Wood Movement

A good woodworker needs to understand wood movement.

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No matter how old the wood is, no matter how well finished, as long as the humidity changes so will the size of the wood. A good finish can slow the movement, but unless it makes an airtight barrier over the wood it will not completely end wood movement.

Wood acts like a sponge. It expands with moisture and shrinks when it's dry. However, unlike a sponge it doesn't change uniformly. Instead, it moves differently according to the grain.

Wood moves the most across the grain (radially in the drawing), and the least with the grain (tangentially).

That means if you have wood grain going in different directions in your project you have to leave room in the design for the wood to expand and contract at different rates.

That's why door panels are made to "float." They may be spot-glued to keep from shifting, but they are mostly glue free so the panel can move inside the frame.

You may think that you can simply buy kiln dry wood and be done with it. However, don't assume that because a board has been kiln dried it will stay at the original moisture content. It won’t.

Most of the moisture is absorbed through the end grain. That moisture swells the cell walls making the wood expand.

It can also cause twisting, cupping & warping. That happens when one area of the wood absorbs (or looses) moisture more quickly than another.

That's why lumber yards often paint the ends of boards or dip them in wax. The idea is to slow the way the end grain reacts to the environment.

I experimented once by making a small, turned box from green apple wood. It was wet enough to spray me with sap as I turned it. I made a perfect cylinder with a fitted top, then tossed it aside. The weather was particularly dry. Overnight it warped into an abstract, bulging thing with a cracked top that perched precariously on the twisted rim.

It was a great example of what happens when wood dries too quickly. Try it yourself sometime, it’s an interesting experiment.

Once you have seen how quickly green wood warps you will understand why sawmills sometimes spray piles of logs with water to slow the drying process. It’s also why museums keep their furniture in a controlled environment.

So, during your planning stages pay attention to grain direction and make sure to take humidity (or lack of) into account when designing.

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