When my piano was last tuned the tuner informed me that the pitch was slightly high – how could this be? I always assumed that pianos went flat over time, not sharp.
You are right in thinking that the general trend, over time, is for the pitch of a piano to drop due to the great tension exerted on the strings and frame – something has to give and if left long enough the pitch will inevitably drop. However, in the short term other factors come into play. Bearing in mind that the piano is constructed from many different materials, humidity and temperature have considerable effect on the tuning and its stability. Too little humidity and the soundboard, being made of wood, will shrink allowing the strings to relax a little of their tension with a resultant drop in pitch. This usually happens during the winter months when the central heating is running and the air is drier. Conversely, if the summer months are particularly humid or damp, the soundboard can easily absorb the extra moisture and swell, forcing the bridges upwards which then have the effect of tightening the strings and raising the pitch. Unless you are particularly anxious for the piano to be at concert pitch at the time of tuning, it is probably best to leave the pitch slightly high as it will fall again when the humidity falls. This annual ‘breathing’ of the soundboard is one of the reasons why piano tuners are so welcomed twice a year!