## 1 Introduction
The first chemical component that changes is hemicellulose as it undergoes deacetylation reactions which ultimately lead to the formation of low-molecular-weight compounds possibly extractable from wood [17; 18; 19]. As for the non-structural compounds, that is, the extractives, during the heat treatment the most volatile ones are released from the wood, while others are completely degraded [20] or transformed into new compounds [21]. These new compounds can be extracted and used in different market sectors because they have important properties, also acquiring a certain value through the development of new products [22]. A better understanding of the differences between extractives during heat treatment is needed to provide their use in different fields such as pharmaceutical and cosmeceutical. For these reasons, the enhancement of secondary wood products remains an open challenge of great interest. The extraction composition is highly variable both within and between species [23] and depends on the age, seasonality and position of the tree [24]. Extractives can be extracted from wood with different extraction techniques that affect the yield and the type of compound extracted. Currently, the most commonly used extraction technique requires the Soxhlet apparatus with solvents or the autoclave with hot water as a simple ecological method. Extractives are generally classified as water-soluble, ethanol-toluene soluble or ether soluble, depending on the solvent used [22]. The type and quantity of extractive compound dissolved in each solvent is different and the choice of the most appropriate solvent depends on the application. Organic solvents are capable of extracting resin compounds, while water is useful for extracting phenolic compounds, glycosides, carbohydrates and soluble salts [25]. Therefore, since extractives are the key constituents that guarantee the durability of wood, the aim of this research work was to evaluate the effect of thermal modification on extraction yield, chemical composition and quality of the extractives, and demonstrate that new green chemistry procedures available could be used to increase the quantity of recoverable extractives. For this purpose, unmodified and thermally modified _Castanae sativa_ and _Fagus sylvatica_ wood was used. The total Italian forestry area is 10.5 Mha, of which the area covered by beech trees amounts to 1.035 Mha, about 10.7% of the total, while that covered by chestnut trees amounts to 0.788 Mha, 8.1% of the total [26]. _Castanae sativa_, or sweet chestnut, is a plant species of the _Fagaceae_ family, native to Europe and Asia Minor, and is one of the most durable wood species in Europe. Due to its good durability, wood is often used for poles and other outdoor applications, both in and above ground and for the manufacture of furniture, barrels and constructions, particularly in southern Europe [27]. One of the reasons for the good duration is the presence of extractives, mainly tannins, used in the past for the production of some medicines, for the production of leather, as well as playing a role in the flavouring of various types of alcoholic beverages [28]. Among other physical and mechanical changes introduced by thermal modification [29], the nature and performance of extractives derived from thermo-modified wood can differ significantly from that of native wood materials. Modified wood extractives can be altered by thermal modification, but new extractives compounds can also form during this process, which are important natural resources in the wood industry [30]. _Fagus sylvatica_, whose colour ranges from blond to reddish, is a wood of medium hardness and rather heavy, widespread in the Italian Alps and Apennines. Easy to work, it is used in carpentry to make rustic furniture and furnishing accessories; a compact wood, but easily attacked by woodworms and fungi if not properly treated [31]. Beech wood is used almost only as firewood; therefore, it is interesting to pay particular attention to the recovery of this wood and its compounds.