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At one time or another, I suspect most booksellers have attempted to straighten warped boards by grasping the corners of the offending board and bending in the direction opposite the warp. Quick and simple. Indeed, with practice, it's possible to develop a refined feel for how far a board can be bent short of creasing or breaking it. The problem with this method is that it places enormous stress on the hinge, especially at the extremities, sometimes to the point of causing noticeable damage or even splitting. Any attempt to correct warping, therefore, should necessarily involve stabilizing the hinge first.
This can be readily accomplished with the use of small blocks of wood and clamps. In the accompanying figure, two blocks of wood have been placed as near as possible to the hinge on opposite sides of the board and clamped tightly in place with spring clamps, though C-clamps could have been used as well. When bending begins, the stress is transferred from the hinge itself to the clamps.
Once the clamps are in place, the board can be bent manually, but I recommend practicing the technique on a book you are prepared to throw away. It's safer to clamp the board into the configuration shown and allow it to return to shape gradually, over a day or so. Adjustable clamps work best for this application because they can be adjusted to any level of tension. Several re-clampings may be necessary to achieve the desired results.
In the case of particularly stubborn boards, it's possible to accelerate the straightening process by misting the concave side of the board with water. This softens the fibers on the side that will be stretched out when clamped. This is not recommended for leather covered boards or any other material that might suffer from a brief moistening. Some experimentation might be necessary.
If the board covering is on the convex side of the board (and the pastedown on the concave side), it's possible that the outer covering material has been stretched and/or separated from the inner board, and restoring it to its original position will produce wrinkling in the cover material. In this case it's probably best to leave well enough alone.
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