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Stamps went self-adhesive because people disliked licking them and postal services wanted stamps that survived humidity and automated handling. Water-activated gum curled and stuck together in damp weather, and peel-and-stick stamps turned out to be faster for everyone. By the early 2000s, nearly all new US stamps were self-adhesive.
Water-activated gum worked fine in a dry office and misbehaved everywhere else. In humid weather, stamps in a booklet stuck to each other before you ever licked them. The taste was unpleasant, and in a busy office licking dozens of stamps for a mailing was slow and unhygienic. There was also a slow decay problem: the traditional gum, made from gum arabic, dried out and cracked on stamps stored for years, so old mint stamps sometimes would not stick at all. Postal services were fielding complaints about all of this for decades. The adhesive was the weakest part of an otherwise durable product.
The idea is much older than most people think. Sierra Leone issued the first self-adhesive stamps back in the 1960s, and the US ran trials through the 1970s and 1980s before committing. Early pressure-sensitive adhesives had problems of their own: they yellowed, they oozed at the edges, and nobody knew how they would age over decades. Postal services are conservative about anything that has to survive in an archive.
The turning point came in the 1990s. Adhesive chemistry had improved, and the USPS started converting its most-used stamps first, then worked through the rest of the lineup. Once customers tried peel-and-stick, there was no going back: they were faster, cleaner, and they stayed stuck in humid sorting plants. By the early 2000s, nearly every new US stamp was self-adhesive. Canada followed a similar path through the late 1990s and early 2000s, and today water-activated gum survives only on a few special issues.
| Water-activated (lick-and-stick) | Self-adhesive (peel-and-stick) | |
|---|---|---|
| How it sticks | Gum moistened with water or saliva | Pressure-sensitive adhesive under a backing |
| Humid weather | Stamps curl and stick together | Unaffected |
| Speed | One at a time, with moisture | Peel and press, much faster |
| Collector removal | Soaks off paper in water | Must be trimmed or carefully peeled |
| Edges | Traditional perforations | Often die-cut to look like perforations |
The classic way to save a used stamp was to soak the envelope corner in water and float the stamp off. Self-adhesive stamps do not cooperate: the adhesive is designed to resist water, so collectors have to trim them or peel them with solvents, and the result never looks like a cleanly soaked stamp. The edges changed too. Many self-adhesives are die-cut, meaning they are cut to shape with a wavy edge that imitates perforations, rather than actually perforated. Purists grumbled. They still grumble.
Mint condition also means something different now. With lick-and-stick stamps, collectors checked that the gum was undisturbed. With self-adhesives, the stamp has to stay on its original backing paper. A self-adhesive stamp peeled off its backing and stuck to an album page is, strictly speaking, used, even if it never saw an envelope.
Not entirely. A few special issues and coil stamps still use water-activated gum, and some countries issue both formats side by side. If you find old lick-and-stick stamps in a drawer, they are still valid postage in the US and Canada as long as they are unused. The gum may need a little help after years in storage: a damp sponge works, and for a stamp whose gum has completely dried out, a glue stick does the job without anyone being the wiser.
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