I have begun experimenting with using Tyvek (tm of DuPont) sheeting
as a bellows or pneumatic covering material.  It's airtight and wears
well.  So far I've made a small pneumatic, 5" by 2" with about a 3/4"
working span.  My observations so far are:

1. The right glue is essential.  Franklin's Titebond II Wood Glue seems
to work well.  This is yellow.  The Franklin's Titebond without the
"II" will not work.

2. Tyvek is rather stiff, and is hard to form around the corners of the
pneumatic boards.  My first attempt was with the construction sheathing
version of Tyvek, which I gave up on as too stiff to form.  I then cut
up a Tyvek mailing envelope, and even that was stiff.  But, by
pre-creasing it to fit the edges of the pneumatic, I was able to get
a good airtight glue job.

3. The stiffness is very apparent in the operation of this small
pneumatic.  A bonus is that cardboard gussets would not be needed if
the pneumatic/bellows operates under pressure instead of vacuum.  Also,
this stiffness would be less apparent in a larger bellows.

4. According to DuPont's web site, Tyvek is made in "hard" and "soft"
forms.  Both of my examples were probably "hard".  The soft version is
used in disposable medical gowns and workers' protective garments.  If
I can get hold of some of that, it may be much easier to work with.
Might also be more receptive to other types of glue.

5. How well the stuff will hold up after 1000s of operations remains
to be tested.  Someone must have built a "torture tester" for pneumatic
cloth -- hopefully not connected to a piano action.  ;-)

I know -- we're supposed to stick with traditional materials.  But I'm
interested in building an organ from scratch, and there have been so
many warnings posted here about defective, porous pneumatic cloth that
alternatives are worth looking at.

Mike Knudsen