Hi All,  Just a few tidbits of information to ponder:

On the Low setting (1), my iron heats up to 230 degrees F.  At Medium
(4), it gets up to 260 degrees F.  At High (7), it's 335 degrees F.
From what I recall, it's been established that the proteins in hide
glue start breaking down at about 150 degrees F.

I believe it has also been established that heat causes rubber to melt
and to harden to some degree once it cools back down.

FYI, I have, on rare occasions, used an iron to fix a questionable glue
seal.  Would I advocate it as S.O.P.?  Perhaps some serious testing is
in order...

Musically,

John A. Tuttle
Player-Care.com
Brick, NJ, USA