(From Jim Harper, who did the technical work, forwarded by Decap
co-owner Michael Woolf -- Down Under in New Zealand.)

It was a mission getting these ranks in our Decap/Mortier to buzz
nicely.  We had brown paper glued on ours as described .  Many of
these paper coverings were actually broken, while others had hardened
and rippled.  Some also had a layer of foil sandwiched between the
paper covering and the outside cap.  A couple worked but were so
brittle they broke into flakes when touched.  Some had a permanent set
on the material such as a bow that of course killed the buzz effect
because the paper can't move.

I tried soft and hard paper (including tracing paper); aluminium foil,
chocolate and cigarette packet foil, etc., but eventually found that what
worked well was unwaxed baking paper glued with hot glue just round the
edges, and not on the centre divider.  Thin strong paper seems to be
the secret.

When dry I pressed each in gently (!) with my finger just a bit to
leave a bit of a stretch in it so it could vibrate.  Very easy to poke
through it in this process.  One of the advantages of hot glue of
course being that mistakes can be easily reversed -- just wet and
scrape off any broken paper and reglue.  Then a bit of careful
adjustment of the outer cap to get the right sound.

The caps had some kind of compressible gasket, maybe kid leather,
to allow gap adjustment.  Foil can be added for "that extra buzz".
A further problem was that, in the process of loosening the antique
screws to adjust the buzz, there were some small leaks around the
threads that affected the pipe's performance, so all screw holes had
to be checked, rebushed if needed, and in the end steel screws were
generally replaced in with new brass screws.

Anyway, while it sounds like a crude sort of arrangement, Mr Mortier
and his workers must have put a lot of thought into this design.  The
bottom line is that this repair using this material (there must be an
equivalent in every country) has worked problem-free on our
Decap-Mortier, for about three years so far.

Michael Woolf
Wellington, New Zealand