The microscopic absorbent cells that make up wood are capable of holding substantial quantities of water. It is common knowledge that wood swells when exposed to moisture, causing piano parts such as keys, to ??stick??. If the piano is moved into a more suitable environment, the wood shrinks back to its natural state and the moving parts free up again. The following exercise demonstrates how much the wood swells when saturated and in what way, with respect to the grain.
A selection of wooden piano parts were selected which had been stored in normal temperature and humidity conditions. They were measured, in various dimensions i.e. length, width and depth. The measurements were recorded and the parts were then soaked in water overnight. In the morning they were dried of excess moisture and re-measured. The new (saturated) measurements were recorded and compared with the original (dry) measurements.
The information was assesed and the change in the components expressed as a percentage.
General observations - In most components, there was significant swelling. The only part that had not changed in size at all was the hammer shank, perhaps because the harder wood (maple) was more resistant to absorption. The keywas found to be the most interesting component, having not swelled lengthwise at all, but had increased in depth by an amazing 4.4 percent! The pilot too, behaved in a similar way, with no increase in length, but 3.8 percent in diameter.
Conclusion - It comes as no suprise that when wood is subjected to moisture, it will swell. On closer examination, the results strongly suggest that while the dimension may be affected only marginally along the grain, there is considerable change across the grain.
How does this affect the piano? The workings of the instrument will be affected several ways. The most obvious outcome would be sticking keys either at the bushings and/or against adjacent keys. Lost motion would be affected too - extreme changes in humidity can cause a hammer initially with lost motion, to be pushed up by the jack (caused by a swelling key) and end up well clear of the rail. Also pitch can be affected as the crown of the soundboard flexes due to seasonal changes in humidity.
It could be argued that soaking piano parts in water is a rather drastic way to conduct an experiment, taking it well outside the extremes that would normally be experienced in a piano. While this is so, it does effectively help to explain why pianos behave the way they do when exposed to damp conditions.
Recommendations - Careful consideration should made with respect to temperature and humidity when deciding on the location of a piano. The installation of a heater or dehumidifier may have to be considered. Further suggestions on piano care are to be found in the numerous articles made widely available to piano owners and technicians.
Brian Holden, 1997