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Impact of juvenile wood on the drying characteristics of Pacific Coast Hemlock structural timber Bradic, Slobodan

Abstract

Large volumes o f relatively small-diameter logs are generated from sustainable sources such as the Pacific Coast Hemlock second-growth forests. The percentage of juvenile wood in this kind of material is higher compared to limited old growth wood supplies. The present scientific knowledge is often limited to individual properties of juvenile wood without including interactions with other drying factors. For that reason, this investigation evaluates the drying quality of the mix of 105 mm square timbers from second-growth western hemlock (Tsuga heterophylla) and amabilis fir (Abies amabilis), commercially known as Pacific Coast Hemlock baby-squares, using two different concepts of interactions. First, the evaluation is based on the influence of juvenile wood presence shown with a pith location at their end-surface and drying target moisture content, while in the second concept the cutting season is added as the third controlled factor. Timber specimens were classified into four groups depending on the presence and location of the tree pith. A total of 640 specimens were dried in a laboratory conventional (heat-and-vent) kiln in four drying runs to 15% and 20% target moisture contents, with one charge of specimens from the summer and other of the fall cutting season. The results showed that timbers with the pith shown in the centre should be avoided in the production of baby-squares because of its high propensity for bow, twist and surface checks, due to the problems with variable shrinkage coupling of juvenile and mature wood within specimens. Specimens with the pith shown close to one of the sides in the cross-section had a lower, but acceptable, quality according to the El20 CFLA grading rules. Because of the higher shrinkage, the lower target moisture content shows a greater risk for twist and surface checks, compared with the higher moisture content. The importance of the cutting season is reflected through its significant interactions with the target moisture content in the case of volumetric shrinkage, and with the pith location in the case of twist. These are consequences of lower initial moisture content in summer-cut specimens that could occasionally reach moisture content below the fiber saturation point, and start to shrink before kiln drying.

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