Wood Expansion and Contraction: Lesson Topic Idea
Have you ever noticed a crack in a piece of furniture or a joint that has spilt? Often times we blame this on the wood we used. We are quick to explain that the wood had too high of moisture content and our wood source did a poor job of drying the lumber. There are other excuses that we sometimes use to help explain the mystery of cracked wood or blown joints. However, I feel the truth of the matter is we are not building furniture correctly.
Wood is unique in that it never really stops moving. The seasonal changes cause it to expand and contract. If we do not allow for this in our projects, the stress is released through cracks and blown joints. Realize that a 12” piece of Red Oak moves about ¼” during a year’s time. If we don’t allow it to expand and then contract within our furniture, it will find a way regardless (I’ve learned this the hard way).
As I walked around the MITES state completion last year I gave an estimated opinion that over 50% of the projects there would be possible victims of wood expansion and contraction. I often wonder if we as teachers don’t know much about wood movement (it has taken me quite a while to understand the concept and I’m still learning) or if we just don’t want to try to explain it to our students (this can also be a difficult venture). Personally I think it is a little of both.
I feel that we could all use a little refresher on the topic of wood expansion and contraction. We want our students to build heirlooms not one hit wonders. Take a minute and read some articles about wood expansion and contraction. If you would like graphs that show the individual wood specie movements or more information just email me.
This SLIDING DOVETAIL joint is one of the best joints to use when dealing this wood movement. On the downside, this joint is difficult to make.
When finishing your work, finish all surface areas with the same amount of coats in all locations. A sealed surface will resist moisture better, where as an unsealed surface will soak in moisture. The results of this dilemma will be a curved surface illustrated below.
Sources to learn more about Wood Expansion and Contraction:
Fine Woodworking – JUNE 2003
Wood Magazine -