Toward the end of the 1960's, when recycling of paper became more and more common, the phosphorescent tagging was becoming ineffective. The reason was that as envelopes began to be produced from recycled content, the fluorescence of the paper began to increase, from the whitening agents used to bleach the pulp. This reaction tended to over-shadow the glow from the tagging, requiring a much stronger and brighter tagging that would be visible to the mail cancelling machines, even when the envelopes were on hibrite paper.
Thus fluorescent tagging came into being. Fluorescent tagging is much brighter than phosphorescent tagging. Generally, most fluorescent tagging glows either a bright yellow, a bright green, or a bright greenish yellow under long-wave ultraviolet (black) light. Australia used a chemical called Helecon (a zinc sulfide compound), which glows a deep orange red. On Australian stamps, Helecon was either incorporated into the ink, the paper, or both the ink and the paper.
The key difference between phosphorescent tagging and fluorescent tagging is that there is no afterglow whatsoever, when the light source is disconnected. Also, unlike phosphorescent tagging, which is usually visible on the surface of the stamp in normal light, fluorescent tagging is generally invisible in normal light. Sometimes you can see shiny bands on the stamp surface, but usually you will not be able to see the tagging clearly without a UV lamp.
When the tagging completely coats the stamp, the paper is often referred to as fluorescent coated paper. An example of this on a stamp from Brazil is shown below:
Here, you can see a light, bright yellow glow over the entire stamp.
On Canadian stamps, the fluorescent tagging is referred to as Ottawa General Tagging. Until the 1980's all general tagging was done on two vertical sides of the stamp only. Starting in the late 1980's many stamps were tagged on all 4 sides. The taggant used initially was called OP-4. It was unstable, and it had a tendency to bleed, or migrate across stamps and onto other stamps and paper that the stamps so tagged came into contact with. Most of the 1972 issues were tagged using OP-4 taggant. Usually this tagging presents the same bluish white glow as Winnipeg tagging, but this is only because most of the colour has migrated over the years.
Later in 1972, the taggant was changed to OP-2, which is completely stable, and glows bright greenish yellow, or bright yellow. The Centennial issues with General tagging used OP-2, as did all issues from the 1972 Krieghoff stamp onward. Initially, the tagging bars used were 3 mm wide, increasing to 3.75 mm on some 1974 issues, and finally to 4 mm wide starting in 1974. There has been a 2 mm width reported in the philatelic press, but I have never seen an example myself.
The picture below shows two examples of the 20c Prairies definitive from the 1972-78 Caricature Issue, with both OP-2 and OP-4 tagging:
The stamp on the right is tagged with 3 mm OP-4 bands, and as you can see, the tagging has a pale, washed out appearance. The stamp on the left is tagged with OP-2 3 mm bands.
Stamps on which the tagging has migrated will show a diffusion of fluorescence on the back of the stamps, like that shown below:
The stamp on the right shows the migration of the tagging. If you look closely you can see that the stamp looks more fluorescent close to the vertical perforations on both sides. A closer look reveals that this uneven fluorescence has been caused by bleeding of the taggant across the stamp. Some examples are much more dramatic than this, but this is fairly typical of what you can expect to see on stamps with this type of tagging over 45 years later.
Some collectors and dealers are of the opinion that stamps showing migration are damaged. I disagree, largely because most stamps would show some migration by now. There is no way to stop the process. It is not a matter of better storage or handling. Therefore, I regard it as a normal characteristic of these stamps.