Stamp Tagging – What Is It & Why Does It Matter?

Did you know the postage stamps on a letter or package you receive are painted in a foreign substance? The practice is called “tagging” and it is not a plot line for a bad TV drama. You might be wondering what is stamp tagging, and why it matters.

Stamp tagging is the process of applying an incandescent substance on stamps. This substance is viewed under UV light. A stamp is tagged to allow an automated machine to locate and position a package for a cancellation and postmark. Tagging helps postal services meet increasing demand.

There are many facets to tagging a stamp, including the type of tagging and how the ink is applied. Additionally, the presence of the ink has a significance to play with stamp collectors and dealers. The following covers what tagging is, how it is done, and why it is important.

Stamp Tagging – What Is It & Why Does It Matter?

Stamp tagging is not something most people know about. Even if you know how luminescence, phosphorescence, fluorescence, or ultraviolet lamps are related to stamps and stamp collecting, you likely do not understand why. A lot of stamp collectors do not even know until they get really into their collection and the history of stamps.

What Does It Mean If A Stamp Is Tagged?

Stamp tagging is the process of coating a stamp (or part of a stamp) with a substance that is invisible to the naked eye and only able to be seen under special lighting. The coating allows the stamps to be seen under shortwave UV light, which helps position the packages for automated canceling.


Zinc-Orthosilicate is the substance used to coat stamps. It is phosphorescent and can only be seen by the naked eye in certain light or if you inspect a stamp very closely.

Shortwave UV Light

Shortwave UV is 200 – 300 nanometers. It is applied to coated and uncoated stamps to help sorters align stamps so that each one can be canceled. This is similar to a black light poster. When ultraviolet light reflects off fluorescent inks on a blacklight poster, the ink absorbs the light. Once absorbed, it re-emits the light as a very bright color.

The same principle applies to tagged stamps. When the reflective ink, called a “taggant,” is applied to a stamp and then exposed under strong shortwave UV light, the stamp emits a bright color in the pattern the taggant was applied. The colors range from yellow-green to bluish-green. The fluorescent glow let machines locate the stamp and position them correctly.

History Of Stamp Tagging

Understanding what tagging is and how it works maps out the history of tagging. The mail canceling process was completed by hand initially, but as mail became increasingly popular, canceling by hand became immensely time-consuming.

Given that one of the selling points of the postal system was the timely delivery of packages, immense delays due to the sorting of mail were becoming an issue. Another problem with the delay in sorting mail was the cost.

The volume of mail and delay in processing was worldwide. The United States Postal Service began to look for a solution to the process of sorting mail in the mid-1950s. The answer most involved agreed on was automation. Automation had transformed manufacturing, the logic went, why not sorting mail?

Several companies produced sorters that would flip letters and packages so that the stamp would be at the front. These machines would also cancel the stamps once they were aligned properly. That alignment, however, was problematic.

Coating To Identify

Automation revolutionized how packages were organized, but it could only go so far. Without a way to identify where a stamp was on an envelope or package, the machine could only arrange letters based on size and shape. A hand sorter still had to locate and position a package so that the cancellation of postage could be done quicker.

Processing a few dozen letters was no big deal, but processing thousands of letters dramatically increased processing time. Even with automation, as mail became increasingly popular, it became obvious an even quicker way of processing and sorting packages was needed.

The answer was to give stamps a coating that could be identified by a machine. Pinpointing the placement of the stamp allowed machines to rearrange and position the package so that a cancellation and postmark could be properly applied to a package.

Almost immediately, the USPS saw improvements in sorting time. This saved money as sorting and marking were quicker, more accurate, and required fewer workers. There was, however, one issue that remained.

That was the postal administration not wanting to dramatically alter the way stamps were designed. To use a visible ink or coating required stamp design to adhere to the limitations of automation. This meant the design on the stamp could not obscure the coating in any significant manner.

The UV Answer

To address that, manufacturers decided to utilize a coating that was nearly invisible unless it was seen through UV light. Machines were outfitted with UV lamps and stamps that were coated with the phosphorescent substance shined a bright yellow-green or greenish blue.

The new machinery would scan the stamps with UV light, identify where the stamp was located on an envelope, flip it if necessary and arrange it uniformly with all the other letters in a batch. The machine would then send the letters down the line where another machine would apply a cancellation mark to the stamp and a postmark to the left of the cancellation mark.

In terms of processing letters, the new system was an immediate success. Stamps were sorted, arranged, and canceled much quicker. Man hours were saved, which resulted in overall savings to the post office. Most importantly, customer service improved as delivery delays were avoided.

Different Types Of Stamp Tagging

Tagging is applied to stamps using 3 different methods. Some stamps are applied with just 1 method. Others are tagged using a variation of the 3. Some are tagged using all 3 methods in one run.

The 3 different methods are as follows:

Overall Tagging (OT)

This method has a goal of applying the liquid taggant over the entire stamp. It is spread onto stamps using a rubber mat or cylinder the stamps are run under or through. Stamps that are tagged in this manner appear “grainy.” With a flexible tagging mat, the coverage of the taggant will not immerse the entire stamp, which can leave some of the margins free of taggant.

The untagged margins were an uncomplicated way of identifying stamps that were pre-phosphored, which covers the entire sheet of stamps entirely. Overall tagging is still used in some cases today. It is usually applied to a stamp between printing and perforating, the timing is to prevent the taggant from slipping through the perforation holes.

