Preservative retention and penetration in Norway spruce wood as influenced by different microwave treatment energy levels

Norway spruce (Picea abies) is often used in construction but is difficult to impregnate with wood preservatives due to pit aspiration, limiting its treatability. The low natural durability of spruce necessitates preservation treatments. Microwave treatment improves the permeability, increasing preservative retention and penetration. However, treatability assessment has often been conducted using visual methods, which can over - or underestimate success. This study used image processing to evaluate MW-treated spruce's treatability with two water-based preservatives. The effect of MW treatment at five energy levels was analysed for sapwood and heartwood. The results showed improved retention values and penetration percentages for the MW-treated sapwood and the heartwood. The retention values of both preservatives increased between 50% and 124% for different treatment combinations. Spruce heartwood MW-treated with 1260 MJ/m3 improved treatability by 24% when analysed with ImageJ. The anatomical analysis showed that the effect of MW on heartwood was more pronounced than on sapwood, leading to the destruction of rays and parenchyma cells that facilitated the absorption and penetration of the preservatives. The study demonstrates the potential of ImageJ for more accurate treatability assessment and suggests that MW treatment if optimised, could improve the natural durability of spruce wood.