Postage stamp gum
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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.
Before postage stamps existed, people receiving letters would have to pay for them. The payment was based on how many papers were in the envelope and how far the letter had traveled. Rowland Hill came up a solution of prepayment. This led to his invention of stamp gum in 1837. The world's first adhesive postage stamp was called the Penny Black. Many early stamps were not gummed, however, and some have been unable to be gummed due to shortage (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  and was made from a mixture of potato starch, wheat starch and acacia gum.
Types and application
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:
- dextrin, produced by heating starch
- gum arabic or acacia gum, derived from the acacia plant
- glue, from gelatin, rarely seen on stamps
- polyvinyl alcohol
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  or gum watermarks.
In recent years, the use of self-adhesive stamps, otherwise known as pressure-sensitive stamps, has become widespread. This relatively new form of stamps has a release carrier backing that has the same pressure-adhesive backing as the stamp itself. Therefore, the stamps can be released easily from the backing and placed onto a postal envelope. The first use was by Sierra Leone in 1964, and the United States tried it later on a 1974 Christmas stamp; this was judged a failure and was not reintroduced until 1989 when it gradually became widespread. In the 1990s, the U.S. Post Office began transitioning from water-based stamps into the use of self-adhesive stamps. By 1995, only 20 percent of the thirty-five billion stamps the Post Office produced every year were self-adhesive, yet by 2013 almost all U. S. stamps issued had become self-adhesive.
A 1965 British study of the transmission of bacteria and viruses on gummed paper found that "Although pathogenic bacteria and viruses were not isolated from sample envelopes obtained from various sources, the gums used in manufacture were found to exert a protective effect against death from desiccation on the bacteria and viruses which had been introduced into them" and it was possible to demonstrate bacterial multiplication in the gum used for the manufacture of postage stamps." The authors added the warning that "postage stamps are often handled very carelessly when issued over the counter, and yet the purchaser will usually lick them without hesitation. The present work shows how readily bacteria can adhere to the surface of gummed paper which has been slightly moistened; and the finger is a suitable source both of moisture and of bacterial contamination."
A 1996 episode of the popular sitcom Seinfeld featured a character (Susan Ross) who was poisoned after licking the flap of too many gummed envelopes. The episode has been linked anecdotally to an increase in worries about the health risks of licking gummed paper and it has been speculated that it may have contributed to the growing popularity of self-adhesive stamps, at least in the United States.
For collectors, gum is mostly a problem. In 1906, trouble had constantly arisen due to the gum on the under face of the stamps. There was an official notice that stated that stamps were going to be prepared with 'hard' gum, and were intended for use in the summer or humid season to prevent the premature sticking together of the stamps, or the sticking to the paraffin paper when in book form. 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. In 1913, a famous stamp called the Clermont steamer had an adhesive that was inscribed; "Hudson-Fulton Celebration". Stamps that had words inscribed on the adhesive were of even more interest to philatelists who made a special point of collecting picture stamps.
However, it has always been desirable for the dedicated philatelist to maintain the integrity of the backing gum, and many collectors take great pains to remove any paper residue that has collected on the gum without removing the gum itself. Such stamps were considered more desirable than stamps without their backing gum.
Stamps printed on the gummed side by mistake have become valuable varieties.
References and sources
- "Rowland Hill's Postal Reform".
- "History of Stamps".
- Williams, L.N. & M. Fundamentals of Philately. State College: The American Philatelic Society, 1971, p.494.
- Mackay, James. Philatelic Terms Illustrated. 4th edition. London: Stanley Gibbons, 2003, p.65. ISBN 0-85259-557-3.
- Patrick, Douglas & Mary. The Hodder Stamp Dictionary. London: Hodder & Stoughton, 1973, p.105. ISBN 0-340-17183-9.
- Gray, Robert. United States Patent, US5685570. Blackburn South, 1997, p. 4
- Williams, p.500.
- Grossman, Anne Jane. Obsolete: An Encyclopedia of Once-Common Things Passing Us By. Abrams Image, 2009.
- S. Selwyn (1965). The transmission of bacteria and viruses on gummed paper. Journal of Hygiene, 63, pp 411-416. doi:10.1017/S0022172400045290.
- Johnson, Stanley C. The Stamp Collector. [S.l.]: Herbert Jenkins, 1920. Print.
- Bells, Mary. “History of Stamps.” About. Web. 20 October 2013.
- “History of Stamps.” The American Philatelic Society. Web. 20 October 2013.
- Johnson, Stanley Currie. The Stamp Collector: A Guide to the World’s Postage Stamps. H. Jenkins Limited. 1920. p 24.
- Poole, Bertram. The Standard Philatelic Dictionary. Beverly, Mass.: Severn-Wylie- Jewett Co., 1922. Print
- Sutton, Richard. The Stamp Collector’s Encyclopaedia. New York: Philosophical Library, 1966. Print.