Decay resistance of sapwood and heartwood of untreated and thermally modified Scots pine and Norway spruce compared with some other wood species

Thermal modification has been developed for an industrial method to increase the biological durability and dimensional stability of wood. In this study the effects of thermal modification on resistance against soft- and brown-rot fungi of sapwood and heartwood of Scots pine and Norway spruce were investigated using laboratory test methods. Natural durability against soft-rot microfungi was determined according to CEN/TS 15083-2 (2005) by measuring the mass loss and modulus of elasticity (MOE) loss after an incubation period of 32 weeks. An agar block test was used to determine the resistance to two brown-rot fungi using two exposure periods. In particular, the effect of the temperature of the thermal modification was studied, and the results were compared with results from untreated pine and spruce samples. The decay resistance of reference untreated wood species (Siberian larch, bangkirai, merbau and western red cedar) was also studied in the soft-rot test. On average, the soft-rot and brown-rot tests gave quite similar results. In general, the untreated heartwood of pine was more resistant to decay than the sapwood of pine and the sapwood and heartwood of spruce. Thermal modification increased the biological durability of all samples. The effect of thermal modification seemed to be most effective within pine heartwood. However, very high thermal modification temperature over 230°C was needed to reach resistance against decay comparable with the durability classes of “durable” or “very durable” in the soft-rot test. The brown-rot test gave slightly better durability classes than the soft-rot test. The most durable untreated wood species was merbau, the durability of which could be evaluated as equal to the durability class “moderately durable”.