The following is an excerpt from the article. Read the full version online from the American Philatelist. 

Luminescent stamps  – stamps that glow – have intrigued stamp collectors for decades. What makes a stamp glow? What types of varieties exist? How do you use an ultraviolet lamp to make a stamp glow? What is the difference between phosphorescent tagged stamps and fluorescent stamps? What is the difference between longwave and shortwave ultraviolet light? These are all questions that pique the interest of collectors who are unfamiliar familiar with tagging.

Luminescence

Luminescence is a broad general term that simply means “the giving off of light.” Luminescent stamps give off light, or glow, when “excited” by ultraviolet light. The properties that cause stamps to glow are largely invisible to the naked eye and ultraviolet (UV) lamps must be used to detect this glow. Post offices in many countries issue luminescent stamps to activate automatic mail-handling equipment or serve as printing security devices. Some of these utilize phosphorescence; some use fluorescence.

Luminescence, as it relates to philately, is both a form of collecting specialty and an expertizing tool. The term breaks down into two main subcategories, fluorescence and phosphorescence.

To fully understand luminescence and its significance to stamp collectors, it is important to know a bit of background on why stamps glow.

Fluorescence

Fluorescent stamps glow while exposed to UV light, but not after the light source is removed. 

As previously mentioned, fluorescence simply means that a stamp or piece of postal stationery exhibits some form of glow when illuminated by longwave UV light. Longwave ultraviolet light is the same type of popular black light that is found in poster shops and roller skating rinks. It has no harmful effects to the eyes.

A number of different factors come into play to make a fluorescent stamp glow. Both organic and inorganic substances (either native to the paper or introduced) can react to light, including optical brighteners, ink components, certain pigments and a host of others (Figure 1). Any of these factors may be intentional or unintentional (Figure 2).

Figure 1. Examples of stamps from Italy with differing fluorescent qualities, including untagged (left), Lumogen fluorescent paper (center) and fluorescent Lumogen paper (right).

Figure 2. A pinkish ink used for this stamp of Russia is highly reactive to ultraviolet light.

Therefore, as a result of any of these factors, even different printings of the same stamp can react differently to UV light, causing collectible varieties to occur (Figure 3). 

Figure 3. These booklet panes of 1981 18-cent Wildlife stamps react quite differently under ultraviolet light.

Some of these varieties include the so-called Hi-Brite papers (caused by optical brighteners) (Figure 4), fluorescent ink types (caused by ink components) (Figure 5), and fluorescent overprinting to aid mail processing or to serve as a security device (Figure 6 and 7). Some of these varieties are intentional, but many are not; they are a result of technological changes in stamp production. Most are highly collectible and some are even rare.

Figure 4. The World’s Fair stamps on this cover range from Hi-Brite (upper left) to virtually dead (no reaction).

Figure 5. Three types of fluorescence found on stamps of Australia. The left stamp is printed using fluorescent ink; the center and right stamps are both on Helicon fluorescent paper.

Figure 6. This stamp from Thailand features a fluorescent overprint.

Figure 7. The £10 Britannia stamp of Great Britain (shown here under both ambient and longwave ultraviolet, and longwave alone) show some of the fluorescent security devices on this gimmick-rich stamp.

One note of caution, however: Fluorescent varieties on used stamps (such as the so-called Hi-Brite paper) can easily be faked, so they are generally bought, sold and traded either as mint examples or on cover. As a result, many fluorescent varieties are not listed by major catalogs and, when listed, are valued only as unused. Phosphorescent tagging (or the lack thereof) cannot be effectively faked, so listings for these items are more plentiful in both mint and used condition. More on this in a bit.

As an expertizing tool, the UV light can also aid in uncovering many types of fakery, including filled thins, regumming, removed cancellations, altered designs and other repairs. The main principle behind this detection technique is that any paper exhibits some form of fluorescent reaction. None turns completely black under UV light, and no two papers match exactly. 

