## 3 Experimental Results
### Mechanical Properties of Subfossil Oak Compared with Aged Oak from Construction and Recent Oak  
The results of compression tests parallel to grain are summarised in Table 4, while the variation in this mechanical property among the different assortment of oak wood and its correlation with the basic density is illustrated in Figure 6.  
An average ultimate compression stress value of 66 \(\pm\) 6 MPa was obtained for the tested recent oak material (NO). This value is in good accordance with the reference literature data indicating an interval of variation of 54-76 MPa for _Quercus robur_, with an average of 61 MPa, while the corresponding values for _Quercus petraea_ are 48-65-70 MPa [38]. The different assortments of subfossil oak presented compression strengths significantly lower (by 20-30%) compared to NO, in the range 46-54 MPa, which agrees with reference literature 33-38-52 MPa [38] The values for the two assortments of aged oak recovered from construction were significantly different, being either by 18% lower compared to NO (54 \(\pm\) 7 MPa for AO_C3), or similar with NO (AO-C1-C2 with average 69 \(\pm\) 7 MPa, statistically not different to NO). Despite this variability of the average values of compression strength parallel to grain among the studied oak wood assortments (Figure 6a), all values correlated well with the basic density of the wood material (Figure 6b). A linear positive correlation with a high correlation factor (R\({}^{2}\) = 0.8955) was obtained for the experimental results of this research. The average values of modulus of elasticity varied in the range 3523-4144 MPa for the assortments of SO, being 15-30% lower than the average value of 4887 MPa determined for NO. Average values of modulus of elasticity varying in the range of 4490-10,342 MPa for subfossil oak from various sources and of different ages compared to 12,351 MPa for recent oak were previously reported [3]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Wood** \\ **Material** \\ \end{tabular} } & \multirow{2}{*}{**NOR**} & \multirow{2}{*}{\
\begin{tabular}{c} **\(\sigma_{c,12}\)** \\ **MPa** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Strain at Break** \\ **Point,** \\ **mm/mm** \\ \end{tabular} } & \multicolumn{2}{c}{**Modulus of Elasticity, \(E_{c}\)**} \\  & & & **Point,** \\  & & **MPa** & **mm/mm** & **MPa** & **MPa** & **MPa** \\ \hline \hline
**SO\_S1** & 15 & 46 \(\pm\) 3 & 0.020–0.023 & 3806 & 4144 & 4808 \\ \hline
**SO\_S2a** & 13 & 46 \(\pm\) 8 & 0.020–0.023 & 2937 & 3765 & 4494 \\ \hline
**SO\_S2b** & 13 & 54 \(\pm\) 5 & 0.023–0.027 & 3432 & 3523 & 3832 \\ \hline
**SO\_S3** & 12 & 50 \(\pm\) 3 & 0.020–0.023 & 3197 & 3585 & 3708 \\ \hline
**AO\_C1\_C2** & 31 & 69 \(\pm\) 7 & 0.023–0.030 & 2236 & 4706 & 5334 \\ \hline
**AO\_C3** & 20 & 54 \(\pm\) 7 & 0.020–0.033 & 1594 & 3854 & 4579 \\ \hline
**NO\_N** & 29 & 66 \(\pm\) 6 & 0.020–0.033 & 4680 & 4887 & 5114 \\ \hline \hline \end{tabular} Notes: NOR = Number of replicates; \(\sigma_{c,12}\) = ultimate compression stress parallel to grain corrected for W 12%; \(E_{c}\) modulus of elasticity calculated for Min = minimum; Mean and Max = maximum values of \(\sigma_{c,12}\) employing the experimental stress–strain diagrams on the elastic domain (between 10% and 40% of the maximum breaking load), according to [43].  
\end{table} 
Table 4: Compression parallel to grain for sub-fossil oak (SO) assortments compared to recent oak (RO) and aged oak recovered from constructions (AO_C): ultimate compression stress \(\sigma_{c,12}\) (average values with standard deviations), strain at break point \(\epsilon\) (range) and Modulus of elasticity \(E_{c}\) (minimum, mean and average values). The fact that the mechanical properties of subfossil wood are lower than those of recent wood has been a common conclusion of several researchers, though differences in the actual values reported were present.