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Moisture Causes Distortion in Drying Wood Causing Cup and Bow Lumber
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Drawing of cup distorted lumber Cupped wood remains flat lengthwise and on the edge, but the sides curve across the width, away from the center.

A distorted board is almost always a sawn board from a sawmill. The distortion, which occurs during drying, may take any of the four forms demonstrated in the images below. The longer and wider the board, the more obvious any distortion, but when cut into furniture parts, the distortion can usually be overcome during the stock preparation stage.

Drawing of lumber with bow distortion In bowed wood, the board remains flat across the width and edges, but the length is curved.

A twisted board usually presents the worst salvage problem. Provided MC is in the 8% to 10% range, if you leave the sawn parts to stabilize in a dry shop for a couple of weeks before preparation, they will normally remain flat after preparation.

posted on December 1, 2008 by Matt Hocking
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