## 3 Results and Discussion
### Strength of joints \"Spruce Wood--PUR Glue\" Effected by Inorganic Preservatives  
The greatest strength of the glued joints of 10.66 MPa was observed in the case of the control set consisting of two specimens of spruce wood not treated with chemical substances. Lower adhesive strength ranging between 10.49 and 8.55 MPa was observed in the first group of test sets in which one specimen was treated with a preservative and the second specimen was not treated with a preservative; the strength decreased by 1.59% to 19.79% (Table 2). A considerable decrease in the adhesive strength between 10.39 and 8.44 MPa was determined in the second group of test sets consisting of two specimens treated with a given preservative of an identical concentration when the strength decreased by 2.53% to 20.83% (Table 2). With a gradual increase in the concentration of all wood preservatives used (c = 0 up to 2%), there was an evident decrease in the strength of glued joints (\(\sigma\)), as it was also proven \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{**Shear Strength (MPa)**} \\ \hline
**Preservative** & & & & \\
**Concentration** & **CuSO\({}_{4}\)** : 5H\({}_{2}\)O** & **ZnCl\({}_{2}\)** & **H\({}_{3}\)BO\({}_{3}\)** & **(NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\)** & **Without** \\
**“c” (\%)** & & & & & \\ \hline \multicolumn{5}{c}{**Reference group of test set (Untreated spruce and Untreated spruce)**} \\ \hline
0 & - & - & - & - & 10.66 (1.24) \\ \hline \multicolumn{5}{c}{**First group of test sets (Treated spruce and Untreated spruce)**} \\ \hline
0.5 & 9.93 (0.93) d & 9.97 (0.72) d & 10.49 (1.25) d & 10.37 (0.68) d & - \\
1 & 9.63 (0.64) c & 9.24 (0.88) b & 9.56 (1.06) c & 10.17 (1.04) d & - \\
2 & 9.56 (0.72) c & 8.55 (0.62) a & 8.80 (0.90) a & 9.35 (1.06) c & - \\ \hline \multicolumn{5}{c}{**Second group of test sets (Treated spruce and Treated spruce)**} \\ \hline
0.5 & 9.36 (0.78) b & 9.65 (0.85) c & 9.58 (1.15) b & 10.39 (1.25) d & - \\
1 & 9.04 (1.07) b & 8.69 (0.55) a & 9.23 (0.63) b & 9.84 (1.24) d & - \\
2 & 8.78 (0.96) a & 8.44 (0.68) a & 8.68 (1.22) a & 8.83 (1.49) b & - \\ \hline \hline \end{tabular} Notes: Mean values are from 10 measurements. Standard deviations are in parentheses. Duncan tests are documented by the indexes (a, b, c, d), evaluating the shear strength of joint values of the first and second groups of the test sets formed from two specimens “one or both treated with preservative” in relation to the reference—control set having the strength value of 10.66 MPa (Significance of indexes: a—very significant decrease > 99.9%, b—significant decrease > 99%, c—less significant decrease > 95%, d—insignificant decrease < 95%).  
\end{table} 
Table 2: Shear strength of joints in the reference (control) set and the test sets—formed from two spruce wood specimens (untreated or treated with inorganic preservative) and PUR glue. with statistically significant linear dependencies \"\(\sigma\) = /a \(\times\) c/ + b\", with the negative \"a\" values and the high coefficients of determination R\({}^{2}\) from 0.85 to 0.999 (Figures 3 and 4). When evaluating the first group of test sets (created by bonding one chemically treated and one untreated spruce wood specimen), following the Duncan test, it was found that there was no significant effect of the 0.5% concentration of any of the four used wood preservatives on the strength of the glued joints. On the contrary, a 2% concentration of preservatives resulted in an apparent decrease in the shear strength of the glued joints at a level of significance of 95% to as much as 99.9% (Table 2). In the first group of test sets, the most evident decrease in the strength of glued joints by 19.79% or 17.45% was observed in the case of test sets in which a specimen treated with a 2% concentration of ZnCl\({}_{2}\) or H\({}_{3}\)BO\({}_{3}\) was present.