The piano hammer is comprised of layers of compressed wool, giving it a resilience that helps it to rebound from the string. In order to produce a tone that is full, rich and balanced, the hammer should have a rounded, egg-shaped end. With wear, the hammer becomes flattened and grooved on the end, producing an edgy, metallic, and brittle tone. Voicing involves restoring the original shape and resiliency of the hammer by filing away layers of felt and needling.