Chastened by their experience with the 1974 Christmas stamp, Postal Service officials resolved to overcome problems experienced with that original pressure-sensitive adhesive issue. When the 1989 stamps were announced, news releases and reports in the philatelic press stated that the adhesive on the 29-cent Eagle and Shield stamp would not migrate, would not stain, and would be soluble in water.
These claims were not precisely true, but as a practical matter they may meet the specifications that USPS officials established. The acrylic-based adhesive on those and subsequent self-stick issues does not migrate through paper as the old rubber-based adhesive did, and it doesn't stain. But over time it can cause paper to become translucent. To prevent that, self-stick U.S. stamps issued since 1989 have a protective primer layer between the stamp paper and the adhesive.
Contrary to published announcements, the adhesive is not soluble in water, but Postal Service specifications required that the stamps be removable from paper by soaking them in water for up to 30 minutes. This works because the primer layer between the adhesive and the paper is water soluble. Even so, soaking in water doesn't always work. The majority of samples tested by the Salm Foundation's scientific laboratory did not float free of attached paper after soaking for 30 minutes in hot water; the lab recommends soaking for up to 45 minutes.
Many factors affect the soakability of self-stick stamps. Although most of them have not been tested in a controlled environment, the following factors have been reliably reported and confirmed: First, the longer the adhesive is allowed to set up, the firmer its bond. Other factors probably accelerate or retard the bonding also, such as ambient temperature and humidity, but time is crucial. Therefore, a stamp should be soaked off as soon as possible. The longer it stays on the envelope paper, the more difficult it becomes to soak it off.
When a stamp fails to separate from the attached paper during a lengthy soak, that is a sign that the adhesive may have penetrated the soluble primer layer and bonded directly to the stamp paper. In that case, it is frequently possible to free the stamp by soaking it in an organic solvent such as naphtha or turpentine. Such a solvent cannot dissolve the acrylic adhesive, but it does cause the sticky substance to soften and swell, after which it usually can be rubbed off the stamp.
Another factor that affects the strength of bonding is the type of paper on which the stamp is fastened. Stamps usually float free from coarse paper more readily than from coated or highly calendered stock. Acidic paper tends to form a weaker bond than alkaline paper, according to laboratory tests. But these are relative distinctions, of less importance than the time given to set up the adhesive.
Some self-sticks have a tendency to curl after soaking, if dried in open air. Collectors who experience this problem are advised to place the wet stamps between two layers of blotter paper, and place a weight on top while they dry.
Autopost and PVI strips lack the water-soluble primer layer between adhesive and stamp paper. They are not soakable in water, and attempts to separate them from envelope paper after treatment in organic solvents are usually disappointing. To collect them off cover in used condition, it is best to leave them undisturbed, and to trim away most of the envelope leaving just neat narrow borders around the stamps, as the 1974 Christmas stamps are normally collected.
In 1996, a U.S. Postal Service authority was quoted as having predicted that the adhesive on self-stick stamps "is likely to turn to powder as it ages," perhaps in 80 to 150 years. Interviewed for this report, the official stated he had been misquoted, and had merely speculated that this transformation might occur, but that no tests had verified it.
A National Archives conservator who specializes in adhesives wrote in response to our questions, "The newer stamps [issued after the 1974 Christmas stamp that had rubber-based adhesive] employ the acrylic based adhesives. These do not yellow, they don't lose their adhesive strength appreciably over time, and they don't dry out and become brittle. They do suffer cold flow which makes for the dreaded 'edge ooze' and also may affect the appearance of the stamp by sinking into the paper. I believe that the difference between these two types of adhesives should be made clear to collectors. The newer stamps definitely do not await the same fate as their 1974 predecessor."
Conservators favor removal of such adhesives whenever possible, but the archives conservator acknowledged that "there are different levels of collecting, and that adhesive removal is not always acceptable when one collects rare or pristine stamps."
Besides potential effects on the stamps themselves, pressure-sensitive adhesive may pose other problems, particularly for cover collectors. Acrylic gums have a tendency to cause certain inks to run, and may cause envelopes to discolor or become translucent. Combined with virtually inevitable cold flow around the edges, these stamps on cover may end up with sticky or otherwise distasteful borders.
