As with everything else which goes into the manufacturing of a piano, the hammer represents the highest refinement of raw material, in this case, wool felt. Basically, the qualities which define a good piano hammer felt are: resiliency, durability, and uniformity and length of fiber.

Resiliency, or the ability of a material to revert back to its original form after displacement, probably marks the biggest distinction between good and bad hammers.

It is the function of the hammer to strike, and then rebound off of the string as quickly as possible. This rebounding action is caused by the hammers' resiliency, as well as the tension of the string which it hits. The higher tension creates, in effect, a harder target, thus throwing the hammer off of the string with a greater rebounding action. A hammer which remains too long on the string after the initial blow, absorbs the energy of the vibrating string, producing a dull sound.

The sound is dull, and muted because the higher harmonics are the first ones to be dampened by too long of a contact time between the hammer and string. These higher harmonics are the first ones to go not only because they are weaker than the lower harmonics, but also because the wavelengths are much shorter, relative to the fundamental and the lower harmonics. As the wavelengths of the higher harmonics become shorter and shorter relative to the amount of hammer surface which is touching the string, they are more easily eliminated completely.

One might compare the hammer-string rebounding action to a man on a trampoline. When the person jumps onto it, there is initial displacement of the webbing, but as it is resilient in nature, the webbing quickly reverts back to it's original form, while throwing the person off in the process. The man would represent the hammer hitting the string, first displacing it, then being catapulted off when original form, or equilibrium is restored.

So what is needed for a good, RESILIENT, musical piano hammer? To answer this question, one must begin with the wool itself, as all wool is not created equal.

There have been many opinions as to the section of the world, and type of sheep which produce the best quality hammer felt. Some say Australian sheep grow the best wool for piano hammers. Others say our own Navajo Indians raise the best sheep for this purpose. This is a debatable and highly questionable topic, which should be the subject of another work. In any case, the following characteristics should be present in any high grade wool which is to be used in the manufacturing of piano hammers.

The woolen fibers are composed of live cells with a springing, flexible, resilient nature. These fibers are woven together, and then tightly packed into the dense material we know as hammer felt.

Woolen fibers, like piano wire, also have elastic limits, and any stretching of these fibers beyond this point destroys their resilient nature. Some felt is stretched over 12" in the weaving process, which is beyond the elastic limit. The end result is a fiber which stays stretched and limp, like a piece of string. The ability to rebound off the strings is lost - the tone is dull, and fades away much too quickly.

Chemically bleaching the wool with sulpher, or chlorine, or any active chemical bleach has a similar effect, i.e. the fibers become shorter, limp, and more or less dead. They no longer have the ability to react to external pressure, i.e. to be displaced by the hammer to string blow and regain its original shape almost immediately.

Hammer felt is tested for this rebounding, or resilient quality by placing the felt in molds for some period of time, and later releasing it. If the felt springs back up, the fibers have life, and the felt is suitable for the production of a good hammer.

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