Air drying lumber is a time-honored method that, properly done, is effective. It normally takes about one year to dry each inch of thickness. Kiln drying speeds the process by strict control of the three factors that cause drying: air movement, temperature and humidity.
Differential shrinkage. A log has a radius and a tangent. If a board is cut so the faces are tangential, the adjacent edges are radial. The shrinkage in the tangential direction is, on average, twice that in the radial direction.
Wood Movement: Quartersawn Board
Because the tangential tissue is symmetrical on each side of the center line through the board, shrinkage is balanced on each face and the board will tend to remain flat.
Types of Distortion

As mentioned in the previous lesson,  there are two ways of removing moisture content (MC) from wood: air drying and kiln drying. Although the two drying methods employ different procedures, the underlying principle is the same for both. To better understand the process of drying wood, imagine the microscopic vessels in the board magnified to the size of drinking straws. These straws, however, differ from ordinary drinking straws in that they are all shapes and sizes. Some have open ends, some have pointed ends; some have thick walls and narrow cavities, while others have thin walls and wide cavities; some are large and some are small. A more consequential difference is that not only is the vessel cavity full of moisture but the vessel wall is also saturated. Moisture in the cavity is called free water, and moisture in the cell wall is called bound water.

Wood dries from the outside, where the evaporated moisture is continuously replaced by moisture migrating from the inside. If the board is dried too rapidly, the outer layers shrink while the interior is still saturated. The result is a complication called case hardening. The key to success is balancing the rate of surface evaporation with the migration of moisture from
the inside.

Three factors control drying: humidity, rate of air circulation and air temperature. Temperature, however, plays a dual role. It influences relative humidity, which affects the rate of evaporation from the wood surface, which in turn governs the rate of migration of moisture outward.

The free water in the vessel cavities dries out first, like emptying a bottle. When all the free water has been removed — this is a theoretical rather than an absolute condition — the wood will have a moisture content of about 30%. This is called the fiber saturation point. Up to this stage, no change in the shape or dimension of the wood has occurred — no shrinkage, no distortion.

The next drying stage is the removal of bound water from the vessel walls, and it’s here that changes begin. As drying continues, the board shrinks in width and thickness but not in length. If shrinkage were to occur equally on both faces and both edges, the board would remain as flat as it came from the saw. However, shrinkage is unequal: A board shrinks about twice as much in the tangential plane as it does in the radial plane. This is called differential shrinkage. And differential shrinkage is the sole reason that wood distorts.

A flat board can distort in four ways: cup, bow, spring and twist. Cupping is almost always associated with flatsawn cathedral figure board. Look at the end grain and you will see which side is nearer the center of the tree. The board has shrunk more on the surface away from the heart, causing it to curl, as though the growth rings were trying to straighten out. If you look at the end grain of a quartersawn board, growth rings are mostly perpendicular to the surface of the board. The tangential shrinkage is the same on both sides, with the result that the board stays flat.

A sprung board is flat but bent, like a river going around a corner. The bend is normally too minor to prevent harvesting useful parts. A bowed board is also flat, but bent, like a road going over a hill. A twisted board is shaped like a propeller. Severely twisted boards are seldom useful.

Wood shrinks and expands in response to water vapor in the air. It takes on water vapor and expands when the humidity is high. It gives back water vapor and shrinks when the air is drier.

Dimensional changes occur on a short-term, or daily, basis and over a long-term, or seasonal, basis as the wood tries to reach moisture equilibrium with its surroundings. Both changes are gradual, but short-term changes tend to affect only the surface tissue, as in the case of a board that cups slightly when left overnight on a bench top. Turn it over, and it gradually returns to flat. Long-term changes, such as the change from more humid summer to less humid winter, explain why doors and drawers that move freely in winter tend to stick in summer.

Neither daily nor seasonal changes in moisture content can be prevented by the application of a finish. A finish may retard the rate of change, and one finish, or the number of applications, may be temporarily more effective than another, but if humidity and temperature changes in the atmosphere persist, wood movement is unavoidable.