Block Tagging (BT)

This tagging method uses a block shape to apply the taggant to each individual stamp. The block size varies with every print run. One susceptibility with this type of tagging is that the rubber mats wear out, this makes the tagging application extremely unique to the cracks, flaking, and breaking that appear in the blocking mat.

Pre-Phospored Tagging (PT)

With this type of tagging the taggant is added before the vignette is printed. The tagging exposure depends on the imagery of the stamp and the coating that is applied to the paper. A pre-phosphored tagged stamp has a coating that appears solid and part of the stamp.

Often, it is difficult to distinguish between an OT and a PT treated stamp because the phosphorous is applied so evenly. The imagery can also appear much bolder because the ink of the image is on top of the taggant. This lets the natural color of the ink come through undiluted.

When coated, the pre-phosphorus soaks into the paper and creates an uneven distribution of color when UV light is applied. Because of this, this tagging is usually fainter than the other 2 methods.

A Fourth Way

Another way of tagging that is used occasionally is to mix the pre-phosphored ink with the printing ink. When this is done the stamp’s tagging will appear very uneven. The distribution of the pre-phosphorous treatment depends on the stamp’s design. The stamp will only shine under UV light where the phosphorus ink was applied.

Patterned Application

Using all 3 of these methods, tagging can be applied in a band or several bands. In some cases, all 3 types of tagging are applied to a sheet of stamps. On other sheets, patterns of taggant are put on the stamps.

How To Find Tagging On Stamps

Before you start looking for the tag marks on any stamps with a UV light, there are a few things you need to cover to ensure you will be successful and most importantly, safe. First, the safety issues.

Vision Safety

UV light can burn your corneas and you will not feel it until the damage is done. Never look directly into a UV lamp or light. If the damage is extensive enough, you can lose your vision altogether. Wear eye protection when possible and limit your time working with a UV lamp.


UV light can also burn your skin. The result is much like a sunburn. Always wear gloves when working with a UV lamp. Wear sunscreen if you are going to be using it for more than a few minutes.

Choose A UV Light

You can find UV lamps online, in hobby stores that have a stamp collecting department, scientific supply stores, and philatelic supply dealers. UV lamps come in long and short-wave models as well as lights that can do both. The differences in purchasing considerations are as follows:

  • Long-wave lamps can be used to detect fluorescent tagging
  • Long-wave lamps are less expensive than shortwave lamps
  • Long-wave lamps can determine the paper type
  • Short-wave lamps burn out more quickly
  • Short-wave lamps can detect the after-glow of phosphorescent tagging
  • Short-wave lamps are required to identify tagging on US stamps

In addition, many lamps come with filters to move between short and long-wave that are activated by flicking a switch. Here are a few other things to consider.

Plug-In vs Portable

Another thing to consider is whether you want to use a plug-in lamp or buy one that is handheld and portable. A plug-in lamp lets you see lots of stamps in one sitting with extraordinarily little adjustment. It is difficult to lug around, though. You can take a portable lamp with you pretty much anywhere but your ability to look at lots of stamps at once is limited.

Ideally, you go with both, but if that is not in the cards, the best lamp is the one that most suits your stamp reviewing habits. If you look at stamps all year long at shows and dealer stores, a portable UV lamp probably makes the most sense. If you look at lots of stamps at once and usually from your workshop or hobby area, then a more permanent lamp is the better choice.

The good thing is that UV lamps are inexpensive enough that purchasing one and saving or the other type will not set you back a ton of cash or take months to be able to afford. Here are a few other things to consider:

  • A plug-in lamp has an immensely powerful bulb.
  • Portable lamps are not that powerful
  • Plug-in lamps usually have short and long-wave capabilities
  • Plug-in lamps are cumbersome, space consuming, and require access to electricity
  • Portable lamps are battery operated and perfect for shows
  • Battery operated portable lamps are less expensive than bulky plug-ins

Stamp Tagging Identification Process

1. Work Quickly And Efficiently

If you have more than one stamp you want to scan under UV light, lay them out in a line with the stamps clearly visible. Do not lay them out on a luminescent surface as that can dilute and obscure UV light.

2. Get Dark

The room you use must be dark. Pitch black is not necessary, but the darker the room the better the tagging will stand out.

3. Review Quickly

Review your stamps as quickly as you can. Separate out stamps that have tagging. Only work in short bursts, even if you are using eye protection. As soon as you are done, turn the UV lamp off and standard lighting back on.

4. Tagging Errors

While tagging revolutionized how stamps were processed, no system is perfect. Errors, as happens with any printing process, occasionally occur with tagging of stamps. Some stamp collectors collect these errors. A UV lamp can help identify tagging coverage errors, mixture mistakes, or stamps where tagging was not applied at all.

The latter example is called “Untagged Errors.” These happen when there is an issue with ink coverage or paper sticks together in the printing process. These types of tagging errors are highly collectible and, in some cases, worth some money. The exception to this is stamps under 10 cents as postal services do not want the machinery triggered by low-value stamps.

Final Thoughts

Stamp tagging lets world postal services mark stamps in a way that does not deface a stamp but still allows for automated sorting and marking. You need a UV lamp to see the tagging application. Stamp tagging is just one more reason stamp collecting fascinates so many people.

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