Silver birch (Betula pendula) and Scots pine (Pinus sylvestris) wood boards (1000 × 100 × 25 mm) were thermally modified in a 340 L pilot-scale device in nitrogen with a special focus on increasing dimensional stability and reducing hydrophilicity. The research expands our understanding of the TM process in a closed system under pressure of nitrogen and its impact on the water absorption capabilities of wood. Several thermal modification (TM) parameters were tested, including temperature (160–180 °C), maximum temperature duration (30–180 min), and TM chamber initial pressure (3–6 bar). TM wood dimensional changes, mass loss (ML), equilibrium moisture content (EMC), and anti-swelling efficiency (ASE) were determined to characterize the TM process intensity and evaluate the hydrophilicity. Birch wood exhibited a higher ML (5.9%–12%) than pine wood (2.6%–9%) after TM. TM caused a shrinkage in the tangential, radial, and total volume of both wood species. The TM birch wood ASE values varied from 22% to 69%, while the pine wood ASE was 27% to 58%. The cell wall total water capacity (CWTWC) of TM wood was greatly reduced. The EMC and volumetric swelling (VS) of TM birch and pine wood were 29% to 67% lower, respectively, at all relative humidities (65, 75, and 95%).

In this study, silver birch (Betula pendula) and Scots pine (Pinus sylvestris) wood planks (1000 × 100 × 25 mm) were thermally modified in pilot-scale equipment. Research extended our knowledge of the thermal modification (TM) process in a closed system under nitrogen pressure, as well as how process parameters affect the chemical composition and mechanical strength of wood. Various TM regimes were selected—maximum temperature (150–180 °C), modification time (30–180 min), and initial nitrogen pressure (3–6 bar). Chemical analyses were performed to assess the amount of extractives, lignin, polysaccharides and acetyl group content following the TM process. The mechanical properties of TM wood were characterized using the modulus of rupture (MOR), modulus of elasticity (MOE), and Brinell hardness. The MOR of both studied wood species following TM in nitrogen was reduced, but MOE changes were insignificant. The Brinell hardness of TM birch wood’s tangential surface was much higher than that of the radial surface, although Scots pine wood showed the opposite pattern. TM birch and pine wood specimens with the highest mass loss, acetone soluble extractive amount, and the lowest xylan and acetyl group content had the lowest MOR and Brinell hardness.

Using a pilot-scale chamber with an interior capacity of 340 L, European aspen (Populus tremula) wood boards were thermally modified (TM) under pressure in nitrogen at a maximum temperature of 160–170 °C, for 60–180 min, and with an initial nitrogen pressure of 4–5 bar. After the TM process, aspen wood was characterised by dimensional changes, mass loss (ML), equilibrium moisture content (EMC), antiswelling efficiency (ASE), cell wall total water capacity (CWTWC), modulus of rupture (MOR), modulus of elasticity (MOE), and Brinell hardness (BH). This work offers fresh insights into the characteristics of aspen wood following a closed TM process in pressurised nitrogen. TM caused ML of 5.4–14.5% and shrinkage in all anatomic directions. The ASE ranged from 22 to 70%, while the CWTWC was reduced from 35% to 11–27%. After treatment, EMC and volumetric swelling (VS) were more than twice as low as in untreated wood. Although MOE values increased and the average MOR reduced following TM, the changes were not important. The TM aspen wood tangential surface’s BH dropped and was noticeably lower than the radial surface’s BH.

The wood of Scots pine (Pinus sylvestris), silver birch (Betula pendula), and European aspen (Populus tremula) was thermally modified in nitrogen under pressure. Three commercial linseed oil-based coatings without or with brown and grey pigments were applied to the specimens. Specimens were placed outside, and weathering stability was assessed for 3 months. The test measured total surface colour change (ΔE) and colonization by wood dicolouring fungi. Following the test, all uncoated specimens demonstrated poor colour fastness and resistance to fungal growth. All tested coatings were unsuitable for protecting untreated wood from dicolouring fungi. The transparent coating was inefficient since it did not significantly prevent untreated or TM wood from fading, and fungal resistance was increased only for a few TM regimes. The colour fastness of specimens with pigmented coatings was enhanced. Specimens with a grey coating exhibited the lowest ΔE and remained consistent throughout the test period. TM specimens with coloured surfaces exhibited greater fungal resistance. However, not all TM aspen and birch regimes had a sufficient growth mark (rating 0 or 1). TM aspen was less resistant to fungi, whereas TM pine displayed very strong fungal resistance across all TM regimes.

This study examines the thermal modification (TM) of European black alder (Alnus glutinosa) wood boards measuring 1000 × 100 × 32 mm. The TM was carried out in a nitrogen atmosphere under an initial pressure of 4 bar at 160 °C for 60, 120, and 180 min, as well as at 170 °C for 30 and 60 min. The TM process resulted in mass loss and volumetric changes with shrinkage observed across all anatomical directions. Water uptake decreased significantly, with the cell wall’s total water capacity dropping from 35% to a range of 14%–27%. Dimensional stability was improved by between 21% and 61%. The TM wood showed a reduction exceeding 50% in both volumetric swelling and equilibrium moisture content relative to the unmodified specimens. A marked decline in the modulus of rupture was observed, especially in samples treated at 160 °C for 180 min and at 170 °C. Conversely, the modulus of elasticity exhibited a slight upward trend, though the changes were not statistically significant. Brinell hardness revealed a pronounced difference between the tangential and radial orientations, with the tangential surface displaying distinctly lower hardness. Chemical analysis indicated a notable increase in acetone-soluble extractives and reductions in the xylan, mannan, and acetyl groups, reflecting structural alterations in hemicelluloses.