The chemical composition of wood in relation to its thermal and hydrothermal treatment is discussed. The degradation and modification processes, which take
place during hydrothermal treatment, are presented. The chemical reasons for the increased dimensional stability, reduced moisture content and improved
biological durability of hydrothermally treated wood are given. Particular attention is given to the chemical reactions that can cause the darker tonality of
hydrothermally treated wood. The literature review of chemical explanations for wood colour changes ranges from the degradation of hemicelluloses through
oxidation reactions to lignin polymerization reactions. Intermediate lignin degradation products are considered to be the main source of colour changes in
hydrothermally treated wood.
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