When the instrument comes into his hands the hammers are still covered with the hard outer skin of the felt head. This must be removed with a sand-paper file. It then is necessary to sound each tone slowly and carefully, first loudly, and then less so. It will be found that some of the hammers produce a harsh and disagreeable timbre, while others may be too soft and mushy. Under all circumstances it is far better to endow the instrument, as far as is possible, with a quality that shall be comparatively mellow and round. Brilliancy cannot be forced artificially without spoiling the whole quality and imparting a thinness and roughness that is most disagreeable.
If the hammers are too hard at the crown, they have the acoustical effect upon the strings of exciting the upper dissonant partials to undue prominence. This occurs from the fact that when the head is hard it rebounds instantly from the string, and thus does not damp any of the dissonant partials. On the other hand, if the head be soft, the felt clings for a fraction of a second longer to the strings and effects the damping to a greater or less degree, according to the relative softness or hardness of the head.
It is thus possible, by discreet manipulation of the felt, to influence the character of the sounds to no small degree. The method is to pick up the felt, when it is to be softened, with a set of felt needles mounted in a handle for the purpose. It is essential to note whether the sound is the same when the hammer strikes with great force as it is when it strikes gently. If, for example, a gentle pressure on the key gives an agreeable quality, but harder strokes on the key destroy this, then we see that the crown of the hammer head is soft enough, but the felt cushion underneath is too hard. Fine needles are, therefore, employed to dig into the lower cushion of felt, while disturbing the consistency of the upper crown as little as possible. On the other hand, when the quality of sound is hard under all conditions, the upper surface of the hammer-head must be treated by picking it with heavier needles. Hardening of the felt may also be undertaken by covering the hammer-head with a damp cloth and then applying a hot iron.