Regardless of origin or technological mastery, every batch of paper has unique characteristics and a specific weave that is virtually impossible to duplicate. Any alteration to this unique pattern (or addition of other fibers) can easily be detected with fluorescent light. Thus, repairs, such as filled thins or other types of repairs, stand out like the proverbial sore thumb under UV light (Figure 8), as do most attempts at removing cancellation ink.

Figure 8. Fluorescence can be used to easily detect many types of repairs and alterations on stamps.

Phosphorescence

Phosphorescent stamps glow even after the source of light is removed. This glow can last anywhere from only a fraction of a second to close to a minute in some cases. The easiest way to tell the difference, however, is by the type of illumination used.

All luminescent stamps glow under shortwave UV light to a greater or lesser degree, but fluorescent stamps also glow under longwave UV light (and usually brighter). Phosphorescent stamps do not glow under longwave light. Different characteristics of the same stamp can be seen when both light sources are used (Figure 9).

Figure 9. Under shortwave UV light, the tagging on these stamps almost appears like two different types of tagging. The bottom image (taken under longwave UV light) clearly shows that the high level of fluorescence on the left pair interferes with what is visible under shortwave light.

Phosphorescent tagging is used by several countries, including the United States and Great Britain. Phosphorescent varieties can only be seen under shortwave UV light. Unlike longwave UV light, shortwave UV light can cause temporary (but painful) eye damage if used improperly or for too long. However, virtually any pair of prescription glasses provides adequate protection for most collecting activities. Even off-the-shelf reading glasses are usually adequate, as the lenses contain UV filters.

Unlike many fluorescent varieties, most stamps with phosphorescent characteristics were intentionally created as such. This is completely true with United States stamps. Phosphorescent stamps are a direct result of U.S. Post Office Departmental policy. How did they come about?

After the close of World War II, the Post Office Department was faced with a huge dilemma. Never before had there been such a high literacy rate among the population, with so much mail traveling through the system. Adding to this burden was the rapidly increasing volume of commercial mail. As a result, mail sorting and processing, which previously had always been a manual task, could no longer be accomplished in that manner.

By the late 1950s, the Post Office began to seriously explore ideas that had been kicked around since the late 1930s. These ideas ranged from magnetics to color recognition systems.

During the late 1950s, a contract was let to the Pitney-Bowes Corporation (of postage meter fame) to begin experimenting with fluorescence and phosphorescence, and the firm created a number of different stamp and postal stationery essays to test the viability of such systems for automated mail sorting and processing (Figure 10). 

Figure 10. Among the early fluorescent tagging tests conducted by Pitney-Bowes for the U.S. Post Office Department is this unique tagged stamp from the 1938 Presidential series.

Fluorescence ultimately was dropped in favor of phosphorescence, and the actual tagging program on live U.S. stamps was piloted by National Cash Register Co., of Dayton, Ohio. Dayton was chosen not only due to the NCR connection, but also because it had a large enough volume of mail to warrant such testing. These tests were to last several years, and the early tagged stamps could be found only in the Dayton area or from the Philatelic Division in Washington, D.C.

Although the first tagged stamp was released Aug. 1, 1963, in Dayton, Ohio, it was not the stamp that was being honored! There was no first-day cancel. The cancel simply read “FIRST DAY OF USE LUMINESCENT TAGGING” (Figure 11). That stamp, a phosphorescent-coated version of the 8-cent Jet over Capitol airmail stamp (Scott C64a), was coated with a calcium silicate suspension that made the stamps glow a bright orange-red under shortwave UV light.

Figure 11. As you can see from the cancel, a first day cover for the first tagged U.S. stamp celebrates the use of tagging rather than the release of the stamp, specifically.

A special phosphorescent tagged airmail label (Figure 12) also was released the same day and was used on many covers. The sole purpose of this early form of U.S. tagging was to separate airmail letters from standard mail. These labels were distributed with the instructions that they should be placed on airmail letters (domestic or those bound for overseas). The tests were a success.

