Ammonia vs. thermally modified timber-comparison of physical and mechanical properties

Gaseous ammonia treatments as well as thermal modification of timber are both applied to modify the surface colour of wood species. Physical and mechanical properties of solid wood after gaseous ammonia treatments have been so far scarcely investigated. Selected physical and mechanical properties, i.e., colour, equilibrium moisture content, wood density, bending strength, bending stiffness, and impact bending strength were investigated for oak, Norway spruce and Scots pine after gaseous ammonia treatment. Obtained data were compared with thermally modified timber data. While wood colour changes were comparable, most properties of the ammonia treated wood did not alter. There was no significant density change of ammonia treated wood; equilibrium moisture content changed moderately. Mechanical properties also remained within acceptable fluctuations. Results proved that gaseous ammonia treatment is a modification that improves the decorative value but has minimal effect on technologically relevant properties of solid wood. In contrast, property alterations of colour-equivalent thermally modified timber were much more pronounced, with possible limitations to some applications.