Wood Defects and Degradation

Collapse

Collapse is the reduction in wood size during drying before fiber saturation. It manifests as a corrugated surface due to crushed wood tissues and sometimes internal lens-shaped cracks. Collapse occurs in some wood species when dried slowly at high temperatures or humidity, or during rapid outdoor drying. It's more intense on radial cut faces than tangential ones. This defect causes significant wood loss during surfacing and reduces mechanical strength. Tangential shrinkage is greater than radial shrinkage, often separating wood tissues parallel to the weaker radial parenchyma cells. The pith is also susceptible to damage, developing cracks and crevices. In eucalyptus, avoid sawing the pith to achieve good quality dried products.

Drying tensions between areas with different moisture content can cause cracks and crevices. Reduce these defects by applying grease or asphalt emulsion to ends, immersing green wood in solutions like sodium chloride, ammonium nitrate, urea, or polyethylene glycol, or shading the wood.

Warping

Warping is the curvature of a wood piece's longitudinal or transverse axis (or both) due to moisture loss. There are four types:

1. Cupping (Figure A):

   Cupping is the transverse axis curvature. It occurs when one side dries faster than the other, for example, due to uneven sun exposure, contact with another object, or one side being varnished. Tangential shrinkage differs from radial shrinkage, increasing cupping. Radial cut faces minimize this defect. Proper drying methods prevent cupping.

2. Bowing (Figure B):

   Bowing is the longitudinal axis curvature. It results from excessive longitudinal shrinkage in wood with juvenile heartwood or reaction wood. Improper stacking, widely spaced separators, and lack of weights can cause bowing.

3. Crooking (Figure C): Similar to Bowing but less severe.

4. Twisting (Figure D):

   Twisting is the curvature of both sides in longitudinal and transverse directions, like a corkscrew. It's common in spiral or interlocked grain but can also occur in straight-grained wood due to uneven growth or stress. Proper weighting during drying prevents twisting.