seasons, especially in the southern United States. Some softwoods may also be discolored in the
same manner. Chemical stain is most common in lumber but it also develops in logs. Sometimes it
appears like a sapstain on the surface of lumber. However, it usually develops just beneath the
wood surface where adequate drying has not occurred (Figure 4-4).
These stains are especially troublesome since they may not show up until the lumber is surfaced,
and they may occur even when chemicals are used to prevent the fungus type sapstains. Chemical
stain can be differentiated from the fungus type. Concentrated oxalic acid will bleach out the chemi-
cal stain, but not those caused by fungi.
4.2.3 Wood Decay. Wood decay is the fungal decomposition of woody material. In its
early or incipient stage, wood decay is difficult to detect even though serious strength loss in the
wood may have already occurred. Toughness is the mechanical property most sensitive to decay,
and a decrease of as much as one-third to one-half of the normal toughness value may occur even
before any weight loss is detected. Toughness is a measure of wood's resistance to impact bending.
Advanced decay is readily detected. Wood with appreciable decay will break brashly (abruptly)
across the grain, whereas sound wood splinters at the break. Brashness, reflecting reduced tough-
ness, can be detected by breaking small pieces by hand, or by lifting pieces of wood from the sur-
face by means of a pointed tool--the "pick test" (Figure 4-5). The wood should be wet, otherwise it
may break brashly even if sound. Brashness also is characteristic of compression wood (as dis-
cussed in Chapter 2), and test results should be interpreted appropriately.
Decay is prevented by keeping the moisture content of wood consistently below the fiber saturation
point (i.e. 20-30 percent), or by preservative treatment of the wood.
18.104.22.168 In Manufactured Products.
Brown rot (Figure 4-6) is the predominant type of decay in softwoods used
above ground. Brown rot fungi remove the carbohydrates or cellulose, leaving a modified lignin
residue. In the early stages of decay, the wood surface lacks luster and appears dull or dead. As
decay progresses, the wood acquires an abnormal brown color, often as if it had been charred.
Cross grain cracking, collapse or crumbling, and abnormal shrinkage finally result.
Dry rot is an erroneous term commonly used to describe well decomposed, brown-rotted wood. It is
applied to wood which is both rotten and relatively dry, thus dry rotted. Actually, the wood was wet
when it decayed and subsequently dried out.
The strength properties of wood attacked by brown rot fungi decrease rapidly even in the early
stages of decay.
White rot occurs when fungi preferentially remove lignin or by removing
both lignin and cellulose together. White rot (Figure 4-7) is important in hardwoods used above
ground. In the early stages of decay, the wood color tends to turn an off-white, sometimes making