Properly seasoned lumber is crucial to furniture making. Moisture-related problems are common to woodworkers; in fact, most of us have a wooden door or drawer that works fine in the winter but sticks in the summer. This is caused by the dimensional change that occurs when solid wood shrinks and swells in response to atmospheric humidity.
Water is a major ingredient in the chemistry of photosynthesis, the method of converting sunlight into sugars critical to plant life. Living trees contain various amounts of water that is drawn from the ground and carried to the leaves. Trees contain less sap in the winter and, like the tides, they are affected by lunar cycles. Trees' water content increases when the moon is full as evidenced by a slight increase in girth. Moisture content from species to species is also a variable. Ash trees are about40% moisture (weight of water to weight of wood with no water), some of the oaks are twice that. We need lumber that is less than 10% moisture content. Wood acts like a sponge, when it dries, it shrinks. When dry, wood becomes harder and, when dried properly, it is more stable. The idea is to dry the wood so when it changes dimension it doesn't crack or "check."
Wood shrinks according to its structure. Think of the tree's growth rings, one ring per year of growth. A board contains these rings too and, when it dries, the rings shrink in their length across the board. They don't shrink as much in their thickness, though, so there is a tendency for the board to distort. We allow, even encourage by our sawing patterns, that distortion to occur in order to avoid splits or checks. The rate of drying must be controlled, particularly for the thicker pieces. Boards lose moisture from the surface before the core. As the surface loses moisture, it shrinks. If the core doesn’t lose moisture at a similar rate the surface may develop checks. It is essential that the moisture content at the surface and the core not vary too much.
Obviously, controlling the rate of drying has a direct bearing on the quality of the lumber. Two other aspects of drying that affect the quality of the lumber are equalizing and conditioning. Equalizing ensures that the moisture content in the center of a board is the same as the outside, or shell. Conditioning relieves the stresses created during drying or stresses that are the result of growing conditions. We've heard sawyers say, "That tree grew next to water" as boards "spring" or warp as they are sawn from the log. A tree growing on a ridge or at the foot of a cliff will develop stresses in response to prevailing wind or uneven exposure to sunlight. Well-conditioned lumber is more stable and translates into better yield in the shop. When drying wood for one's own use, it is our experience that greater care is taken. Drying is an art and a science.
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