I've seen some postings on gluing rubberised cloth with hot glue and
using an iron.  Permit me to add some notes to the subject.

Rubber became useful after the invention of the vulcanisation process
by Mr. Goodyear in 1939.  Before that all bellows were made from
leather.  The best quality for this purpose comes from sheep.  This
type of leather was (and is) quite expensive.

Vulcanising rubber did offer the possibility of making a replacement:
rubberized cloth.  This was a much cheaper product, easier to produce
in a variety of types and qualities.  Most common was (and is) the
two-layered rubber/cloth, next came the version with a rubber layer
with on each side a layer of cloth.  Rubberised cloth soon was by far
the most common used product for bellows, almost totally replacing the
leather.  However, when it comes to quality and durability, leather
still is the first choice, especially for tougher operational
conditions.

There is no problem in using an iron to hot-glue rubberised cloth or
leather.  You will not damage anything, since you will continuously move
the iron, or just shortly put it on the working spot, allowing it to
distribute just enough of its heat to keep the glue just enough melted
to be applied properly.  It does ask for some experience, but is not
difficult.

Within limitations: the hotter the iron, the faster you will have to
work.  It's a matter of balance.  But unless you do keep the iron fixed
on one spot too long, you will not burn anything.  Nor will you do harm
to the bellows cloth.  Keep in mind that using hot glue already does
bring some heat into the cloth, but this is not enough to cause
problems.  In fact, using a hot iron does also help to get rid of the
water in the glue solution by vapourising it, thus allowing the glue
itself to bind faster.

It is not advisable to try to fix a questionable glue seal by re-heating.
If it's only one small leak, it might be better to glue it over with
some fresh glue, if possible, or, if needed, also using a piece of
bellows cloth to cover the weak spot.  However, most times if any
leakage does occur, it is better to renew the cloth totally, since one
leak almost for sure will rapidly be followed by another, probably
next to the re-glued spot.  And re-heating old hot glue is risky, too,
because quite a lot of heat is needed.  So you might not only risk
damaging the glued parts, but also force to decompose (burn) the glue
even before melting it.  (Indeed, like John A. Tuttle wrote, by
breaking down the proteins in the glue).

My very first job was at a rubber factory.  I do remember some things.
Normally rubber is vulcanized at 140 degrees Celsius.  This compares
to 284 degrees Fahrenheit.  The elasticity of rubber is based on a
network (like a fishing net) of rubber molecules connected by molecules
(bridges) of a vulcanisor (like sulphur).  Short heating of rubber
(as done when using an iron when applying bellow cloth using hot glue)
will not significantly harm this molecular structure -- the rubber will
melt nor harden.

Time is the biggest threat (for anything, including) rubber, for over
time the flexibility of the molecule bridges will go back or even get
lost.  Sunlight is second best in threats, because the intermolecular
connections of (especially natural) rubber do break down under the
influence of ultraviolet radiation.

Jan Kijlstra