The evaporation of water from wood takes place largely through the ends, i.e., in the direction of the longitudinal axis of the wood fibres. The evaporation from the other surfaces takes place very slowly out of doors, and with greater rapidity in a dry kiln. The rate of evaporation differs both with the kind of timber and its shape; that is, thin material will dry more rapidly than heavier stock. Sapwood dries faster than heartwood, and pine more rapidly than oak or other hardwoods.
Tests made show little difference in the rate of evaporation in sawn and hewn stock, the results, however, not being conclusive. Air-drying out of doors takes from two months to a year, the time depending on the kind of timber, its thickness, and the climatic conditions. After wood has reached an air-dry condition it absorbs water in small quantities after a rain or during damp weather, much of which is immediately lost again when a few warm, dry days follow. In this way wood exposed to the weather will continue to absorb water and lose it for indefinite periods.
When soaked in water, seasoned woods absorb water rapidly. This at first enters into the wood through the cell walls; when these are soaked, the water will fill the cell lumen, so that if constantly submerged the wood may become completely filled with water.
The following figures show the gain in weight by absorption of several coniferous woods, air-dry at the start, expressed in per cent of the kiln-dry weight:
Absorption of Water by Dry Wood
| White Pine | Red Cedar | Hemlock | Tamarack | |
| Air-dried | 108 | 109 | 111 | 108 |
| Kiln-dried | 100 | 100 | 100 | 100 |
| In water 1 day | 135 | 120 | 133 | 129 |
| In water 2 days | 147 | 126 | 144 | 136 |
| In water 3 days | 154 | 132 | 149 | 142 |
| In water 4 days | 162 | 137 | 154 | 147 |
| In water 5 days | 165 | 140 | 158 | 150 |
| In water 7 days | 176 | 143 | 164 | 156 |
| In water 9 days | 179 | 147 | 168 | 157 |
| In water 11 days | 184 | 149 | 173 | 159 |
| In water 14 days | 187 | 150 | 176 | 159 |
| In water 17 days | 192 | 152 | 176 | 161 |
| In water 25 days | 198 | 155 | 180 | 161 |
| In water 30 days | 207 | 158 | 183 | 166 |
Rapidity of Evaporation
The rapidity with which water is evaporated, that is, the rate of drying, depends on the size and shape of the piece and on the structure of the wood. An inch board dries more than four times as fast as a four-inch plank, and more than twenty times as fast as a ten-inch timber. White pine dries faster than oak. A very moist piece of pine or oak will, during one hour, lose more than four times as much water per square inch from the cross-section, but only one half as much from the tangential as from the radial section. In a long timber, where the ends or cross-sections form but a small part of the drying surface, this difference is not so evident. Nevertheless, the ends dry and shrink first, and being opposed in this shrinkage by the more moist adjoining parts, they check, the cracks largely disappearing as seasoning progresses.
High temperatures are very effective in evaporating the water from wood, no matter how humid the air, and a fresh piece of sapwood may lose weight in boiling water, and can be dried to quite an extent in hot steam.
In drying chemicals or fabrics, all that is required is to provide heat enough to vaporize the moisture and circulation enough to carry off the vapor thus secured, and the quickest and most economical means to these ends may be used. While on the other hand, in drying wood, whether in the form of standard stock or the finished product, the application of the requisite heat and circulation must be carefully regulated throughout the entire process, or warping and checking are almost certain to result. Moreover, wood of different shapes and thicknesses is very differently effected by the same treatment. Finally, the tissues composing the wood, which vary in form and physical properties, and which cross each other in regular directions, exert their own peculiar influences upon its behavior during drying. With our native woods, for instance, summer-wood and spring-wood show distinct tendencies in drying, and the same is true in a less degree of heartwood, as contrasted with sapwood. Or, again, pronounced medullary rays further complicate the drying problem.