Everything in a piano is either made of wood or depends on wooden parts to function. Wet, humid conditions, swells and warps wood and dry, parched conditions will shrink and crack wood. Both high humidity and low humidity badly affect pianos, and even worse is this swinging back and forth, which is what we have in Kansas City ~ "hot, humid summers" and "cold, dry winters."
Dryness causes the most severe damage to pinblocks, bridges, and soundboards. However, those cracks would never develop, without first going through a period of damp, humid conditions.
For example, let’s take the soundboard:
The soundboard already has a vast amount of pressure exerted on it; and wet, humid conditions cause it to swell, pushing the wood grain with enormous force. As a result, the soundboard sometimes develops a “pressure ridge.” To view these pressure ridges on grand pianos, lower your eyesight to just above the level with the soundboard ~ to the point where the light glares off the soundboard. Locate the “pressure ridge” and run your finger across the ridge. You will be able to feel the ridge with your touch.
Manufacturers do not consider a pressure ridge a crack and do not warranty pressure ridges. However, if the piano goes through a period of dryness, this is where the crack will develop. “Pressure ridges” can be seen on many pianos still sitting on the dealer’s showroom floor. Which is one of the many reasons you should never sign a purchase contract without having a trusted professional piano tuner inspect the instrument.
Soundboard cracks usually do not cause much of a problem with the tone and overall sound of your piano unless the soundboard ribs loosen and begin to buzz and rattle. Unless, there is a rattle or buzz the soundboard will still perform, but the crack does devalue your piano.
A more severe problem that may develop as a result of “humidity swings” is pin-block damage. The pinblock is what holds the tuning pins tight. The quality and condition of the pinblock determines the tightness of tuning pins and is one of the most critical factors in tuning stability. For many manufacturers, including Steinway, the minimum specification allowed for tuning pin tightness, to pass inspection and exit the factory is only 50-inch pounds. While technicians, like myself, would like all new pianos to measure 75 to 95-inch pounds many pianos leave the plant with just the minimum specification
A piano is still tunable at 50 to 60-inch pounds, but once the pinblock loosens to the extent the tightness of the tuning pins only measures 25 to 30-inch pounds, that piano is no longer considered a tunable piano. After a piano has gone through a few seasonal changes, it is not uncommon for the pinblock to loosen five to ten-inch pounds in the first five years of service. To avoid getting stuck with one of those 50-inch-pound pianos, never, never, never buy a piano without a professional piano tuner appraising the quality of the piano before purchase.