We prepared hammers from a total of five different materials that were heavier, lighter, harder, and softer than felt. If the hammer was even 1 mm wider than normal, it would touch adjacent hammers and become difficult to move. If the hammer protruded vertically to any degree, the string could not be struck forcefully enough to obtain sufficient sound volume, result in a soft, weak sound. Accurately forming the shape of the hammer was the most difficult part of the experiment.
Blue urethane foam is a lighter material than felt. It produced a soft, muffled sound. When the corrugated cardboard surface hit the strings, it made a bright, crackly sound. Next was the rubber eraser. Because of its weight, we thought that it would hit the string forcefully and make a loud sound, but the sound was unexpectedly soft. The leather was split thin and rolled up in layers before being formed into the hammer shape. It produced a fairly good sound. Leather is moderately elastic, and may come close to the properties of felt. The last was Paper clay. It was the hardest and heaviest material used, and made a sharp, high-pitched sound, similar to that of a cembalo or a Taisho koto (Japanese transverse harp). The drawback is that the hammer was too heavy, and it was difficult to play repetitions because some parts of the action can't return to their at-rest position.
Of these materials, leather produced the sound closest to felt. However, even when you vary the strength used to depress the key, there was almost no change in the tone of the note. With felt, the harder one depresses the key, the louder the sound, and the impression of the note produced changes from a soft feeling to a strong, powerful feeling.