I was certain that someone would respond to the use of Tyvek as a
bellows material.  D. L. Bullock did in MMD 051018.  Included in his
discussion was a general statement that all new materials prove to be
unsatisfactory.  Sponge Neoprene was mentioned in that group.  It is
true that Sponge Neoprene can be found in late Aeolian plastic valves
and that the Sponge Neoprene is unsatisfactory in that application.

When I was first establishing myself as a restoration tech, I too,
experimented with various materials.  Primarily, I focused on area that
originally used leather; gaskets, valve facings, and bellows.  I felt
that leather had big problems, but at the time most machines were new
(early 1900's), no other material was any better or as good.  Leather
is never 100% air tight, it rots, and it corrodes nearby metal parts.

In researching Sponge Neoprene, I discovered that many varieties of
that material exist.  One, which is classified as SC-43, skin both
sides, seemed to be promising as a valve material.  The experiments
with that material showed it could be mass produced cheaply to make
100% air tight valves.  At this point I must mention that an 100% air
tight valve is a good thing, but not necessary.  If the pumping system
has adequate capacity, some valve leakage can be tolerated.

The end of this story is that I offered to my customers the option of
having valves made of 100% air tight Sponge Neoprene -- it was their
choice.  Many took the option.  The original sheet of Sponge Neoprene
that was used to make valves still sits in my shop, out in the open for
all to see.  It is very much in tack and supple.  It is 30 years old.

Bob Taylor