Lumber defects are nothing more than imperfections in the media we use to build things. And, no matter what wood you use, some or all of them are going to be a little less than perfect. Defects can be either "man made" or naturally occcuring. The point of this page is to identify the types of defects that are out there, and maybe point a few fingers.
Man made defects can happen anywhere along the milling chain of events, from tree to table saw; from felling to fillister plane. But, with only a couple of exceptions, more times than not, man made defects can be attributed to wood movement. No matter what species of wood you use, you're absolutely going to experience movement or the change in wood dimension in one or more planes due to the change in moisture content. And, most of these resolve to one of two types of problem... Either the wood was not dried with proper care or the wood distorts or warps due to internal stresses because of inconsistent rates of moisture loss.
Naturally occurring defects are more "organic" so to speak. Natural defects are always environmental, be they soil composition, changes in weather conditions, foreign invaders of one type or another, or growth patterns. They will almost certainly show up as changes in color, but can also represent as a physical defect as in wormholes.
Some defects, especially the natural defects, can be part of the design aesthetic and create stunningly beautiful pieces. Others can be worked around or incorporated to impart a certain "feel" for a piece. All of the man made problems though, will have to be corrected through the milling process. And, depending on the severity of the problem, some boards may not be salvageable and have to be discarded. Further, some lumber, because of the way it's initially cut, may be prone to continued warp and distortion, even after milling.
|Bow||A curve along the face of a board that usually runs from end to end.|
|Check||A crack in the wood structure of a piece, usually running lengthwise. Checks are usually restricted to the end of a board and do not penetrate as far as the opposite side of a piece of sawn timber.|
|Crook||Warping along the edge from one end to the other. This is most common in wood that was cut from the centre of the tree near the pith.|
|Cup||Warping along the face of a board across the width of the board. This defect is most common of plain-sawn lumber.|
|Shake||Separation of grain between the growth rings, often extending along the board's face and sometimes below its surface.|
|Split||A longitudinal separation of the fibres which extends to the opposite face of a piece of sawn timber.|
|Twist||Warping in lumber where the ends twist in opposite directions.|
|Wane||The presence of bark or absence of wood on corners or along the length of a piece of lumber. Wane in the form of bark is more commonly associated with rough milled lumber. In the case of construction lumber (i.e., pine 2'x4's), it can be bark or missing wood.|
|Machine Burn||Discoloration of the wood due to overheating caused by friction, and either scorching the wood or the resins within it. Machine burn is caused by stopping or not feeding the wood across the blades at the correct rate of speed. Although species like cherry, pine, or walnut are more susceptible than others, machine burn can almost always be prevented by using sharp blades and correct feed rates.|
|Blue Stain||A discoloration that penetrates the wood fibre. It can be any colour other than the natural colour of the piece in which it is found. It is classed as light, medium or heavy and is generally blue or brown.|
|Pitch||An accumulation of resinous material on the surface or in pockets below the surface of wood. Also called gum or sap.|
|Loose Knot||A knot that cannot be relied upon to remain in place in the piece. Caused by a dead branch that was not fully integrated into the tree before it was cut down.|
|Tight Knot||A knot fixed by growth or position in the wood structure so that it firmly retains its place in the surrounding wood.|
|Spalt||Typically found in dead trees, spalting is any form of wood discoloration caused by fungi. There are three types of spalting that are typically incorporated into woodworking as design elements, pigmentation (or sapstain), white rot, and zone lines.|
|Wormhole||Small holes in the wood caused by insects and beetles.|