As will be surmised by the reader, early covering of pianoforte hammers with felt was a tedious operation. The material was cut up separately for each hammer, and glued on to the wooden head by hand. But this method suffered under the grave disadvantage of affording no correct guide to the true thicknesses and no means other than furnished by hand-cutting, of graduating these thicknesses with exact evenness. It remained for an American (or Americanized German) inventor to devise a machine that should glue up and finish an entire set of hammers from one continuous sheet of felt, of which the thickness was graduated in the making and which could be relied upon to contain the correct amount of felt, of any desired total weight, the whole graduated with great evenness. Not merely was this so, but the methods of manufacturing felt for other special purposes connected with the manufacture of pianofortes has been entirely revolutionized, so that today we have dampers, punchings, and many other parts of the mechanism of pianofortes successfully manufactured out of various kinds of felt, which vary greatly in consistency, thickness, and other properties.
To return, however, to the special case of hammer-felt. We find that the original methods of working together the layers of carded wool has been much improved, chiefly through the efforts of the inventor mentioned above. It is now a matter of slight difficulty to turn out sheets of felt for the making of piano hammers that shall have any desired weight, size and graduation of thickness.
The weight of the sheets of felt varies largely according to the nature of the hammers that are to be produced. Thus it is customary, as may be guessed, to provide full-size concert grands with hammers of greater thickness, while the smaller styles of instrument have progressively smaller hammers. The actual weights vary according to the caprice or calculation of manufacturers and designers. An average weight of felt for hammers suitable to be placed in a nine-foot concert grand is from 16 to 18 lbs. per sheet. This does not include the “underfelt,” which is glued directly to the wooden head of the hammer and is used to give greater resiliency to the lower hammers. It is generally omitted at the higher treble end.