Assessing the Fire and Thermal Behavior of Thermally Modified, Acetylated, and One-sided Surface Charred Beech and Birch Wood

Abstract

The fire and thermal behavior of European beech (Fagus sylvatica L.) and silver birch (Betula pendula L.) woods were comparatively evaluated after being modified by thermal treatment (200 °C), acetylation, and one-sided surface charring. A small-flame ignition test (EN ISO 11925-2), gross heat of combustion via bomb calorimetry (EN ISO 1716), and thermo-gravimetric analysis (TG/DTG) were used for assessment. Thermal modification resulted in moderate mass loss (~3% to 4%), while acetylation achieved high weight percentage gain (>20%) and the strongest reduction in equilibrium moisture content; surface charring had only a negligible effect on bulk moisture properties. While none of the applied modifications altered the Euroclass-related reaction-to-fire classification, statistically significant differences in early-stage flame spread were observed. Acetylated wood exhibited increased flame spread, whereas surface-charred specimens showed a pronounced reduction, particularly for beech. All modified materials displayed higher gross heat of combustion compared with reference wood, reflecting increased carbon-rich constituents; however, this increase did not directly correspond to improved fire performance. TGA revealed similar degradation behavior for reference and thermally modified wood, whereas acetylated and surface-charred materials exhibited fundamentally different thermal responses because of chemical substitution and polysaccharide degradation with carbon-rich char formation, respectively. The study highlighted surface charring as an effective modification for reducing flame spread and demonstrates the necessity of combining complementary fire-testing methods to capture modification-specific fire behavior.