LUMBER DEFECTS
Most lumber defects adversely affect a board’s appearance, strength, workability or ability to take a finish. Sometimes, however, irregularities or abnormalities can actually make a piece of wood more desirable, especially when they produce a popular, distinctive figure like bird’s-eye or burl. Of course, intended use is the final arbiter; what may be a blemish to one woodworker is another board’s selling point. Knots, for example, would be a significant defect in boards intended for a tabletop, but they are an essential feature of some types of paneling.
Lumber defects are either natural, man-made or the result of poor seasoning. All wood harbors natural defects that are caused by growing conditions or qualities of the species itself. The same type of defect may be present in different woods. Some imperfections are found in all species. Loose knots, for example, are caused simply by the way trees grow. They are the remnants of broken branches that have become encased by the growth of new wood. Other natural defects include gum in hardwoods, pitch in softwoods and reaction wood in all species. Natural forces such as fire, wind, fungi and insects can also cause defects in wood. A common defect of this sort is blue stain.
Several defects occur when wood is exposed to the air and allowed to dry.
Because wood does not shrink uniformly in all dimensions, warping can result when the moisture content of lumber drops below a certain level. (Refer to the Drying and Storing Wood chapter for more information on proper seasoning of wood.) Some common seasoning defects are checks, bow, cup, twist, crook and split. Keep in mind that these defects can also occur in boards cut close to the pith of a log. Some com
mon defects are explained in the chart below and opposite.
Although it is virtually impossible to buy wood that is completely defect-free, you can increase your chances of obtaining the best lumber for your needs by selecting your wood carefully (page 42). Another point to consider is that you can salvage some lumber with defects if you have access to the necessary tools and learn how to use them (page 53).
DEFECTS IN WOOD
|
TYPE |
CHARACTERISTICS |
REMEDIES |
|
Tight knot |
Appears as a whorl that is intergrown with the surrounding wood tissue. Knots form as the girth of the tree increas- |
Does not seriously weaken - a board; can be cut out or |
|
es, gradually enveloping the branches. If the branches are still alive at the time of their envelopment, the knot integrates with the wood in the tree trunk |
used, as appearance dictates. |
|
|
Loose or dead |
Appears as a whorl encircled by a dark ring. When a |
Remove knots before working |
|
knot c£> .......... |
branch dies the remaining stump is eventually enveloped ^ by the trunk. But the dead stump cannot integrate with the tissue surrounding it, creating a loose or dead knot. |
with the lumber. |
Flatten bowed boards on the jointer (page 55), or cut into shorter pieces, then use the jointer.
Cup may correct itself if both faces are allowed to dry to the same moisture content. Cupped boards can be salvaged on the band saw (page 54) or jointer (page 55).
Board can be salvaged on jointer (page 55), or cut into shorter boards.
Board can be salvaged on jointer or table saw (page 55).
Similar to checks, appearing as separations along the growth rings. Also known as ring check or ring shank. Results from improper drying of wood or felling domage.
Appears as a dark streak across the faces or edges of lumber. Occurs when planer knives are dull or spin on one part of board for too long.
Appears as a discoloration of the surface on otherwise normal-looking wood. Results from molds that flourish when lumber is dried or stored in warm, moist or poorly ventilated conditions. Species like holly and English sycamore are prone to blue stain.
THE STRESS OF UNEVEN GROWTH
