Ron Ginger has discovered that the felt sold at fabric stores today is
mostly polyester.  This is usually thin and not of uniform thickness,
lumpy, and not very springy.

The only source I have found for decent felt is

  Organ Supply Industries
  P.O. Box 8325
  Erie, PA 16505-0325
  http://www.organsupply.com/index.html

Their 1/16" red wool felt, product No. 6102.01, is just the right
thickness.

The valves should be faced with kidskin from

  The Leather Supply House
  412 West Marion Street
  South Bend, IN 46601
  http://www.leathersupplyhouse.com/

Their product Code CGK, Goat-Kid 1 oz. = 0.016"+, will make a composite
of the correct total thickness.  The originals seem to have been done
with some very thick tan pouch leather, no longer available, but this
thin kid works just as well.

Tape a strip of the kid, nap down, to a flat board and apply hot glue.
Quickly stick a strip of felt on that, and weight this temporarily with
a heavy book.  It seems counter-intuitive to use hot glue, which dries
very hard, but that's the way composite valves were made in the old
days.  Hot glue will not soak into the felt and stiffen it.

After the glue is fully dry, pull the sheet over the edge of a table;
the thin glue layer will break up into a million individual crystals,
leaving the composite flexible and soft, as if the glue wasn't there at
all.  If any modern adhesive is used, it will not jell, but will remain
liquid until dry, and soak into the felt.  That leaves the material
stiffened with a permanent, rubbery consistency, spoiling the whole
job.

After the valves have been cut out, carefully burn a small amount of
shellac in a tin cup, down to a very thick, syrupy consistency.  Apply
a drop of this to the face of the key, and rub it around with the
finger.  Stick the felt side of the valve to this.  Again, the burned
shellac will adhere the seat to the key very firmly, without soaking
into the felt.

One can do this right in the instrument.  The spring action of the keys
will clamp the seats to the keys, until the shellac is fully dry.

Richard Vance