Abnormal discoloration of wood often denotes a diseased condition, indicating unsoundness.  The black check in western hemlock is the result of insect attacks.[29] The reddish-brown streaks so common in hickory and certain other woods are mostly the result of injury by birds.[30] The discoloration is merely an indication of an injury, and in all probability does not of itself affect the properties of the wood.  Certain rot-producing fungi impart to wood characteristic colors which thus become criterions of weakness.  Ordinary sap-staining is due to fungous growth, but does not necessarily produce a weakening effect.[31]

[Footnote 29:  See Burke, H.E.:  Black check in western hemlock.  Cir.  No. 61, U.S.  Bu.  Entomology, 1905.]

[Footnote 30:  See McAtee, W.L.:  Woodpeckers in relation to trees and wood products.  Bul.  No. 39, U.S.  Biol.  Survey, 1911.]

[Footnote 31:  See Von Schrenck, Hermann:  The “bluing” and the “red rot” of the western yellow pine, with special reference to the Black Hills forest reserve.  Bul.  No. 36, U.S.  Bu.  Plant Industry, Washington, 1903, pp. 13-14.

Weiss, Howard, and Barnum, Charles T.:  The prevention of sapstain in lumber.  Cir. 192, U.S.  Forest Service, Washington, 1911, pp. 16-17.]

CROSS GRAIN

Cross grain is a very common defect in timber.  One form of it is produced in lumber by the method of sawing and has no reference to the natural arrangement of the wood elements.  Thus if the plane of the saw is not approximately parallel to the axis of the log the grain of the lumber cut is not parallel to the edges and is termed diagonal.  This is likely to occur where the logs have considerable taper, and in this case may be produced if sawed parallel to the axis of growth instead of parallel to the growth rings.

Lumber and timber with diagonal grain is always weaker than straight-grained material, the extent of the defect varying with the degree of the angle the fibres make with the axis of the stick.  In the vicinity of large knots the grain is likely to be cross.  The defect is most serious where wood is subjected to flexure, as in beams.

Spiral grain is a very common defect in a tree, and when excessive renders the timber valueless for use except in the round.  It is produced by the arrangement of the wood fibres in a spiral direction about the axis instead of exactly vertical.  Timber with spiral grain is also known as “torse wood.”  Spiral grain usually cannot be detected by casual inspection of a stick, since it does not show in the so-called visible grain of the wood, by which is commonly meant a sectional view of the annual rings of growth cut longitudinally.  It is accordingly very easy to allow spiral-grained material to pass inspection, thereby introducing an element of weakness in a structure.

There are methods for readily detecting spiral grain.  The simplest is that of splitting a small piece radially.  It is necessary, of course, that the split be radial, that is, in a plane passing through the axis of the log, and not tangentially.  In the latter case it is quite probable that the wood would split straight, the line of cleavage being between the growth rings.