Abstract
Abstract 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.
Highlights
Among hardwood species native to Europe, oak heartwood demonstrates some of the highest levels of natural durability, along with black locust and sweet chestnut
It is generally known that the composition of extractives varies widely from species to species, and the total amount of extractives in a given species depends on several factors, such as silvicultural and growth conditions, heartwood age, chemical oxidation, genetic disposition, or even the dissolvent capacities of the solvent(s) used (Mosedale et al 1996; Vivas et al 1996; Doussot et al 2002)
If we consider the variability of extractive content and compare subfossil oak C with average values of sessile oak (7.3% and 31.7 mg GAE g−1 for total extractive and phenolic compounds, respectively) from the above-mentioned experiment, we can assume that the content dropped at least by 50%
Summary
Among hardwood species native to Europe, oak heartwood demonstrates some of the highest levels of natural durability, along with black locust and sweet chestnut. The most widespread European oak species are sessile oak [Quercus petraea (Matt.) Liebl.] and pedunculate oak (Quercus robur L.), which are both classified as Classes 2 (durable) to 4 (less durable) in terms of durability according to EN 350:2016 in soil-contact situations. A lower service life or durability of wood in contact with the ground is related to the long-term presence of sufficiently high moisture; this facilitates the activity of wood-degrading agents and different wood-decaying microflora, including soft-rot fungi and bacteria. The natural durability of wood can be further reduced by the leaching of extractives (Aloui et al 2004; Fojutowski et al 2014) or their detoxification by soil bacteria (Wakeling and Morris 2014)
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