Woodworker's Journal 2008-32-6, страница 41

Shrinkage

Quartersawn board

Wood is Hygroscopic

Generally speaking, if you buy or own a board of distorted wood, it will be cut up into selected parts for the job at hand. It is inevitable although often surprising how much distortion is minimized as the parts are cut into smaller pieces. The next task is planing to thickness and size (see "Marking Up for Success: Prepare the Stock" at www.woodworkersjournal.com), which entirely removes all distortion. However, wood is hygroscopic: it loses or gains moisture (shrinks or expands) until its MC reaches equilibrium with the humidity of the air. This point is called equilibrium moisture content (EMC).

Woodworkers long ago learned to cope with wood's nature to shrink and expand. For example, one of the earliest and ingenious solid wood furniture designs is the frame-and-panel, which allows a panel to change size within a dimensionally stable frame.

Dimensional changes occur on a short-term, or daily, basis and over a long-term, or seasonal, basis. Both are gradual, but short-term changes tend to affect only the surface tissue as in the case of a board that cups slightly when left overnight on a bench top. Turn it over, and it gradually returns to flat.

Neither daily nor seasonal changes in moisture content can be prevented by the application of a finish.

Air-drying lumber is a time-honored method which, properly done, is effective. It normally takes about one year to dry each inch of thickness. Kiln-drying speeds the process by strict control of the three factors that cause drying: air movement, temperature and humidity, all of which, when air drying, are subject to the vagaries of weather.

Quartersawn Board. Because the tangential tissue is