Understanding Wood – Hoadley
Chapter 6 – Water and Wood
Chapter 6 of Understanding Wood explores the relationship between water and wood. Any aspiring furniture-maker should gain at the very least, a basic understanding of these principles.
The following three topics are the focus of this lesson.
To put it simply, wood has a tendency to absorb moisture in damp environments and release it during dry spells. The term “moisture content” refers to the ratio of water by weight vs. the weight of the same piece when all water has been removed. The term “equilibrium moisture content” refers to the normal moisture content for the piece of wood at a given relative humidity.
Freshly cut or “green” wood is not suitible for most applications so excess moisture must be removed. This serves several purposes. Future shrinking is minimized by pre-shrinking. Properly dried wood is stronger and easier to glue or finish. It also costs much less to ship due to less weight.
Most wood is dried in kilns. This is a carefully monitored process whereby wood is placed in an enclosure and hot air is circulated through the lumber. Wood intended for interior use is typically dried well under 10% moisture content. Lumber intended for construction will range from about 15-19% moisture content.
Air dried lumber simply refers to wood that is allowed to dry naturally to the equilibrium moisture content level of the geographical location and the season. The process does require some control such as carefully stacked piles with spacers (called”stickers”) between boards. It is protected from direct sunlight, rain and the end grain is painted to promote even drying throughout the boards.
Since equilibrium moisture content is very different from one locality to the next it is not possible to give exact numbers regarding the EMC of wood. A chart listing average monthly EMC values for various US cities can be found in this USDA document on page 15.
This plane is oriented tangential to the growth rings. Wood has the most dimensional change along this direction. A decent approximation for most species of wood is 1/8″ movement in a 12″ wide board.
This plane is oriented along the length of the grain. The amount of shrinkage and expansion that occurs along the longitudinal plane is negligible as far as the furniture-maker is concerned.
This plane is oriented perpendicular to the growth rings. There is less expansion and contraction in this direction and is roughly half that of the tangential plane.
Types of Warping
Flatsawn wood tends to warp in this manner due to uneven shrinkage and swelling in the tangential and radial planes. As the board becomes drier, the growth rings will tend to flatten out.
Anything that causes more tension on one side of the board than the other can cause it to bow. Since no tree grows perfectly symmetrical, this is a common thing to see in lumber.
This is as common as a bowed board. Again, it is due to uneven distribution of stress as the wood loses or gains moisture.
It is similar to a bowed board but since it is bending down the width of the board there must be some serious tension involved. Usually boards like this will tend to twist and bend into a new shape when it is cut. Avoid using boards like this in places where stability is needed.
This is caused by some sort of localized stress such as a knot. The board looks straight on either side of the defect but takes a sharp curve right at the defect.
This is seen when a board is cut in such a way that the growth rings are going diagonal through the board. Uneven dimensional change between the radial and tangential planes causes two opposing corners to shrink and swell more than the other pair.