Wood warping is a deviation from flatness in timber as a result of stresses and uneven shrinkage. Warping can also occur in wood considered "dry" (wood can take up and release moisture indefinitely), when it takes up moisture unevenly, or – especially – is allowed to return to its "dry" equilibrium state unevenly, too slowly, or too quickly. Many factors can contribute to wood warp; wood species, grain orientation, air flow, sunlight, uneven finishing, temperature – even cutting season and the moon's gravitational pull are taken into account in some traditions (e.g., violin making).
The types of wood warping include:
- bow: a warp along the length of the face of the wood
- crook: (also called wain) a warp along the length of the edge of the wood
- kink: a localized crook, often due to a knot
- cup: a warp across the width of the face, in which the edges are higher or lower than the center of the wood
- twist or wind: a distortion in which the two ends do not lie on the same plane. Winding sticks assist in viewing this defect.
Wood warping costs the wood industry in the U.S. millions of dollars per year. Straight wood boards that leave a cutting facility sometimes arrive at the store yard warped. This little understood process is finally being looked at in a serious way. Although wood warping has been studied for years, the warping control model for manufacturing composite wood hasn't been updated for about 40 years.
A researcher at Texas A&M University, Zhiyong Cai, has researched wood warping, and was working on a computer software program in 2003 to help manufacturers make changes in the manufacturing process so that wood doesn't arrive at its destination warped after it leaves the mill or factory.
- Cai, Zhiyong, Dickens, James R (2004). "Wood composite warping: Modeling and simulation". Wood and fiber science. Soc Wood Sc Tech. 36 (2): 174–185.