## 4 Discussion  
To be able to have a conclusive discussion regarding the results obtained in this work, all results are grouped in Table 7. Also, this table includes the comparison values obtained for paulownia and balsa wood to those obtained for linden wood, corresponding to one of the relationships (Equation (10)). The first analysis of the results shown in Table 7 was a comparison between the characteristics of the three species, namely how close the characteristics of paulownia and balsa are to those of linden wood. It was clearly observed that paulownia wood was much more appropriate as a replacement for linden wood with a total of 526.6 points, thus being 4.4 times more appropriate than balsa wood. Paulownia wood had a density lower than that of linden by 34%, but higher than that of balsa wood by 1.62 times. It was observed that the experimental value of balsa wood density was closest to that obtained by other researchers [1], with the difference only 1.6% lower. In the case of linden wood, the difference between the two values was higher than 9.7%, and in the case of paulownia wood, the difference was higher than 21.6%. From the point of view of density, the three analyzed species are different as the differences are greater than 5-10% between them, paulownia wood being more appropriate as a replacement for linden [35; 36]. Some authors [17] found a wood density of about 300 kg/m\({}^{3}\) for paulownia, which was very close to the value of 304 kg/m\({}^{3}\) of this research. The analysis of color parameters with regard to the natural color of planed wood showed that from a color point of view, the three wood species are very appropriate (L = 21.8-25.1; a = \(-\)(35.2-37.6); and b = 3.6-4), with the points of all three species being positioned in the fourth trigonometric quadrant. Appropriate values were also found by other authors [5; 18], and a similar value of paulownia was obtained by others [17]. The water absorption of these three wood species was dependent on their porosity; it increased with a decrease in the density of these species. The exception to this rule was paulownia wood, which had a lower absorption than linden wood. The explanation for this phenomenon could be attributed to the microscopic structure of this wood species \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Characteristics**} & \multicolumn{2}{c}{**Linden**} & \multicolumn{2}{c}{**Paulownia**} & \multicolumn{2}{c}{**Balsa**} \\ \cline{3-8}  & & **Value** & **Value** & **Eq. 11\%** & **Value** & **Eq. 11\%** \\ \hline
1. & Unit density, \(\rho_{0}\), kg/m\({}^{3}\) & 461 & 304 & \(-\)34.0 & 118 & \(-\)74.4 \\ \hline
2. & Luminance & 21.8 & 25.1 & +15.1 & 24.7 & +13.3 \\ \hline
3. & a* value & \(-\)35.2 & \(-\)37.3 & \(-\)5.9 & \(-\)37.6 & \(-\)6.8 \\ \hline
4. & b* value & 4.0 & 3.6 & \(-\)10.0 & 3.6 & \(-\)10.0 \\ \hline
5. & Absorption, \% & 38 & 27 & +7.8 & 85 & \(-\)123.6 \\ \hline
6. & Tangential shrinkage (TS), \% & 4.4 & 2.5 & +43.1 & 1.4 & +68.1 \\ \hline
7. & Radial shrinkage (RS), \% & 3.1 & 1.3 & +58.1 & 1.6 & +48.3 \\ \hline
8. & Ratio, TS/RS & 1.4 & 1.8 & \(-\)28.5 & 1.2 & +14.2 \\ \hline
9. & Compressive strength, N/mm\({}^{2}\) & 52.9 & 42.5 & \(-\)2.3 & 9.1 & \(-\)82.7 \\ \hline
10. & Modulus of resistance, N/mm\({}^{2}\) & 92.3 & 62.1 & \(-\)32.7 & 14.1 & \(-\)84.7 \\ \hline
11. & Modulus of elasticity, N/mm\({}^{2}\) & 6782 & 4852 & \(-\)28.4 & 927 & \(-\)86.3 \\ \hline
12. & Brinell hardness (R), N/mm\({}^{2}\) & 20.4 & 14.4 & \(-\)29.4 & 11.1 & \(-\)45.8 \\ \hline
13. & Ecological coefficient O\({}_{2}\); CO\({}_{2}\) & 1 & 5.5 & +550 & 5.2 & +520 \\ \hline
14. & Quality coefficient, MOR/\(\rho_{\text{(g/cm${}^{3}$)}}\) & 200.2 & 204.2 & +1.9 & 119.4 & +0.8 \\ \hline
15. & Quality coefficient, \(\sigma_{\text{c}}/\rho_{\text{(g/cm${}^{3}$)}}\) & 114.7 & 139.8 & +21.8 & 77.1 & \(-\)32.7 \\ \hline  & Total points & ---- & ---- & +526.6 & ---- & +117.7 \\ \hline \hline \end{tabular}
\end{table}