Why does a string break?
The main reason a string will break is because it is defective. When a string is made at the foundry, a block of steel is pulled through a smaller and smaller hole. During that process, should there be a slight power surge in the plant, the string will become slightly thinner for a very small segment of the wire. In most cases, the difference is so minute that it will never be a problem. But sometimes the “dimple” in the wire is enough to cause the string to break when it is pulled up to tension. Most of the time this is caught at the piano factory, but sometimes the defective wire will survive the manufacturing process, and decide, completely on its own, to break 1, 5, 10 or even 50 years later, especially when that defect is at one of the pressure points. (Which was described in an earlier article). Thus, when a string breaks for no apparent reason, blame it on the manufacturing process.
Very hard and excessive playing will also cause a string to break. There are two things that happen at the same time. When a key is played, the hammer hits the string, which then vibrates up and down. At some point the string comes to an apex, the point where the string cannot go any higher or lower. When the hammer strikes the string again at exactly the same time the string is at the upper apex, there is no other place for the string to go. The string will then be stretched beyond its tensile strength, and breaks.
As the string vibrates, it bends at the agraff or cape bar. As the string bends at these pressure points, it creates friction, which causes the string to heat up. (Try bending a paper clip a couple of times). When the string heats up, the molecular structure of the string breaks down and the tension of the string will become greater than the tensile strength of the string. Combine this with the hammer hitting the string at its apex, the result is a broken string.
Under normal playing conditions and when the strings are “healthy“, the strings do not heat up enough to break. The best way to avoid breaking strings during heavy playing is to not keep the sustain pedal down all the time. The dampers actually hold the strings down, and it won’t be over stretched.
A string will also break when rust has corroded it, especially by salt air. Due to our constant high humidity and proximity to the oceans, we obviously have a big problem with rusty strings here in Hawaii. Rust and corrosion literally eats away at the steel and breaks down the molecular structure of a wire.
As was mentioned in an earlier post, there are several pressure points on a string where a string comes in contact with a metal part of the piano, like bridge pins and tuning pins. It is at those pressure points where the rust causes a problem. When enough rust has corroded the string, the tension of the string will become greater than the tensile strength of the wire, and it breaks.
There are three ways to prevent rust on strings. On an upright, a damp chaser inside the piano, which creates heat, will keep away the moisture, and prevents a lot of rust. In addition to the damp chaser, there is also a product called Rust Blocker that will help keep rust off the strings. Rust Blocker emits a vapor that settles on the strings and prevents moisture from adhering to the strings, thus preventing rust.
On a grand piano, a damp chaser under the piano will help, as will Rust Blocker. But the best way to keep moisture off the strings is with a string cover, which is put over the strings, under the lid. In extreme circumstances, it might also be advised to put a cover over entire piano, especially in a large room or auditorium.
Unfortunately, those preventive measures can only do so much. Pianos in homes next to, or within 100 yards of the ocean, almost don’t have a chance. The salt air will corrode the strings, and there is not much that can be done to stop it.
To reiterate, the three reasons a string breaks are: playing too loud, defective wire and rust. Knowing what causes a problem is the first step is the first step in prevention.