Gum

A stamp has two sides, a front and a back. I won't say both are equally important, but both are important. A stamp is not just a pretty face.

In recognition of this fact, collectors have stopped putting hinges on mint stamps and some experts have stopped signing stamps - they provide a photocertificate instead. I think both are doing the right thing.

The back of a stamp is also a source of information. There are some issues for which forgeries can be identified simply from the paper or gum used. For example, the commonest forgery of the First Yessayan set of Armenia, which is very close in design to the originals, always has a very shiny gum which you never find on the originals, where the gum is matt. The paper is also thinner and more brittle. But you can check just the gum and if it's shiny, you know immediately the stamp is a forgery.

There are rare forgeries of rouble value Denikins (South Russia) which are very hard to distinguish from Originals until you turn over. The gum is also thick and brown as on a large proportion of Originals ( a small proportion of Originals have white gum), but it is applied differently, with a brush so that the gum looks like varnish, with streaks. When you find such varnished stamps (they are rare), it is worth then looking at the face to confirm that you are handling the forgery.

With used stamps, it is very tempting to record things like perforation or watermark on the back of the stamp. That way, the information does not get lost or mislaid. I can tolerate this, but not when the back of the stamp ends up covered with the history of the world.

Of course, there are stamps where the back has something printed on it, like Latvia's Map stamps. I am surprised that there are still collectors unwilling to acquire two copies, one to display the front, one to display the back. It seems a no brainer.