The Technique of FURNITURE MAKING

Shrinkage factors

Shrinkage factors along the length can usually be ignored for they are only fractional, but both tangential and radial shrinkages can be con­siderable, according to the particular species. Unfortunately, the greatest degree of shrinkage takes place between the critical moisture con­tents of air dried wood (20 per cent) and fully conditioned wood (9 to 10 per cent). For example, English oak, which is usually classed as a medium stable timber, will shrink approx­imately 5/16 in (8 mm) for every 12 in (304 mm) of width across the face of a tangentially cut board, and 3/16 in (4.5 mm) for a radially cut board, if dried down from 20 to 12 per cent moisture content; while beech, which has a large shrinkage value. will shrink 3/8 in (9.5 mm) and 7/32 in (5.5 mm) respectively. There is, however, no conformity between species, and other woods will exhibit less tangential and more radial shrinkage.

Dimensional changes for some of the more commonly used types are given below. As the moisture content of wood at any given humidity rate varies with the species, the values are based on conditioning from an outdoor humidity rate of 90 per cent down to an environmental humidity of 60 per cent.

Approximate comparative movement

Distortion, i. e. warping, twisting, etc., is not dependent on shrinkage values, but generally speaking timbers whose tangential and radial shrinkages are near to each other distort very little.