worms, and fungi, which can cause defects either
The fiber-saturation point varies from 25 to 30
before or after the lumber is cut. Improper
percent, but for general purposes is accepted as 30
seasoning causes other defects and blemishes.
percent. Below the fiber-saturation point, the
imbibed water extracts from the cell walls, causing
The most common defects are knots. Knots
a reduction in the thickness of the walls.
occur in most kinds of lumber and are the result of
branch growth. An interwoven knot forms while the
Wood shrinks across the grain when the
tree is alive. Its annual rings are interwoven with
moisture content lowers below the fiber-saturation
t h o s e of the trunk of the tree. Usually an
interwoven knot is solid and is not a serious defect.
shrinking and swelling of the wood cells, changing
If the limb dies before the tree is cut, the wood
the size of the cells. Therefore, the lowering or
formed in the trunk of the tree makes no further
raising of the moisture content causes lumber to
connection with the limb, but grows around it. This
shrink or swell.
produces a dead knot. This may be loose enough to
drop out or may be tight enough to hold its shape
The loss of moisture during seasoning causes
and position when the tree is being sawed into
wood to be (1) harder, (2) stronger, (3) stiffer, and
lumber. A spike knot is a long, thin knot caused by
(4) lighter in weight. There are two methods for
the way the tree was sawed. Small, solid knots are
seasoning lumber--air drying and kiln drying.
not objectionable in most of the lumber used aboard
ship. If lumber has loose or large knots, you should
Air-dried lumber is exactly what the name
cut it into smaller pieces to eliminate these defects.
implies. It is wood placed in a shed or in the open
to dry. This method takes up to 7 years to season
Heartshake and windshake (fig. 3-5) are other
lumber defects. Heartshake is caused by the action
of the wind and is a lengthwise separation of the
A faster method of drying is known as kiln
annual rings. Windshake is also a defect caused by
drying. The wood is placed in a kiln and treated
the action of the wind, which causes the tree to
with steam. The time required for drying varies
from 2 or 3 days to several weeks. Often a
combination of air-dried and kiln-dried methods is
A CHECK is a crack or separation, usually
used to dry lumber.
short, caused by the uneven shrinking of the wood
cells in seasoning. Do not confuse these with pitch
Lumber is dry enough for most uses when the
pockets. Pitch pockets are small enclosed spaces in
moisture content reduces to about 12 or 15 percent.
the wood filled with sap or pitch (rosin).
However, lumber used for patterns should be drier.
The moisture content should be 8 or 10 percent for
WARP or WARPAGE is a lumber defect in
hardwoods and 10 to 12 percent for softwoods. As
which a board distorts from a true, flat surface; it is
an HT, you will learn to judge the dryness of a
twisted, bowed, or cupped warped. The varying
wood by its color, weight, smell, and feel. Looking
amount of moisture in the wood changes the
at the shavings and chips also helps identify wood.
diameter of the cells. This causes the board to
shrink or swell in width as well as in thickness, but
LUMBER DEFECTS AND BLEMISHES
not in length. Redwood is an exception because it
will swell or shrink in all three dimensions.
A defect in lumber is any flaw that affects the
strength, durability, or utility value of the lumber.
A blemish is a flaw that mars only the appearance
of the lumber. A blemish that affects the utility
value of the lumber (such as a blemish in wood
intended for fine furniture or cabinet work) is also
You will seldom find a piece of lumber that
does not have a defect or blemish of some sort.
Some defects and blemishes are the result of decay
Figure 3-5.--Defects in logs.
in the growing tree. Others are the result of insects,