Thermal modification at elevated temperatures changes the chemical, biological and physical properties of wood. One of the main targets of thermal modification is to increase the biological durability and decay resistance of wood. Although the effects of thermal modification on wood have been widely studied, the significance of sapwood and heartwood on the resistance of thermally modified wood has been paid less attention. In this study the effects of thermal modification at four different temperatures (170°C, 190°C, 210°C and 230°C) on resistance against soft- and brown-rot fungi of sapwood and heartwood of Scots pine and Norway spruce were investigated using accelerated laboratory test methods. Natural durability against the soft-rot micro-fungi was determined according to CEN/TS 15083-2 by measuring the mass loss and modulus of elasticity (MOE) loss after an incubation period of 32 weeks. Test material was also exposed to brown-rot fungi Coniophora puteana and Poria placenta essentially according to a mini-decay test using two exposure periods (6 and 10 weeks). On average, the soft-rot and brown-rot tests gave quite similar results. However, the brown-rot test gave slightly better durability classes than the soft-rot test. The untreated heartwood of Scots pine was more durable against decay organisms than the sapwood of Scots pine and the sapwood or heartwood of Norway spruce. The differences between sapwood and heartwood were more significant within pine than within spruce material. The thermal modification significantly increased the decay resistance of all pine and spruce samples. The higher the level of thermal modification, the better biological durability and resistance against decay will be achieved.

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