Home

Education in wood Dynamics

To better understand the basics of furniture design a little education is necessary at this point. Wood is a dynamic material, continually shrinking and swelling in response to seasonal and local changes in humidity. The paradox is that if wood is allowed to shrink and swell un-restrained it will remain healthy - similar to breathing; however, if wood is constrained and forced to exist in a specific rigid dimension it will be crushed and torn apart as it tries in vain to follow its natural cycle. To further complicate the matter the magnitude of fluctuations in a board are different depending on which axis you are measuring. So the overall length (longitudinal dimension) of a hardwood board may only change by .1% and it's thickness (radialy) by 2%, but the width (tangential to the growth rings) by as much as 5%. So, if you mate two boards, one longitudinally and one across the grain you produce a laminate that has 50x as much shrink/swell cycle on one side as the other - a disaster in slow motion.

This is why plywood always eventually delaminates - wood fibers fixed in opposing axis - and why I avoid its use entirely in fine furniture. Large panels such as table tops are especially susceptible to size fluctuations because of the accumulated cross grain width of the boards. To counter this I use a wood joint called a sliding dovetail to attach them to their rigid frame members below. This solution allows the top to dimensionally fluctuate unconstrained while still holding firmly to the longitudinal forms supporting it.

The seasonal shrinkage and swelling of wood also plays havoc when you have very different materials in intimate contact like metal screws and wood. In regards to humidity, metal is a dimensionally stable material and as we have learned, wood isn't.  This is why you see loose screws on hinges in old furniture, admittedly a difficult thing to avoid. Because of this issue I avoid metal fasteners in all my joints preferring instead to use traditional wood framing joints, including mortise and tenons, bridle joints, dovetails, scarf joints, etc. These joints are not only stronger now but will last much longer and are very attractive in and of themselves.