We have quite the sticky subject today — that concerning all that is used to adhere stamps to covers or cards. The term ADHESIVE in philately can either refer to the gum on the back of a stamp, be it water-activated or self-adhesive, or the stamp itself affixed to prepay postage in contrast to a pre-printed design as on postal stationery. Stamps have also been issued without any adhesive at all and were affixed to envelopes by glue or other means.

It has been said that stamp gum, in its never-hinged state, is the most valuable substance on Earth.

The earliest human use of adhesive-like substances was approximately 200,000 years ago. Two stone flakes were discovered in central Italy partially covered with birch-bark tar and a third stone dating from the Middle Pleistocene era. This is thought to be the oldest discovered human use of tar-hafted stones.

The birch-bark-tar adhesive is a simple, one-component adhesive. Although sticky enough, plant-based adhesives are brittle and vulnerable to environmental conditions. The first use of compound adhesives was discovered in Sibudu, South Africa. Here, 70,000-year-old stone segments that were once inserted in axe hafts were discovered covered with an adhesive composed of plant gum and red ochre (natural iron oxide) as adding ochre to plant gum produces a stronger product and protects the gum from disintegrating under wet conditions. The ability to produce stronger adhesives allowed middle stone age humans to attach stone segments to sticks in greater variations, which led to the development of new tools.

The first references to adhesives in literature first appeared in approximately 2000 BCE. The Greeks and Romans made great contributions to the development of adhesives. In Europe, glue was not widely used until the period 1500–1700 CE. From then until the 1900s increases in adhesive use and discovery were relatively gradual. Only since the last century has the development of synthetic adhesives accelerated rapidly, and innovation in the field continues to the present.

In 1750, the first British glue patent was issued for fish glue. The following decades of the next century witnessed the manufacture of casein glues in German and Swiss factories. In 1876, the first US patent (number 183,024) was issued to the Ross brothers for the production of casein glue.

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. At the time, he wrote that his prepaid postage adhesive would be “covered at the back with a glutinous wash.” This led to his invention of stamp gum in 1837; the first postage stamps used starch-based adhesives when issued in 1840.

The first U.S. patent (number 61,991) on dextrin (a starch derivative) adhesive was issued in 1867. Natural rubber-based sticky adhesives were first used on a backing by Henry Day (U.S. Patent 3,965) in 1845. Later, these kinds of adhesives were used in cloth backed surgical and electric tapes. By 1925, the pressure-sensitive tape industry was born. Today, sticky notes, Scotch tape, and other tapes are examples of PSA (pressure-sensitive adhesives).

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.

Stamp gum - on the back of a U.S. mint, never hinged stamp (#C10, 1927)
Stamp gum – on the back of a U.S. mint, never hinged stamp (#C10, 1927)

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.

Under-gum printing on the back of a U.S. stamp (#3184, 1998)
Under-gum printing on the back of a U.S. stamp (#3184, 1998)

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 nineteenth 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 expertization.

The first self-adhesive stamps were issued by Sierra Leone in February 1964 and Tonga in April 1969 in an attempt to avoid the tendency of traditional water-activated stamps to stick together in humid conditions. They also made die cutting into fanciful and unique shapes easier.

Pressure-sensitive adhesives are manufactured with either a liquid carrier or in 100% solid form. Self-adhesives for stamps are made from liquid PSAs by coating the adhesive on a support and evaporating the organic solvent or water carrier, usually in a hot air dryer. The dry adhesive may be further heated to initiate a cross-linking reaction and increase molecular weight. 100% solid PSAs may be low viscosity polymers that are coated and then reacted with radiation to increase molecular weight and form the adhesive (radiation cured PSA); or they may be high-viscosity materials that are heated to reduce viscosity enough to allow coating, and then cooled to their final form (hot melt PSA, HMPSA). The stamps are usually issued on a removable backing paper.

The United States Postal Service’s first foray into self-adhesive stamps was in 1974 with the 10-cent Dove Weather Vane (Scott #1552), produced by Avery Dennison, that soon became discolored due to the instability of the adhesive. Another such stamp wouldn’t be issued by the United States until 1989. Stamp collectors criticized the format, as the rubber base adhesive used tended to progressively yellow the stamps. They also found them difficult to remove from covers, and to save in mint condition, though self-adhesives of recent years have improved in these respects.

U.S. Christmas self-adhesive stamp, on its backing liner (2014)
U.S. Christmas self-adhesive stamp, on its backing liner (2014)

The British Post Office first issued self-adhesive stamps on October 19, 1993, with the introduction of books of 20 First Class stamps, later a 2nd class stamp was introduced. In later years, other issues were produced in the self-adhesive format. Die cutting tools for the UK self-adhesive stamps were manufactured by Arden Dies of Stockport, Cheshire, using tools designed by Robert Clapham. Outside of the philatelic community, the stamps have been welcomed as more convenient; by 2002, virtually all new USPS stamps were issued as self-adhesives.

More recent USPS self-adhesive stamps are not readily removable from the envelope or cover backing by traditional water soaking. Some collectors of used stamps have discovered that although not readily removable by water, the self-adhesives can be removed with Bestine (a hexane solvent), Benzine (Petroleum Ether), or a natural based citrus solvent containing d-limonene (e.g., Pure Citrus Orange is an air freshener product that works for this purpose).

Many collectors are only interested in owning stamps in the pristine, mint, state in which they were originally sold at the post office. They are willing to pay a premium for stamps on which the gum has never been disturbed. For investment purposes there is nothing like the never hinged stamp with full gum.

Gum on stamps does have a number of disadvantages. It may crack, curl, become glazed and brittle, discolor, eat into the paper, attract vermin, stain and possibly harm the paper of the stamp itself, possibly even destroying the stamp over time. For this reason, there is a growing movement among collectors to abstain from the practice of collecting never-hinged stamp, even to the point of collecting stamps with no gum.

Unused stamp of the Confederate States of America showing cracked gum and thins
Unused stamp of the Confederate States of America showing cracked gum and thins

Other than mint stamps with full original gum, stamps are also described as lightly-hinged (LH) which show a slight mark where the stamp hinge was once attached. The gum is not greatly disturbed. Some stamps still have pieces of a stamp hinge adhering to its back which are described as having hinge remnants (HR). Heavily-hinged stamps may have been hinged badly or hinged more than once and may have a large area of missing gum and/or multiple hinge remnants.

Unused stamp of Siam showing cracked and discolored gum with thin and hinge remnants (#1, 1883)
Unused stamp of Siam showing cracked and discolored gum with thin and hinge remnants (#1, 1883)

Hinges may also turn brown with age causing discoloration to the stamps. Creases may occur in the gum due to careless handling. Thinned areas (“thins”) can also occur when removing hinges as the gum sticks to the hinge. Some gum is over-sensitive and can cause mint stamps to stick to each other if stacked prior to mounting in an album.

Gum used on some stamps contained sulfuric acid which destroy the paper over time and unused examples should be collected with the gum removed. Inferior gum used in the manufacture of stamps can result in damage when stamps are later subjected to less than ideal conditions, such as high heat and humidity.

Yes, gum is often the source of condition problems among stamps. It has been said that the long-range health and preservation of stamps would be better without gum. Short of soaking the gum from your stamps, the next best thing you can do is protect them by proper storage in albums or stock books and by not subjecting them to high humidity, sunlight or swings in temperature.