The Treatability of Selected Local Timber Species with Alkaline Copper Quaternary (ACQ) Preservative: Correlating Wood Anatomy, Physical Properties, and Preservative Performance

Abstract

This study evaluated the ACQ treatability of 24 local timber species through vacuum pressure impregnation and correlated preservative retention and penetration with physical and anatomical wood properties. Retention ranged widely from 0.478 kg m⁻³ (Bridelia retusa) to 14.980 kg m⁻³ (Cullenia ceylonica and Cupressus macrocarpa). Based on retention and Chrome Azurol S (CAS) penetration assessment, species were classified into four treatability classes. One-way ANOVA confirmed highly significant differences in density (F₂₃,₄₈=199.79; p<0.001), moisture content (F₂₃,₄₈=48.04; p<0.001), and ACQ retention (F₂₃,₄₈=82.75; p<0.001) across species. Pearson correlation analysis revealed a very strong positive relationship between fibre wall thickness and wood density (r=0.955; p<0.05) and a strong negative relationship between fibre lumen diameter and density (r= −0.903; p=0.001), demonstrating that fibre morphology is the dominant determinant of wood density. Neither density (r= −0.411; p=0.163), moisture content (r = 0.005; p = 0.987), total ray area (r=0.097; p=0.789), total vessel area (r= −0.021; p=0.932), fibre wall thickness (r=0.082; p=0.725), nor fibre lumen diameter (r=0.302; p=0.183) produced statistically significant correlations with ACQ retention, indicating that preservative uptake is governed by the combined interactive influence of multiple anatomical and chemical wood properties rather than any single parameter. Cullenia ceylonica, Cupressus macrocarpa, and Ficus amplissima were identified as the most effectively treatable species, classified as very easy to treat (Class 4, retention >10 kg m⁻³) with full penetration exceeding 25 mm. These findings provide a species-specific treatability database for Sri Lankan timber and highlight the need for multi-variable modelling in future preservative research.