Not all preservation problems are caused by the adhesive. In many respects, self-stick U.S. stamps are more stable than other contemporary stamps when artificially aged and when subjected to rub tests. Unlike some recent postage stamps, the printed image on all self-adhesives tested by the Salm Foundation's laboratory proved to be stable. None of the ink rubbed or soaked off in standard tests, even at elevated temperatures designed to simulate five years of aging. However, the ink on some stamps can be removed with a normal eraser, according to reports from collectors.
Hot extraction laboratory tests revealed that nearly all the self-adhesives are alkaline or about pH neutral. The only ones that showed a significant level of acidity were the two plastic ATM issues. There is no reason to believe that acidity is worrisome for plastic stamps as it would be for paper stamps, but care should be taken not to store the plastic stamps in close proximity to others.
These observations on preservation apply only to regularly issued U.S. postage stamps, and probably also to self-stick federal Duck stamps. They do not apply to foreign self-adhesives, Autopost and PVI strips, state revenue stamps, Christmas seals, or miscellaneous postal labels. The problems associated with these are often more severe, and less predictable than those that face the stamps which are the subject of this report. For example, many Autopost stamp imprints have faded to near invisibility in the decade since they were issued, and their long-term prospects for preservation are probably bleak.
For those who want to keep their stamps in mint condition, certain precautions are advisable. They should not be lifted from their backing paper, which creates a more significant change in the pressure-sensitive gum than hinging causes to traditional water-activated gum. Instead, one should remove each adjacent stamp and use them as postage, then trim away all but a small border around the stamp to be saved. These can then be mounted in the usual way, but the mounts should be examined from time to time for evidence of cold flow sticking. If the stamp face clings to the mount, the mount should be discarded and replaced.
The exceptions to this advice are the linerless coils. For those stamps, USPS provides silicone-treated paper strips to use as backing, on which single stamps or strips may be attached before inserting into mounts.
There are two methods of saving the self-adhesives that are printed on both sides. For collectors who want to show the front-and-back feature itself, simply save the full pane as issued. Do not remove the stamps from the backing. This is what collectors do now if they want to show the printed design on the liner backing, or other special features such as the special die-cut Steamboats pane. Some collectors save two of each, so they can show both front and back, side by side.
For collectors who just want to keep mint singles or blocks of self-adhesives that are printed on both sides, peel away the other stamps, front and back, and use them for postage. Then trim a neat border around the ones to be saved while they are still adhered to the backing liner, and mount in the normal way.
A separate problem is storage of large uncut press sheets, which are shipped in cardboard tubes or large cardboard envelopes, neither of which is safe for long-term storage. The safest holders are sleeves made of inert archival materials such as Mylar D; next best are interleaved layers of glassine inside boxes or folders made of acid-free cardboard.
For collectors of used U.S. stamps off cover, the techniques described above for removing the pressure-sensitive adhesive are essential to master. Once the gum is gone, these can be stored like any other used stamps, although the plastic ATM stamps should not be kept in contact with paper stamps. The most important point to remember is to remove the adhesive as quickly as possible, because the longer it sets up, the tighter it clings to the stamp.
[Reprinted with permission; slightly condensed from the original.]
This is the second and final part of Report No. 5 in the Arthur Salm Foundation's series on philatelic product research. This series is produced by the Arthur Salm Foundation, which was formed by the Collectors Club of Chicago. Previous reports in this series are available without charge to anyone who sends a large self-addressed envelope bearing $1.01 in stamps to the Collectors Club of Chicago at 1029 N. Dearborn St., Chicago, IL 60610.
The Salm Foundation commissioned Ken Lawrence, an experienced researcher and expert on self-adhesive stamps, to write this report. Lawrence pioneered the application of forensic science to stamp and cover expertizing. He and his spouse, Kathleen Wunderly, have established a consulting firm, Collectors Advisory Team, which provides customized services to the philatelic community.