In the early days of stamp collecting, the concept of keeping stamps in mint condition was unheard of. There were no such things as stamp hinges or mounts. In order for collectors to put their stamps in their albums, they resorted to means that we would consider horrific today, such as licking them and sticking them in the album, gluing them into the album, or taping them into the album. In later years, in some cases, collectors would take pieces of gummed paper, sometimes part of the sheet margins, fold the pieces in half, then attach them to the back of the stamp and to the album page, to hold the stamp in place in the album. Obviously, that made a mess out of early mint postage stamps. In a lot of cases, the stamps were discolored and completely ruined. In other cases the stamps could be carefully "soaked" from the album pages, pressed, and then saved as mint examples without gum.
Many countries, in the 1840's and 1850's, gummed their postage stamps with thick, yellowish gums of various compositions. After many years, some of these gums can turn dark, become brittle, and in a few instances, can even crack and physically break the stamp in half. In a lot of instances, the catalogs recommend that the gum be removed from these early mint condition stamps, and in many cases, the prices in the catalog for early mint condition stamps are for examples without gum.
I have had to soak mint Confederate States stamps in the past, to prevent the gum cracks from destroying the stamps. Many of these old, thick gums do not dissolve in water. In certain instances, once the stamp is wet and gum is moist, I've had to use the edge of a pair of tongs and physically "scrape" the gum off the back of the stamp. This is also true with some mint Austrian stamps of the late 1800's. The gum should be removed from a stamp that is being destroyed by gum creases. When the stamp is soaked, it can start to curl up, unless all the gum is scraped off of it. Once the stamp has curled into a small roll, it can be next to impossible to get it to flatten out again!
Some countries, even in the 20th Century, have distributed new stamps to their post offices without having any gum at all. In these cases, either the postal clerk or the postal patron had to use their own adhesives, when affixing the stamps to letters.
In many Asian countries, where there is excessive heat and humidity, stamps were also issued without gum, and one of the fixtures in many of the post offices there was the "glue pot", which was used to affix the stamps to the envelopes.
One special exception I want to address has to do with a couple German stamp issues of the 1930's. On Scott #B68, the OSTROPA souvenir sheet, and on Scott #C57-58, the Airship Hindenburg airmail set, the gums used for them contained sulfuric acid. Over time, this will cause the paper to stain a dark brown and eventually disintegrate. Being left in mint condition will destroy them, and collectors MUST remove the gum, in order to preserve them. A collector should NEVER BUY ONE OF THESE WITH ORIGINAL GUM.
Flown covers, with the Hindenburg
stamps on them, are frequently seen with brown stains around the edges
of the stamps, sometimes with the stains going all the way through the
paper of the envelope, showing on the back side. I have also read
stories of people that have acquired stacks of the rare OSTROPA souvenir
sheets, in the original post office packaging, and when they were
opened, the souvenir sheets just fell apart into tiny pieces. It would
be very sad to put a mint original gum OSTROPA souvenir sheet you paid
$1,000 for into your album, and over a period of years, watch it turn
brown and disintegrate.