## 3 Results and Discussion
The presented diagram indicates a significant thickness reduction of fir and linden samples whose hardness is not significantly increased. In hardwoods, i.e., ash, walnut, oak, and beach, a smaller change in thickness causes a greater increase in hardness. For each type of wood, the diagram enables one to determine the penetration depth needed to achieve the desired hardness. As both sides of the workpiece are almost equally deformed, the roller's penetration depth equals approximately half of the measured thickness reduction. Only the samples with no visible imperfections were selected to ensure the homogeneity of the test material, as recommended in the literature. Namely, according to [PERSON] [2], on a macroscopic scale (up to 200 mm), such samples can be considered homogeneous. As normal force affects both sides equally and the sample width is equal on both sides, as well as the diameters of rollers, the stresses and deformations are expected to be approximately  
Figure 7: The change and percentage increase in hardness of wooden workpieces after roller pressing. equal on both sides. The experiments have proven that the proposed method of wood processing does not cause changes in the shape of the samples; thus, based on findings and literature sources, it can be concluded that the deformations of the samples were almost equal on both sides. Figure 8: Microstructure linden sample after roller pressing: (**a**) at a depth h/2, (**b**) at a depth h/4, (**c**) the surface layer. Figure 9: Change in the hardness of wooden workpiece after roller pressing, depending upon the penetration depth, i.e., the thickness reduction. In the case of fir and linden, the achieved improvement of hardness was not very significant. The maximum increase in hardness of these two types of wood was observed after processing them by applying the force of 1177 N. The hardness of fir was increased by 34.3%, while the increase in hardness of linden was 25.2%. A further increase in the pressing force does not produce a greater hardness, so it has no practical significance. As shown in Figure 10, the pressing force, when increased from 1177 N to 1570 N, destroys the wood structure, which reduces the hardness of the fir sample. Results of the elastic modulus measurement (Tables 1 and 3, Figure 11) indicate that all wood samples had an increase in the elastic modulus after being processed by roller pressing. The most significant relative increase in the elastic modulus of 66.73% was measured on a linden sample, processed with the force of 1177 N. Elastic modulus of a fir sample increased by 37.24% after being processed by applying the force of 1177 N. As discussed before, a further increase in the pressing force has no practical significance because it only leads to the damaging of the wood structure (Figure 10). From the measurement results, it can be concluded that the most significant relative increase in the elastic modulus was achieved on softer types of wood, i.e., on samples that, under the action of a specific pressing force, suffered the largest plastic deformations, which caused the largest increase in density (Figure 12). The diagrams in Figures 7, 9, 11 and 12 and Tables 2 and 3 clearly show that the relative increase in hardness is not associated with an identical relative increase in the elastic modulus. For example, the diagram in Figure 12 indicates that the elastic modulus of oak, ash, and walnut samples increased after the treatment with pressing forces of 392 N and 785 N. Larger values of pressing force did not produce a significant increase in elastic modulus, but they did improve the hardness (Figure 9). Therefore, it can be concluded that the proposed method has great potential in the processing of hardwood elements (ash, beech, oak, and walnut) for the furniture industry, where hardness is considered a primary quality criterion. Figure 10: Structure of the fir sample after the processing by applying the force of 1570 N: (**a**) cracks, magnified 5\(\times\), (**b**) crack, magnified 20\(\times\), (**c**) damaged macrostructure.