Investigations into pouch sealants like John Tuttle's are always to
be applauded.  In my experience many sealants, especially those that
set, like rubber solution and RTV silicone, _do_ make leather pouches
materially stiffer, and this is a big problem in instruments where the
suction level required to play pianissimo is barely greater than that
required to shift the valve.

It may be that an increase in pouch stiffness is compensated for
to some degree by total airtightness, but stiffness must be minimized.
British Aeolian, which certainly used rubber solution to seal its
pouches, seem to have used just one thin continuous surface coat,
rather than saturating the leather with several coats of solution.
It's possible they used pre-coated sheets to make the most flexible
pouches.

Non-setting sealants, especially liquids, may not make pouches
appreciably stiffer, but leather is inherently porous, and any
compound that does not set, however much is applied, must eventually
be expelled by prolonged suction.

But in a stack a pressure differential occurs across a pouch for only
relatively small proportions of time.  So subjecting treated pouches
to prolonged suction is not a representative test.  More important is
what pressure is required in a particular pouch to raise intermittently
the weight of a valve over its travel in a given time -- i.e.,
instantaneously.

In practice Hydrophane is not expelled from best quality leather
pouches and it renders them effectively airtight and very supple
for upwards of twenty years.  I've found nothing else that makes
leather pouches as sensitive.  It's harmless to glues or wood and
it's necessary to make pouches quite wet with it initially.  Two or
even three generous applications are indicated.

The question is, though, if sealants are to be applied to _new_
pouches, why use leather?

Patrick Handscombe
Wivenhoe, Essex, UK