A truck loaded with newly felled logs destined for a sawmill to be converted into boards ready for drying could just as aptly be called a water tanker as a logging truck. That’s because practically half the weight of each log could be due to water. We call this its moisture content (MC).
Since the properties of timber depend a great deal on the amount of moisture it contains, knowing what that amount is allows us to dry and use it to the best advantage.
Determining Moisture Content
MC is a variable and its weight is expressed as a percentage of a constant, which is the oven-dry weight of the wood.
Weight of water x 100
______________________ = %MC
Dry weight of wood
If you have a scale or balance accurate to about half a gram, the oven-dry method of determining MC is simple. Take a sample from a board, weigh it wet, dry it, weigh it dry, and do the math.
Initial weight of sample – dry weight of sample x 100
Dry weight of sample
This was the simplest method of determining MC prior to the advent of moisture meters. All that is required now to get fast results is to follow the manufacturer’s instructions. Although I don’t consider meters absolutely essential, they are nonetheless very desirable, for they provide a real indication of whether the wood will perform as expected.
Depending upon whether it is softwood or hardwood, there can be a considerable range of MC in a newly-felled tree. As well, the sapwood will contain more moisture than the heartwood. The average MC is about 75%, but it can exceed 200% in some species! The desirable MC for furniture-making is 8% to10%, so clearly a lot of moisture must be removed.
Options for Removing Moisture
Two ways of removing moisture are air drying and kiln drying. Done correctly, both methods will deliver wood that is sound, flat and without stain mark from stickers. Done incorrectly, both will deliver distorted, split and/or casehardened material best suited for firewood. Although the two drying methods employ different procedures, the underlying principle is the same for both. To better understand the problem 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. No wonder, then, that the MC in a newly-felled tree can range from 75% to more than 200%. Moisture in the cavity of the newly-felled log is called free water and moisture in the cell wall is called bound water.
Drying wood is a slow process. It 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 (RH), 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 MC 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: it shrinks about twice as much in the tangential plane as it does in the radial plane. This is called differential shrinkage.
Differential shrinkage is the sole reason that wood distorts. A flat board can distort in four ways: cup, bow, spring and twist. Various splits or shakes may also occur. These distortions are further complicated by the fact that trees grow in a spiral fashion. The amount of spiralling varies from hardly perceptible to the highly visible, such as may be seen in the splits or cracks in a wooden utility pole.
Wood is Hygroscopic
Generally speaking, if you buy or own a board of distorted wood, it will be cut up into selected parts for the job at hand. It is inevitable although often surprising how much distortion is minimized as the parts are cut into smaller pieces. The next task is planing to thickness and size, which entirely removes all distortion. However, wood is hygroscopic: it loses or gains moisture (shrinks or expands) until its MC reaches equilibrium with the humidity of the air. This point is called equilibrium moisture content (EMC).
Woodworkers long ago learned to cope with wood’s nature to shrink and expand. For example, one of the earliest and ingenious solid wood furniture designs is the frame-and-panel, which allows a panel to change size within a dimensionally stable frame.
Dimensional changes occur on a short-term, or daily, basis and over a long-term, or seasonal, basis. Both 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.
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.
Kilns Regulate Drying Variables
Kiln operators have a vested interest in minimizing any sort of degrade to the charge of wood being dried. They strive to avoid distortion, splits and case hardening. The early days of kiln drying often produced batches of less than satisfactory quality. As operators learned how to better regulate the three drying variables, they developed and applied a rigorous series of adjustments over given periods of time. Operators now follow a kiln schedule for every species of commercial lumber. More than one species may have the same kiln schedule so they can be dried together. Kiln operations today are so well understood that results are predictable. The outcome is that we have better and more consistent lumber products to work with than our predecessors. Kiln drying has become a science; air drying is still an art.