, and more accurately, humidity are critical elements in the care of any musical instrument. The more extreme their changes, the more damaging the results. Relative humidity is the amount of moisture contained in the air as a percentage of the maximum amount of moisture that the air is capable of holding.

The air’s moisture content varies with weather conditions and activities within the home, while the amount of moisture the air is capable of holding varies with the temperature. Wood is a hygroscopic material-that is, its cells readily absorb moisture from the surrounding air when relative humidity rises, and lose water when relative humidity falls.

This brings about a dimension change across its grain as the wood expands and then contracts to accommodate the varying levels of moisture. The worst possible situation for a piano is one where it exposed to hot, humid summers and dry, cold winters.

The soundboard, being the largest single area of wood and often coated with a thin finish, is particularly sensitive to climatic change. Despite being made of spurce-a wood placed in the most stable category of timbers, its dimension still change up to ten times more across the grain than along the grain.

Since the soundboard is made up of several planks glued together and fixed around its edge, this movement is restricted. The only way it can relieve its expansion is upward, naturally increasing the soundboard’s crown. When the atmosphere dries, the air retracts the moisture and the soundboard shrinks, so its crowns flattens.

As the soundboard moves up and down, so does the bridge, altering its resistance to the pressures of the strings. This causes a change in their tension and tuning is lost. Unfortunately, as the strings lie across the soundboard and the pressure change is more acute in its middle, the tuning will not go sharp or flat evenly over the piano’s compass.

Small movements of the soundboards are natural in all makes and models of piano; the problems arise when those changes become extreme. If the humidity is allowed to rise beyond safe levels, pressure ridges develop at the joints of the soundboard’s individual planks and these will crush the delicate cells within the soft spurce.

If the piano is subjected to excessive dryness, the soundboard shrinks to a point where it cracks along the grain of the timber. This is altogether more serious, since a split soundboard, as well as affecting a piano’s tuning stability, causes its tone and sustain to deteriorate.

Although most evident in the soundboard, humidity changes affect all the piano’s wooden parts. In older pianos solid beech wrest planks can shrink and split causing the wrest pins to become too loose to be able to hold the string tension. Many of the problems found in an action and keyboard develop from excess humidity. If the key swell, they bind on the pins that guide their movement and become unable to return freely. Action parts too can become sluggish, due to a tightening of the center pin bushing.

If the piano encounters very damp condition then more than just its wooden parts are at risk:  the strings and tuning pins may rust too. Many of the numerous glue joints within the strung back, casework, and action begin to soften. Even the piano’s tone is affected, because of the hammer felt absorbs moisture, it loses its hardness and so produces a softer, more mellow tone.

To avoid these problems a piano should be kept in conditions of controlled relative humidity. This should be a constant 42 percent, but never fall below 35 percent or rise above 65 percent. Temperature should ideally remain a constant 68 degrees Fahrenheit (20 degrees Celsius).

Hygrometers can be used to monitor the relative humidity within a room, and humidifiers used to counter atmospheric changes. In areas of lesser climatic variation the immediate vicinity around a piano can be adequately stabilized by the use of specialist devices such as the Dampp-Chaser system.

These can be installed reasonably economical in both upright and grand pianos and should be fitted by a competent professional. Technicians are no longer advise keeping a jar of water in the bottom of an upright piano, a rattle-inducing and possibly damaging exercise that has very little effect on the humidity within the entire piano.