WOOD - Materials for Civil and Construction Engineers - page 410

Pitch Pockets are well-defined openings between annual rings that
contain free resin. Normally, only Douglas fir, pines, spruces, and
western larches have pitch pockets.
Bark Pockets are small patches of bark embedded in the wood. These
pockets form as a result of an injury to the tree, causing death to a
small area of the cambium. The surrounding tree continues to grow,
eventually covering the dead area with a new cambium layer.
Checks are ruptures in wood along the grain that develop during sea-
soning. They can occur on the surface or end of a board. Surface
checking results from the separation of the thinner-walled early
wood cells and is confined mostly to planner surfaces. Cracks due to
end checking normally follow the grain and result in end splitting.
Splits are lengthwise separations of the wood caused by either mishan-
dling or seasoning.
Warping is a distortion of wood from the desired true plane. The four
major types of warping are bowing, crooking, cupping , and twisting .
Bowing is a longitudinal curvature from end to end. Crooking is the
longitudinal curvature side to side. Both of these defects result from
differential longitudinal shrinkage. Cupping is the rolling of both
edges up or down. Twisting is the lifting of one corner out of the
plane of the other three. Warping results from differential drying
due to the production environment or from the release of internal
tree stress.
Raised, Loosened, or Fuzzy Grain may occur during cutting and dressing
of lumber.
Chipped or Torn Grain occurs when pieces of wood are scooped out of
the board surface or chipped away by the action of the cutting and
planning tools.
Machine Burn is an area that has been darkened by overheating during
cutting.

10.7 Physical Properties

Important physical properties include specific gravity and density, thermal
properties, and electrical properties.

■ 10.7.1 Specific Gravity and Density

Specific gravity of wood depends on cell size, cell wall thickness, and number
and types of cells. Regardless of species, the substance composing the cell
walls has a specific gravity of 1.5. Because of this consistency, specific gravi-
ty is an excellent index for the amount of substance a dry piece of wood ac-
tually contains and is nearly constant within each species. Therefore, specific