Wood Formation
June 2, 2009 by blogtopia
Filed under Definitions
Wood, in the strict sense, is yielded by trees, which increase in diameter by the formation, between the existing wood and the inner bark, of new woody layers which envelop the entire stem, living branches, and roots. Technically this is known as secondary growth; it is the result of cell division in the vascular cambium, a lateral meristem, and subsequent expansion of the new cells.
- Growth rings
- Knots
- Heartwood and sapwood
Knots in Wood
June 1, 2009 by blogtopia
Filed under Definitions
A knot is a particular type of imperfection in a piece of wood; it will affect the technical properties of the wood, usually for the worse, but may be exploited for artistic effect. In a longitudinally-sawn plank, a knot will appear as a roughly circular "solid" (usually darker) piece of wood around which the grain of the rest of the wood "flows" (parts and rejoins). Within a knot, the direction of the wood (grain direction) is up to 90 degrees different from the grain direction of the regular wood.
In the tree a knot is either the base of a side branch or a dormant bud. A knot (when the base of a side branch) is conical in shape (hence the roughly circular cross-section) with the tip at the point in stem diameter at which the plant’s cambium was located when the branch formed as a bud.
US plywood grades
June 1, 2009 by blogtopia
Filed under Definitions
Plywood grades are determined by a veneer quality on the face and back of each panel. The first letter designates quality of face veneer (best side), while the second letter denotes the surface quality of the back of the panel. The letter "X" indicates the panel was manufactured with scrap wood as the center plies, not "exterior" as is commonly thought. The A-D rating is only good for construction (softwood) plywood, not for hardwood plywoods such as oak or maple.
"A": Highest grade quality available. Can be defect free or contain small knots, providing they are replaced with wooden plugs (the fillers having a "boat" or an "American football" shape) or repaired with synthetic patch. This grade may contain occasional surface splits that are repaired with synthetic filler. The surface is always sanded and provides for smooth paintable face quality.
Drying defects
June 1, 2009 by blogtopia
Filed under Definitions
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).