducted on non-typical wood structures of veneer surfaces in the peripheral area of each defect (10-30 mm radius around the defect). The defects were different for each wood species, including knots (Figure 1), tensile wood, irregular annual rings, deflection of wood fibers (spiral grain), discoloration (Figure 2), decay (Figure 3) etc., since, as with the material of wood, its defects, as well, are unique. The number of 3 to 10 different veneer sheets were measured from each veneer's package and for each wood species, in order to investigate the potential differentiation of roughness as a function of different wood depths on the defects' development areas. Figure 1: Dead knots in oak wood sliced plain-cut veneer (**a**), and in chestnut wood sliced veneer (**b**). The roughness parameters of the prepared veneer surfaces were evaluated using a fine stylus type profilometer, Mitutoyo Surftest SJ-301 (Figure 4), with the profile tracing method using the diamond stylus of the device, according to ISO 4287:1997 [17]. The stylus technique was determined to be used, since compared to the other methods, such as pneumatic, laser, and acoustic emission, it is accurate, practical, and repeatable [5]. The measuring speed, the diameter of the pin and the upper angle of the pin tool were 10 mm / min, 4 \(\upmu\)m, and 90\({}^{\circ}\), respectively. The sampling length was of 2.5 mm, and the evaluation length was of 12.5 mm (five times of the sampling length). The values of the surface roughness parameters were determined to be within \(\pm\)0.01 \(\upmu\)m. The measurements were implemented in a direction perpendicular to the direction of grain orientation. Figure 4: Roughness test using the Mitutoyo Surftest SJ-301 profilometer (**a**); an example of the roughness spectrum on a poplar sliced veneer sheet of typical structure (**b**). Figure 3: Polar wood sliced plain-cut veneer sheet with live knot and decay (**a**), and cherry wood veneer with irregularities of annual rings (**b**). Figure 2: Ash wood sliced veneer with discoloration and irregularities of annual rings (**a**), and live internal knot with spiral grain around the knot (**b**). The roughness measurement points were randomly selected by marking them on the surface of the samples, in order to cover the whole area. Three roughness parameters, the mean numerical deviation from the midline profile along the entire length of the stylus movement (mean arithmetic deviation of profile--Ra), the average height between the peak-valley derived from five identical lengths of the profile (mean peak-to-valley height--Rz), and the distance between peak and valley points of the profile, which can be used as an indicator of the maximum defect height within the assessed profile (maximum roughness--Ry), have been widely used in previous studies [18; 19; 20], where detailed information about these roughness parameters has been presented. These parameters, employed also in the current study, have also been used previously in the quantification of surface quality of veneers [4; 21], and other wood composites, and they are defined by the respective roughness standards [17]. Prior to each measurement, the instrument was calibrated and the roughness measurements were performed at room temperature (\(20\pm 2\)\({}^{\circ}\)C) [22; 23].