[DoctorBeaver (OP)]

Licking stamps makes them sticky, so how does steam make them unstick again?

[Karen W.]

This is a good question and I happen to have no answer, but need one as I have several full sheets of old stamps I need to unstick from themselves without leaving watermarks will steam leave watermarks if I use it to remove the stamps from each other! I also do not undestand how it works..

[paul.fr]

Licking stamps makes them sticky, so how does steam make them unstick again?

oops, quoted but posted before typing a reply!

two guesses.

#1 the steam breaks or loosens the glues bond
#2 the steam dilutes the glue, thus weakening it.

[eric l]

The glue on the backside of stamps is usually dextrin.  As a dry powder, it is not more sticky than sugar - in fact, dextrin is a dextrose polymer.  When wet, it can bind itself to the cellulose, which is the main component of paper.  The stamps is fixed to the envelope because the water penetrates into the envelope and evaporates afterwards, but the hydrogen bounds between the dextrin and the cellulose remain.
By steaming, you have dilution and you can remove the stamp just like you could before the glue had dried when applying it on the envelope.
Nowadays, you'll also find sticker type stamps (referred to as "auto-adhesive").  They use an other type of glue, and a backing sheet that is easily removable. In my experience, they are hard to unstick by steaming. 

[Karen W.]

Interesting I might try it anyway!

[DoctorBeaver (OP)]

Thank you, Eric