Moisture Content in Guitar Wood

Guitar Wood: Environmental Factors Affecting Moisture Content

Once the wood has reached equilibrium, the moisture exchange with the environment does not stop. As the humidity rises, the wood takes on moisture and grows in dimension. As the air becomes drier, the wood gives up moisture to keep up with its surroundings and shrinks in dimension. This can be witnessed in the shop by placing a freshly milled piece of thin wood flat on the workbench. You will notice the edges of the wood will curl up, making the wood appear concave; or the piece will bulge up in the middle, creating a domed look. The first scenario indicates that the air is dryer than the wood and the top surface of the wood is giving up moisture and shrinking. The second scenario indicates the air contains more moisture than the wood and the top surface of the wood is taking on moisture and swelling. In both cases the dimensions are changing. The introduction of moisture can also cause the wood to warp and twist.

Always be sure to inquire when purchasing guitar wood if it has been dried or not, and whether it was dried in a kiln or air dried. If the wood has been air dried, you will also want to know how long it has been drying. Wood that has not been dried fully will need to sit until equilibrium is reached. Wood tends to give up moisture much faster through the end grain than the face grain, and that is why you will sometimes notice end checks or cracks. To avoid this, coat the ends of the wood with paraffin wax or even glue, in a pinch, to retard the loss of moisture through the end grain. Also, all wood, when not in use, should be stickered. Stickering is placing a small piece of wood under each end of a board, 1/8 inch of height will work just fine, so that air can freely circulate around the entire piece (see page header).

Extreme changes in humidity can have extreme effects on a guitar. Dimensional changes are most pronounced in the largest dimension of a particular piece of wood. For instance, the top or back of a guitar is a piece of wood roughly 2mm thick but almost fifteen inches wide in the lower bout and almost twelve inches in the upper bout. The width of the top and the back will move considerably with extreme changes in humidity. Typically in widths less than 3 inches movement is negligible, but 12-15 inches is another story. Checks and cracks can develop from drastic changes in humidity toward the dry side; buckling is the result of swelling from humidity. The finish applied at the end of making a guitar, especially shellac used in French polish, will also add some barrier to the moisture.