[ John Farmer wrote in 090606 MMDigest:

> ... how do you decide whether to seal or not?

Our method has always been not to seal and set up the stack on
a test bench with suitable vacuum supply and test it by running
a test roll through it.  If it goes through the repetition test
sequences at a speed a lot faster (say 25%) than the recommended roll
tempo then it passes.  If some or all notes do not pass, then I agree
you have to dismantle the stack again, at least as far as the primary
pouches.

Our experience has been that the majority of stacks perform fine
without sealing.  This is all it has to achieve in the instrument,
after all.  I am referring to standard player piano actions rather than
those for reproducing instruments, where a little more thought is given
to equalising the performance throughout the musical scale by setting
the gaps between pouches and valve stems more accurately, etc.

We also test the pouch board by sucking and blowing (with the mouth)
each pouch via its tubing nipple and assessing (a) its leakage and (b)
its flexibility by how quickly it responds.  Don't ask me to quantify
the measurement of leakage -- you just get to know what feels right
after you have done a good many.  The mouth is a sensitive air pressure
provider and you quite quickly get used to 'feeling' the leakage.

Experience will tell you what is adequate and what is too much.  It is
quick and immediately reveals faults such as a pouch rim not quite
stuck down properly, etc.  Okay, it's not scientific but it's quick and
effective.

I personally doubt that the original manufacturers did any more than
this.  The machines were made down to a price, and a complex laboratory
setup would have taken far too much time.  Sometimes I doubt if they
even did that much checking, having had an Aeolian Duo-Art stack with
a pouch in the middle somewhere that did not move at all!  The fault
was that the hole into which the brass nipple tube was inserted had
never been drilled right through...

One occasion where we did have to seal was in the case of the xylophone
primary pneumatics on a Philipps Pianella.  The pneumatics are tiny
hinged ones just like piano striking pneumatics in shape but much
smaller and covered in pouch skin.  They go in parallel with the stack
secondary pouches and there was sufficient leakage in them for them not
to close reliably in operation.  Sealing them with egg white
immediately solved the problem.

David Evans - Revelstoke Nickelodeon Museum
Revelstoke, British Columbia, Canada