Wood is hygroscopic, it absorbs moisture from the air - and, like almost everything, it releases moisture into the air. This causes the moisture levels in the wood to seek to always be in an equal balance with the moisture levels of the air.
When the relative humidity levels in the air are higher than the moisture levels in the wood, the wood begins absorbing moisture out of the air, eventually coming back into balance with air moisture levels. This absorption causes the wood to swell.
If the air relative humidity levels fall below the moisture levels in the wood, the wood's moisture begins to evaporate into the air, eventually falling back into balance with that of the air's levels. This loss of moisture results in the wood shrinking. Since air can change relative humidity levels faster than wood can adjust to those changes - wood is in a constant "game" of follow-the-leader with the air's levels.
One way we can help to slow down this exchange is by applying a finish to wood. Wood that has a good finish is much slower to exchange moisture with the air. Unprotected wood is not only faster to absorb and release moisture as relative humidity levels change, but will absorb and release more moisture as well since it reacts quicker to air level changes. While no finish can prevent complete moisture exchange, an undamaged finish (i.e. a finish that isn't chipped, scratched, or missing) can greatly slow this exchange and results in much more stable wood movement.
The chart below shows a basic idea of the seasonal difference between moisture absorption and release from unprotected wood (in blue) and protected wood (in red). Most environments differ from each other, but this chart can give a good general idea.
It can be noticed in this example how a protecting finish has slowed the absorption and release enough that the moisture levels in the wood are much more an average of the seasons. This is because - while the wood begins absorbing moisture in the spring and summer it is still in the process of rising to equalize that of the air, when the air's relative humidity falls in the autumn and winter; at which time the wood then begins a slow process of losing moisture to the air. This slowed down process of the wood, caused by the finish, results in much less wood movement and so much more stable wood (which means less damage to pianos and other wood furniture).
On the other hand, a badly worn or missing finish offers no real protection and allows the wood's moisture levels to rise and fall much more quickly. Chips, dings, and scratches in a finish can add up, and result in a finish that is no longer providing the protection needed. The results seen in wood of a badly worn finish can be cracking, warping, cupping, etc..
This is one reason Parks & Sons Piano Service not only suggests keeping an instrument's finish in good shape, but also controlling the environment's moisture and temperature levels around the piano.Some problems associated with repeated, uncontrolled fluctuations in relative humidity include:
Joint failure is a common problem in areas of uncontrolled humidity and furnishing with deteriorated finishes.
When wood fibers swell, they do so mostly across the grain and very little in length. Figure 1 represents wood grain at a "normal state", while figure 2 represents the same grains swollen with moisture. Notice how the swelling is equal (i.e. the round wood grains remain round when swollen because they expanded equally in both directions).
Most joints (like a mortise and tenon) may fail because the wood that makes up the joint is swelling and shrinking in contradictory directions. Eventually this causes the glue to fail and the joint comes apart (see fig 3).
Yet another problem that arises from swelling and shrinking is known as compression shrinkage. Most of us have seen this at one time or another. This type of swelling can lead to warping, cracking, cupping, etc., of the wood.
This form of damage takes place when part of the wood is restrained from normal swelling and shrinkage movement, while another are is not. As the wood swells the fibers in the restrained areas become compressed across the area of restriction, but continues normal swelling in the unrestricted directions. Areas not restricted attempt to swell in a normal pattern (i.e. retaining shape). The result is that, when the wood shrinks again, the fibers in the area of restraint retain their deformity and cause the shape of the wood itself to change.
Cupping is probably something most of us have seen, especially in older wood floor that routinely were wet mopped, outdoor decking on which the rain and sun effect the top surfaces mostly, and very old tables which routinely have been wiped clean with wet cloths over decades. In all of these situations the cupping always is in the direction from which the moisture was introduced.
The above two figures provide a very simple representation of a piece of wood that has been made wet on its top surface. Notice that the lower fibers are not effected since no water has reached them. In the first figure, the upper fibers can only swell freely up and down since the lower fibers are acting as side restriction (somewhat like a clamp). These top fibers become distorted and once they lose moisture and shrink (the figure to the right) they retain the distortion, causing the wood itself to distort. These figures are very exaggerated to help make the point. In reality the process is incremental, each time the fibers swell they distort a little. Slowly the board cups or warps. If the pressure is great enough the board may split or crack.
These forms of wood movement can be the cause for nails working loose, screws that no longer fit their holes, split leaf table tops that no longer lay flat, veneer pulling free from its substrate, soundboards splitting, and many other problems.
By controlling both relative humidity and temperature, and making sure that the finish on our wooden items is in good condition, we can alleviate many problems and keep our pianos (and furniture) functioning and looking nice for much longer.