Hasta La Ideas

This is the fourth post of an ongoing series about wood. Understanding its nature, the way it moves and changes, and the implications for designers. Check back every Wednesday for the next installment.

The first three posts are here:
» How Logs Are Turned Into Boards, Part 1: Plainsawn
» How Logs Are Turned Into Boards, Part 2: Quartersawn
» How Logs Are Turned Into Boards, Part 3: Riftsawn

My front door that used to open easily now sticks and requires a massive shove. The heat of summer is here and with it comes the impact on wood.

When we think of wood—constructed into a table, a bookshelf, a door—most of us assume it will remain more or less the same. After all it’s dead, right? But as I mentioned in my first post of this series, wood moves. And in some cases it can move significantly.

Understanding why it moves in the first place can help us know how to allow for such movement in design. So in this entry we’re going to get a bit science-y (and not so much about the pretty pictures). I should point out that you could literally spend entire semesters studying this stuff, so reading this one entry is not going to make you an expert; what we are doing here is providing some basic information and terminology, so that you will know what to search for, should you decide to research further for your specific application.

It is the cellular structure of the tree, and the living systems within the board, that influences its movement, long after it’s been cut. Because the wood in living trees is very, very wet. Its cells are swollen with sap, which is mostly water, but also some minerals and sugars. Water within cells is called “free water” and it can be squeezed from the wood, but water can still remain within the cell walls. This is called “bound water” and it is only when the bound water is removed (by drying) can the wood shrink and become strong.

Take a look at these images. Below left is a magnified photo of wood. You can see vessels that look like bundles of drinking straws running through the wood. Below right, the graphic illustrates the difference between bound and free water. It’s like the free water is sitting inside the straws, but the bound water is moisture trapped within the walls of the straws themselves.

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