Liquid water sorption in the longitudinal direction in wood samples of Norway spruce (Picea abies (L.) Karst) was measured with computed tomography (CT) scanning and image processing and then evaluated using multivariate discriminant analysis. The objective was to determine whether heartwood from young spruce has appropriate properties, in terms of low water sorption, for use in exterior products.

CT images of the cross-section of a green stem showed inhomogeneity in heartwood density (moisture content) and the border between sapwood and heartwood were diffuse. The variation in moisture content in the green stem could be divided into sapwood, heartwood and incompletely developed heartwood (intermediate wood).

Test pieces consisted of sapwood, heartwood and intermediate wood 32 mm thick and 200 mm long and dried to 12% MC. Measurements were performed after 1, 3, 7 and 14 days of liquid sorption in end grain and drying under ambient conditions. PLS-DA (discriminate analysis) models show that after sorption for 1, 3, 7 and 14 days, 71%, 71%, 78% and 77%, of the observations were predicted correctly. The MC gradients from intermediate wood were more similar to MC gradients from heartwood than sapwood during sorption and drying. MC gradients from sapwood take longer time to level out to an MC below 20% than heartwood and intermediate wood. Since capillary water height between heartwood and sapwood is small, the conclusion is that age or annual ring with of the tree must be taken into account when durable heartwood for outdoor use is chosen. Rapidly grown young spruce may be mixed with old, suppressed spruce with better properties for outdoor use.