This is an excerpt from “With the Grain: A Craftsman’s Guide to Understanding Wood” by Christian Becksvoort.
A commonly encountered misconception is that wood breathes. As we know, heartwood is composed entirely of dead cells, while sapwood has some living cells, which die after the wood is cut. Nonetheless, wood is an organic substance that by its nature responds to climatic changes. Moisture is absorbed or given off as the seasons dictate.
When the relative humidity rises, the wood fibers absorb moisture that penetrates from the outside and causes the wood to swell. As the humidity decreases, excess moisture is given off by the fibers to be reabsorbed by the surrounding air. Wood is constantly trying to maintain a balance between its moisture content and that of the surrounding environment. This balance is called the equilibrium moisture content (EMC). Simply expressed, it is the amount of moisture present in wood at a given temperature and relative humidity over a period of time (Fig. 4-9).
Relative humidity is expressed as a percentage of the amount of moisture that the air is capable of holding at a given temperature. Warm air can hold more water vapor than cold air. For instance, at 86°F (30°C) and 100 percent relative humidity can hold five times as much water vapor as air at 43°F (6°C) and 100 percent relative humidity. Hence, it is a good idea for a well-equipped wood shop to have a thermometer and hygrometer.
Numerous considerations influence the air drying of lumber, among them:Climatic conditions. Generally speaking, very little drying of lumber is possible during the winter, particularly in those areas where the temperature remains below freezing. Moisture close to the surface can evaporate by the process of sublimation, whereby the water goes from a solid state (ice) directly to a gaseous state (vapor) without becoming a liquid. In areas where winter temperatures are relatively mild, some drying will occur, as long as rainfall and humidity are not excessive. Drying rates are variable and often very localized. The location of the drying pile and even its orientation to the sun and prevailing wind all influence the rate of evaporation.
Species. The wood species makes quite a difference when it comes to length of drying time. Specific gravity is a fairly good general indicator of drying rate. The lower the specific gravity, the faster the drying time. The softwoods and lighter species of hardwoods dry faster under favorable conditions. The percentage of sapwood and heartwood also plays a part. For example, sugar maple dries faster than Northern red oak with roughly the same specific gravity, but sugar maple has more sapwood. Figure 4-11 lists the approximate air- drying times of some native woods.
Thickness. The old rule of thumb “one year of drying for each inch of thickness” has no basis in fact. First, it does not take species into account. Second, drying time is a function of the square of the thickness. This means that 8/4, or 2″ (5 cm) stock takes four times as long as 4/4, or 1″ (2.5 cm) stock. In fact, for some species the drying time is even longer than the square of the thickness. This is one reason (along with the differential between radial and tangential shrinkage, described in Chapter 5) why it is next to impossible to dry entire logs without serious cracking or checking.
Grain orientation. Quartersawn wood is slower to dry than plain-sawn wood. The ray cells aid in drying, and although they appear more prominent on quartersawn wood, not nearly as many are exposed on the face of the board.
Pile construction and foundation. The actual method of stacking the wood has a lot to do with the drying rate. Adequate space left around each board aids in drying. Many smaller piles dry faster than one large pile. The pile foundation should be well off the ground to allow for free air movement underneath. Weeds and debris should not obstruct the air flow. Finally, the ground should be well drained, with no standing water.