Drying defects are the most common form of degrade in timber, next to natural problems such as knots (Desch and Dinwoodie, 1996). There are two broad categories of drying defects (some defects involve both causes):
- defects that arise due to the shrinkage anisotropy. This leads to warping: cupping, bowing, twisting, spring and diamonding.
- defects that arise due to uneven drying. This leads to the rupture of the wood tissue: checks (surface, end and internal), end splits, honey-combing and case-hardening. Another such defect is collapse, often seen as a corrugation, or "washboarding" of the wood surface (Innes, 1996). Collapse is a defect that results from the physical flattening of fibres, above the fibre saturation point (thus not a form of shrinkage anisotropy).
Australian and New Zealand Standard Organisations (AS/NZS 4787, 2001) have developed a standard for timber quality. Their five criteria for measuring drying quality:
1. moisture content gradient and presence of residual drying stress (case-hardening);
2. surface, internal and end checks;
3. collapse;
4. distortions;
5. and discolouration caused by drying.
This standard also indicates how to assess each of these drying quality criteria and provides a classification to express drying quality.
Source: Wikipedia
Incoming search terms:
- timber drying quality and case hardening and moisture content gradient