Why do pianos go out of tune? There are three main reasons. The first and foremost one is pressure. As I’ve said many times before, a piano is a ‘pressure bomb’. A basic little spinet will often have up to 32,000-33,000 pounds of pressure on the strings which are bowed across the cast iron plate and the sound board. This is a huge amount of pressure. To put it in perspective, a large vehicle will often weigh about 4000 pounds. That is only about 12.5% of the pressure on a small spinet! This aspect of pianos makes them totally unique in the world of instruments. There is nothing else like them.
For a piano to be properly in tune the pressure has to be balanced across all of the approximately 220 strings in a piano. When this is done the piano is “in tune”. What must be remembered here is that each string is high quality steel stretched near it’s maximum capacity. The string is hooked onto the bottom of the cast-iron plate at one end and looped over a tuning pin at the top of the piano on the other end. Specifically, a string starts on a tuning pin, stretches down to the bottom of the piano and goes around a hitch pin and back up to the top on another tuning pin. While it is one wire, the hooking around the hitch pin at the bottom makes them into two distinguishable, separate, tunable strings. The hitch pin this wire is hooked around is usually cast into the cast iron plate. The tuning pin at the top is anchored into the pin block which is a multi-laminated plank of hardwoods constructed and designed for maximum strength.
Changing the pressure on these strings is what knocks a piano out of tune. The two primary reasons are Excessive Humidity, or changes in humidity, and Excessive Dryness and changes in dryness.
Each tuning pin holding the highly pressurized strings is anchored in wood (the pin block). Excessive humidity will permeate the pin block and decrease or eliminate the ability to hold enough pressure for that note. When that happens that note is untunable and needs work done to make it tunable. When this is bad the only remedy is often replacing the tuning pin with one having a larger diameter. Small changes in humidity going either way can quickly throw a string or a piano out of tune as the pressure holding ability of the pin block is constantly being changed.
Excessive dryness can cause the pin block in a piano to dry out and get brittle with the same eventual effect of making the piano untunable. Small changes in dryness are really the same as changes in humidity just on the other end of the spectrum. The same effect though, an out of tune instrument.
Pianos love constancy. Relatively moderate but stable temperature and humidity are ideal. Sudden changes create a pressure imbalance rapidly. Keeping your piano in a relatively stable environment should provide the stability and constancy you need to have the proper degree of tuning stability.