This research investigates the thermal modification (TM) of European black alder (Alnus glutinosa) wood boards (1000 x 100 x 25 mm), treated under nitrogen atmosphere at 4 bar starting pressure. TM was performed at temperatures of 160°C and 170°C for 30, 60, 120, or 180 minutes. Despite growing interest, detailed studies on the impact of TM on black alder wood, particularly under nitrogen pressure, are scarce.

The TM process resulted in mass reductions ranging from 4.6% to 8.6%, with shrinkage observed across all anatomical directions. Water retention decreased significantly, with the cell wall’s total water content dropping from 35% to a range of 14%–27%. Anti-swelling performance improved, with efficiency between 21% and 61%. Notably, the treated wood exhibited more than a 50% reduction in volumetric swelling and equilibrium moisture content compared to untreated samples.

Regarding mechanical properties, a decrease in the modulus of rupture was noted, particularly for treatments at 160°C for 180 minutes and 170°C. On the other hand, the modulus of elasticity saw minor increases, though they were not substantial. Brinell hardness tests highlighted a significant contrast between the tangential and radial surfaces, with the tangential surface demonstrating notably lower hardness.

In conclusion, TM substantially improves dimensional stability and moisture resistance in black alder wood. The dark brown color developed during TM enhances its visual appeal, making it a competitively priced alternative to more expensive wood materials. This study expands the knowledge of TM applied to black alder, demonstrating its potential for use in sustainable wood product industries.