The aim of the thesis was to investigate how thermal modification of spruce and ash wood affects the uptake of the Burnblock fire retardant and the flammability of the wood after treatment. Spruce wood was examined in its unmodified state and after thermal modification at 180 °C and 230 °C, while ash wood was examined in its unmodified state and after modification at 180 °C. Control, impregnated, and impregnated-then-leached specimens were prepared. Fire-retardant uptake and its loss during leaching were determined gravimetrically, while its distribution was assessed using micro-X-ray fluorescence spectrometry and hyperspectral imaging. The fire performance of the specimens was evaluated using a single-flame source test and by measuring the damaged surface area. Wood mass loss increased with increasing thermal modification temperature. Ash wood modified at the higher temperature partially ignited during the process and was therefore excluded from further analyses. Leaching caused a considerable loss of fire retardant in all groups; however, thermally modified specimens retained more fire retardant than unmodified specimens. During the single-flame source test, the impregnated specimens generally did not continue to burn. The greatest tendency to burn and the largest damaged surface area were observed in the control spruce wood modified at the higher temperature. We found that thermal modification affected fire-retardant retention and the response of wood to flame exposure. Impregnation with Burnblock effectively limited continued burning, although its protective effect decreased after leaching.
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