Natural durability of subfossil oak: wood chemical composition changes through the ages

In recent years, subfossil oak has become increasingly popular, particularly in the manufacture of small wooden products. Due to the long period of its underground preservation, detailed knowledge of its properties is essential to properly use this material. In this study, subfossil oak samples dated to approximately 1000, 2000 and 3000 years BP and recent oak samples were chemically analyzed to determine the contents of extractives, the main wood components, and inorganic elements. The results were then evaluated in light of their natural durability. The mass loss of subfossil oak was 2–3 times lower than that of the recent sample, but the age of the subfossil oak itself had no influence on its durability. The long-term leaching process of water-soluble ellagitannins, together with their hydrolysis and bonding in ferric tannate complexes, were responsible for the decreased durability. The oldest subfossil oak had the lowest amount of phenolic compounds and the highest content of inorganic elements. Optical emission spectrometry proved an increase in inorganic elements 5–7 times higher than recent oak content, with the highest increase found for calcium and iron. Compared to recent oaks, subfossil oaks manifested decreased content of carbohydrates and correspondingly increased lignin content. Our results revealed that subfossil oak cannot be considered a suitable material for exterior use under aerobic conditions.