There are several factors that can alter the tuning of a piano: humidity, temperature, moving, excessively hard playing, aged or worn materials, and abuse.
• Humidity fluctuations is the number one cause of changes in piano tuning. When the soundboard, pinblock and bridge are in a moist environment, the wood cells absorb the moisture and swell up, and as they expand they pull the strings tighter, causing the piano to go sharp. Logically, if the piano moves to a drier atmosphere, the wood shrinks, the strings loosen and go flat, and sometimes cracks and splits may open and start buzzing. Every seasonal change — every wet season and dry season — alters a piano’s tuning, and on a smaller scale.
• Temperature is somewhat related to humidity. When the steel strings heat up, they loosen and go flat. When they get cold, they tighten and the piano goes sharp. Every winter and summer, day and night, wood stove crackling or doors and windows wide open, the temperature is in constant flux. A piano on stage in an auditorium or sitting in a church spends all week in a cold environment, until Sunday or the evening of the concert when the heat is turned on, the hot stage lights shine down and the warm audience enters and breathes hot humid air into the room. The tuning makes a major shift.
• Moving always knocks a piano out of tune. The position of the tuning pins in the pinblock and the wires over the bridge pins is quite sensitive. Any time a piano is tilted or jostled, there is bound to be a shift in these positions. If a piano is moved from one environment to another, then there may also be a change in humidity and temperature.
• Excessively hard playing causes a piano to go out of tune when the force of the hammer is strong enough actually to stretch the speaking length of the string enough to pull it over the friction points.
• Many materials can become aged or worn. Those specifically related to tuning stability are generally wood, although some metal parts can give as well. In particular, the pinblock can become split, or the tuning pins can become loose in their holes from repeated tunings or wood shrinkage. The bridges can split, usually along the line of the bridge pins; as the strings are tightened during tuning, the pins then move and do not keep the piano wire in place. Sometimes V-bars, agraffes, and hitch pins can bend or move as well.
• Abuse of the piano or the tuning can also be a cause of a piano’s going out of tune; avoiding abuse is common sense for proper piano care and maintenance.