Seven main natural wood distortions occur after logs have been processed into lumber. The natural wood or lumber defects include a bow, crook, cup, twist, spalling, knots and cracks. Some lumber defects can be salvaged when cut properly.
Wood or lumber distortions occur from internal stresses of the wood and how the moisture is absorbed and released from the lumber. Identify a distortion by visually inspecting the surface, edges and ends of each piece of lumber.
A bow is seen when a board lays flat in the middle and both ends curve away from the horizontal. The bow may be very slight or it may be very drastic. Salvaging a piece of bowed lumber requires the bowed ends to be cut off.
A crook is identified by placing the board on edge and looking to see if one or both of the ends curve off the horizontal. The board may rock like a rocking chair if one end is pushed. Salvage a crooked board in two ways. Cut off the end that is crooked and use the remainder. Or cut the crooked edge off the board with a table saw by laying the flat edge against the fence. Flip the board over once the edge is flat and trim the opposite side. The board will be significantly narrower than purchased.
A twist occurs at either end or both ends of a board. The board will turn up or down. A twist is very noticeable when the board is laying on a flat surface. Salvage the board by cutting off the twisted end. The board may also be put under a wood plain to remove the twist. Then either trim the board to a narrower width or cut off the end.
Identify spalling by looking for a gray to green discoloration in the wood. The discoloration is permanent and cannot be sanded from the wood. It is caused by a fungus growth. Spalling may not matter depending on the woodworking project. Avoid purchasing lumber with spalling for fine wood projects.
Identify checking by looking for splitting at the end of the board. The splits will occur along the grain of the wood. Checking occurs when the board has not been cured properly. Salvage the wood by cutting off the checked ends. The only problem with using a piece of wood that shows checking is the wood may continue to split with age.
Knots are seen in many boards purchased. Avoid hardwood with knots due to the density of the wood surrounding the knot and the possibility that the knot may dislodge from the wood. The area does not cut easily and may split away from the rest of the wood. Wood with knots can still be used for many projects.
The Free Library: Alligatoring, checking, cracking, flaking and spalling
Woodworking: Understanding Wood Distortations
Good Wood Forest Products Inc: How to Avoid Wood Distortions After Buying