Wood as a Building Material - Carpenter's and Builder's Math, Plans, and Specifications - page 63

No method is known for estimating the amount of reduction in
strength from the appearance of decayed wood. Therefore, when
strength is an important consideration, the safe procedure is to
discard every piece that contains even a small amount of decay.
An exception may be pieces in which decay occurs in a knot but
does not extend into the surrounding wood.
Blue Stain
In the sapwood of many species of both softwoods and hardwoods,
there often develops a bluish-black discoloration known as blue
stain .Itdoes not indicate an early stage of decay, nor does it have any
practicable effect on the strength of the wood. Blue stain is caused
by a fungus growth in unseasoned lumber. Although objectionable
where appearance is of importance, as in unpainted trim, blue stain
need cause no concern for framing lumber. Precautions should be
taken, however, to make sure that no decay fungus is present with
the blue stain.
Nuclear Radiation
Very large doses of gamma rays or neutrons can cause substan-
tial degradation of wood. In general, irradiation with gamma rays
in doses up to about 1 megarad has little effect on the strength
properties of wood. As dosage increases above 1 megarad, tensile
strength parallel to grain and toughness decrease. At a dosage of 300
megarads, tensile strength is reduced about 90 percent. Gamma rays
also affect compressive strength parallel to grain above 1 megarad,
but strength losses with further dosage are less than for tensile
strength. Only about one-third of the compressive strength is lost
when the total dose is 300 megarads. Effects of gamma rays on
bending and shear strength are intermediate between the effects on
tensile and compressive strength.
Weathering
Without protective treatment, freshly cut wood exposed to the
weather changes materially in color. Other changes due to weather-
ing include warping, loss of some surface fibers, and surface rough-
ening and checking. The effects of weathering on wood may be
desirable or undesirable, depending on the requirements for the par-
ticular wood product. The time required to reach the fully weathered
appearance depends on the severity of the exposure to sun and rain.
Once weathered, wood remains nearly unaltered in appearance.
The color of wood is affected very soon on exposure to weather.
With continued exposure, all woods turn gray. However, only the
wood at or near the exposed surfaces is noticeably affected. This very