Evaluation of boron distribution from preservative pastes made from ulexite, copper and fluoride in Scots pine wood

Boron diffusion from preservative pastes made of either ulexite or copper or fluoride compounds was evaluated. The pastes produced were applied to sapwood blocks prepared from Scots pine and conditioned to target moisture contents (MCs) of 30%, 60%, and 90%. The pastes were inserted into the treatment holes in the wood blocks and boron diffusion was determined at three assay zones across the blocks after 7-day, 30-day, 90-day, and 180-day-incubition period at room temperature. Ethylene glycol was also added in some of the paste formulations to improve boron diffusion. In general, boron diffusion from the pastes was improved with increased MC of wood. No steady amount gradients with distance away from the treatment hole were observed for the amount of boron diffusion in the assay zones; however, there were few exceptions. Amount of boron from the pastes with ulexite in wood exceeded well 0.1% boric acid equivalent (BAE) boron threshold in most assay zones and incubation times. Expectedly, boron diffusion from the pastes with ulexite was significantly lower than those from the pastes with borax + copper because of higher water solubility of borax than that of ulexite. The ulexite-based preservative pastes may be useful particularly when wood is kept at high MCs at extended periods.