Figure 12. These labels, released on the same day as the 8-cent tagged airmail stamp, are tagged with a phosphorescent coating to facilitate the separation of airmail from first-class mail.

Within a few months, the purpose of experimental phosphorescent tagging was expanded to include the automatic facing and canceling of mail. This means the machines were now capable of not only sorting airmail from standard mail, but to line up all envelopes so the stamps were in the same corner and then cancel them! A new form of taggant was introduced for this purpose. It was a suspension of zinc orthosilicate, which glows a bright greenish yellow under shortwave UV light (Figure 13). Thus, airmail and standard mail could still be separated (by the color of the taggant) and all letters could be faced and canceled automatically.

Figure 13. Zinc orthosilicate was used on these stamps (the so-called “LOOK” coil). This taggant glows a bright greenish-yellow under shortwave UV light.

Since both types of taggant were mineralogical derivatives, they could not be dissolved to form a solution; they had to be contained in a suspension. A transparent lacquer was chosen as the carrier (Figure 14). Also, as mineralogical derivatives, the two compounds were highly abrasive. This came into play later, with different forms of tagging created to avoid wear and tear on post-printing processing equipment.

Figure 14. Phosphorescent tagging, a mineralogical derivation, must be suspended in a translucent lacquer.

This tagged lacquer was applied by a color station on the printing press (or sometimes as a separate press run) after all other colors were printed.

On Oct. 26, 1963, the first full printing run of tagged stamps was produced. It was the 5-cent City Mail Delivery commemorative (Scott 1238, Figure 15). As a result, the first untagged errors also were produced, when no tagging (in error) was applied to some stamps. Even though the taggant itself is invisible under standard light, its absence as an untagged error is classified as a form of color-omitted error. As with any color-omitted error, there can be no traces of taggant present on a stamp, and many so-called tagging errors are not completely missing the taggant. This is easily missed with the use of an inadequate lamp.

Figure 15. The first full-run U.S. stamp to be tagged was the 5-cent City Mail delivery stamp (center). This development also led to the first verifiable untagged error (block).

Tagging continued to be an experimental application for several years. Most issues that were released with tagging also exist as issued varieties without tagging, and some of the tagged varieties have become scarce.

By January 1967, stamp production was converted to printing all stamps (except those intended to be precanceled) as phosphorescent tagged issues. By 1969, most post offices had the special facing and canceling equipment designed for use with tagged stamps (Mark II facer-cancelers). By 1973, even the high-value Prominent Americans series stamps had been released as tagged stamps.

There now are numerous types of applications of taggant, each a separate and collectible type, and some stamps exist with more than one type, again, creating collectible varieties, some of which are scarce.

Until recently, tagging on U.S. stamps was also considered a security device against counterfeiting, as no counterfeits were known to ever use a phosphorescent coating. This was a way to quickly and accurately detect counterfeits in the field, as well as in the offices of the Postal Inspection Service. Even the most convincing postal counterfeits from China were able to be detected easily, because they had no tagging. There are, of course, other determiners, but this was the fastest.

Unfortunately, all that changed this past summer (2022), when convincing counterfeits began appearing that had a phosphorescent coating, some of which were seemingly better applied than on the genuine examples (Figure 37). As of this writing, there is now no other way to detect counterfeits quickly without close examination and comparison with known genuine stamps, but this is a recent development. There will surely be more to this story over time.

Figure 37. Counterfeit stamps used to be untagged, making them easy to detect. Now, most have tagging. From left: untagged counterfeits, tagged counterfeits (with strong tagging) and tagged genuine examples of the current U.S. Global Forever stamp.

All in all, tagging on U.S. stamps forms a fascinating and colorful collecting specialty that can be as inexpensive or expensive as you want it to be. There’s also the constant thrill of the chase involved because there’s always a chance of making a new discovery. What does all this mean? It means there’s a whole new world of beautiful collectible stamps available by simply turning on your UV lamp. Just take the time to get to know the stamps and the listings. There are more discoveries to be made!