Here are those two rules:
1) Wood moves.
2) Short grain will getcha.
While both these rules are imperative concepts for a woodworker, only the first one--wood moves--offers anything that makes for interesting reading, so I'm just gonna stick with that one.
Here's the short way to explain this: even after a tree has been chopped down, cut into boards, kiln-dried, turned into a piece of furniture and slathered with some kind of finish, that wood still collects and sheds moisture just as it did when it was a living tree. That's the reason you may have encountered a door that won't stay closed in the winter and can't fit in its frame in the summer. Summer months tend to be wetter, humid months, so the wood swells. Winter months are dry, so the wood shrinks.
To the left is a picture a of a homemade hygrometer I have in our house.
It was made by gluing the brown wood--a wide strip of end grain-- to the blond wood--a long, skinny strip of long grain. Wood movement really only comes into play across the wood and not end to end. So what's happening here is that the end grain piece is taking on or giving up moisture according to season, and getting longer or shorter respectively. The long grain piece it's glued to doesn't want to let it do that. The result, now that it's June, is the short grain has expanded so much it's bending the whole piece to the right. Just four short months ago it was actually bent the other way.
Isn't that something!
Here's the more fun way to talk about this: Graham taught us--drilled into us, actually--that you can't argue with wood. You're not gonna beat it. If wood wants to move there is no amount of space-age glue, screws or prayer that's gonna hold it steady.
Don't believe it? I've heard that back in the day demolition crews building the railroads would occasionally encounter stones in their path that were so large that no responsible amount of dynamite could move or break it. In that situation the crew fetched up a big old wedge of white oak. The wedge was driven into an available crack and buckets of water were poured on. After a day or two, that wedge would start to swell and swell. The rock would sit, and the wedge would swell. Then the wedge would swell some more and then, inexorably, there would be a loud Crack! and a rock the size of a house would simply split in two. (That's why the game was originally Rock, Wood, Scissors.)
Perhaps this tale is apocryphal (and I made up that part about rock wood scissors.) In any case, I do know I've seen plenty of examples of wood doing exactly what it wants. One always has to allow for movement. When making a frame and panel door, you always have to leave a little bit of room for that panel to shrink and swell or you're gonna end up with a warped door or a cracked frame. A table top has got to have room to expand and contract or it's gonna curl up on the sides in winter, bow up in the middle during summer, or quite possibly both.
Finally, here's one last thing about Graham's rules on the board: Life at the craft center was an interesting thing. We were out in the middle of nowhere. A whole lot of time was spent in the studio. Tiny games were invented as a creative, but low-pressure way to maintain one's sanity. One day I walked into the wood studio and noticed that someone had changed that first rule on the board. It said:
Wood MooseA few days later rule #2 had changed. This time, instead of "short grain will getcha" it said:
Short Granny'll Getcha!
This went on for months. I don't know how many iterations we eventually came up with, but by the end not even the code-busters that cracked The Enigma would have been able to figure out where it all started.
