In philately, gum is the substance applied to the back of a stamp to enable it to adhere to a letter or other mailed item. The term is generic, and applies both to traditional types such as gum arabic and to synthetic modern formulations. Gum is a matter of high importance in philately.
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The use of gum was part of the original proposal by Rowland Hill, and most stamps have been gummed. Many early stamps were not gummed, however, and some have been unable to be gummed due to shortages, for instance the typewritten Uganda Cowry stamps of 1895. Extreme tropical climates were also a problem for Curaçao and Suriname. Some stamps, intended only for sale to stamp collectors have been issued without gum, for instance the United States Farley's Follies souvenir sheets of 1933. On the first stamps of Great Britain the gum was called cement [1] and was made from a mixture of potato starch, wheat starch and acacia gum.[2]
Originally, gumming took place after printing and before perforation, usually because the paper had to be damp for printing to work well, but in modern times most stamp printing is done dry on pre-gummed paper. There have been a couple of historical instances where stamps were regummed after being perforated, but these were unusual situations.
On early issues, gum was applied by hand, using a brush or roller, but in 1880 De La Rue came up with a machine gumming process using a printing press, and gum is now always applied by machine. The gum is universally spread as uniformly as possible.
The greatest manufacturing problem of the gumming process is its tendency to make the stamps curl, due to the different reaction of paper and gum to varying moisture levels. In the most extreme cases, the stamp will spontaneously roll up into a small tube. Various schemes have been tried, but the problem persists to this day. On Swiss stamps of the 1930s, Courvoisier used a gum-breaking machine that pressed a pattern of small squares into the gum, resulting in so-called grilled gum. Another scheme has been to slice the gum with knives after it has been applied. In some cases the gum solves the problem itself by becoming "crackly" when it dries.
The appearance of the gum varies with the type and method of application, and may range from nearly invisible to dark brown globs. Types of gum used on stamps include:
Some stamps have had gum applied in a pattern resembling a watermark, presumably as an additional security device. German stamps from 1921 had a pattern of wavy lines while Czech stamps from 1923 had gum showing the initials of the Czech Republic, CSP. These patterns have been called gum devices [2] or gum watermarks.[3]
In recent years, the use of self-adhesive stamps has become widespread. The first use was by Sierra Leone in 1964,[4] and the United States tried it on a 1974 Christmas stamp, although the experiment was judged a failure and not repeated for many years. Traditional gums remain in use, although differentiated by calling them water-activated. All Israeli postage stamps feature a water-activated adhesive that is certified Kosher.
For collectors, gum is mostly a problem. It is rarely of use in differentiating between common and rare stamps, and being on the back of the stamp it is not usually visible. Nevertheless, many collectors of unused stamps want copies that are mint, never hinged which means that the gum must be pristine and intact, and they will pay a premium for these. While not so much of a problem for modern issues, the traditional way of mounting stamps in an album was with the use of stamp hinges, and some experts claim that very few unused stamps from the 19th century have not been hinged at some point in their existence. This means that old unused stamps are inevitably under suspicion of having been regummed, and the detection of regummed stamps is an important part of philatelic expertisation.
Stamps printed on the gummed side by mistake have become valuable varieties.