Nature of Wood
Knowing the Material
Wood comes from trees.
Everyone knows that, though most of us forget that wood was originally designed to feed and support buds, leaves, and flowers--not to build furniture. This makes it a great material for supporting things, but gives it other qualities that woodworkers sometimes wish it didn't have.
Wood has "grain"--a directional quality to the material--that results from its use as a conductor of sap up and down the trunk of the tree. If you wanted to conduct liquid, say a soft drink, upwards from a cup to your lips, what would you use? A straw!
So does the tree: picture its structure at the cellular level as a box of hollow soda straws.
The tree uses the straws to conduct sap from the roots to the leaves and back. And because of their long, thin shapes, they're very strong; the tree also uses them to hold up its branches and leaves, and this explains why wood has such great strength along the grain compared to its strength across the grain.
Imagine that instead of plastic, the straws are made of a porous material that can soak up water like a sponge. Now you have a better picture of wood's structure. Sap, being mostly water, not only flows through the straws, but saturates the spongy material of the straw walls themselves.
Like sponges soaked in water, they stay large and swollen as long as they remain wet. As soon as we cut through the straws, or cells--as we do when we cut a tree--they begin to dry out.
The sap that fills the hollow wood cells is the first to evaporate. While it does, the wood's shape is unchanged, because the cell walls are still saturated with water. The cell walls cannot dry out, in fact, until the sap flowing through the cells has evaporated. Only then can the wood tissue itself begin to dry.
Speaking of sponges, the wood is now a lot like the sponge you wrung out and left on the sink the last time you did dishes: it's not dripping wet because all the water inside its pores has been squeezed out, but it's still very moist. Its shape is unchanged. Over the course of the night, however, it will dry out, and in the morning you will find a shrunken, shriveled cake, hardly recognizable as the same sponge.
Wood behaves in a similar way. Wood tissue shrinks as it dries, and this changes its shape. But because of each cell's long, thin straw-like shape, most of its shrinkage goes to make it thinner, or narrower, rather than shorter.
This is why a piece of wood will shrink across the grain, but not along the grain, as conditions dry out. It will become narrower, but will never change its length. This difference in shrinkage between crossgrain and longgrain directions is the biggest problem faced by anyone building with wood, and there are many ways to solve it.
This drying and shrinking causes another problem, though, when we attempt to dry out a freshly cut log. Because the wood loses moisture from the outside of the log first, the outer portion will dry before the center. The result: tensions are created as some parts of the log shrink and try to pull away from other parts. Unless the drying is done very gently and slowly, the wood just cracks apart. (Even then, splits and cracks will occur unless the original round shape of the log is altered in some way by cutting.
You would think, then, that as long as we dry the wood thoroughly, without cracking or spoiling it, then we can build things to our heart's content. We can forget about the wood changing shape ever again.
It'd be great if that were the case. The bad news is that wood really loves water. Even after we've dried it so very carefully for weeks and months and even years, our wood still has moisture in it. Why? Because the air itself has moisture in it--water vapor--that the wood absorbs. Wood is so moisture-loving (or hygroscopic), in fact, that it continues absorbing moisture out of the air even through the varnishes, lacquers and other sealers we use as finishes (though they can slow the absorbtion down a bit).
The air's moisture content varies from one time to another, and it is newsworthy enough to be reported on the daily weather forecasts. Its moisture level, or relative humidity, varies during the course of each day and from one day to the next, as well as seasonally throughout the year.
When the air's relative humidity rises, the wood begins to re-absorb moisture from the air and become wetter. Eventually it reaches a point of equilibrium, where it neither gains from nor loses moisture to the air. This will last only until the humidity of the air changes again, and the wood will then moisten or dry out to compensate. Every time it does, it will either expand or contract as a result.
This process will continue as long as the wood exists. No matter what length of time the wood spends drying, it will always be subject to changes in size and shape with the changing relative humidity in the air.
When the air gets colder in the winter and can't hold as much moisture, things dry out even more--especially in heated homes. The summertime brings the opposite problem: the air is damp and sticky, and wood responds by taking on moisture and swelling. When it does, the furniture and fixtures in our houses, the doors and trim, the floors, and the wood frames themselves, are all affected.
For furniture or woodwork, this means that a thing that has been built using tight, perfectly fitted joints to hold itself together is constantly becoming a different size! Since nothing short of putting furniture in plastic bags will stop the process of moisture cycling, we must get around this problem in other ways. By using ingenious methods of construction which have been developed over centuries, cabinetmakers have managed to minimize the destructive effects of atmospheric humidity.
You can read about some of them in "About Building."