2. Drying should proceed uniformly at all points, otherwise extra stresses are set up in the wood, causing warping, etc.
3. Heat should penetrate to the interior of the piece before drying begins.
4. The humidity should be suited to the condition of the wood at the start and reduced in the proper ratio as drying progresses. With wet or green wood it should usually be held uniform at a degree which will prevent the surface from drying below its saturation point until all the free water has evaporated, then gradually reduced to remove the hygroscopic moisture.
5. The temperature should be uniform and as high as the species under treatment will stand without excessive shrinkage, collapse, or checking.
6. Rate of drying should be controlled by the amount of humidity in the air and not by the rate of circulation, which should be made ample at all times.
7. In drying refractory hardwoods, such as oak, best results are obtained at a comparatively low temperature. In more easily dried hardwoods, such as maple, and some of the more difficult softwoods, as cypress, the process may be hastened by a higher temperature but not above the boiling point. In many of the softwoods, the rate of drying may be very greatly increased by heating above the boiling point with a large circulation of vapor at atmospheric pressure.
8. Unequal shrinkage between the exterior and interior portions of the wood and also unequal chemical changes must be guarded against by temperatures and humidities suited to the species in question to prevent subsequent cupping and warping.
9. The degree of dryness attained should conform to the use to which the wood is put.
10. Proper piling of the material and weighting to prevent warping are of great importance.