Canadian Tagging (Ottawa/Winnipeg)

Generally, I can tell P2 and OP4 tagging without having to resort to a UV light. I do, however, have an item where the tagging is very difficult to see unaided and doesn't even react to UV light (My light has both long and short waves). Would this be WPG tagging or is there any other explanation? Is it possible that the stamps went through the tagging process but the machine had run out of the active material? In such a case, would this be classed as an untagged error (which is included in Unitrade's listing)?

On some of the early tagged stamps they used a tagging that would migrate. These stamp's tagging could actually transfer to other stamps in an album even through the album page if my memory is correct. This migratory tagging was abandoned early on. So if you are dealing with early tagged stamps this could be the reason the tagging is absent.

Well that is probably the answer but I find it strange that there is not a single trace of the tagging, other than an almost invisible colourless band, on any of the four stamps on a PB.The stamp in question is Unitrade 594 (10c landscape definitive) which was produces using OP4 migratory phosphor as well as OP2 and W2B.

I wonder if encasing those stamps in, say, crystal mount, would have kept the Win tags strong.

Hello itma,
It is very common for the migratory OP4 stamps to now show almost no evidence of tagging at all. In fact they tend to show under the UV lamp as a violet colour much like the Winnipeg (W2B) tagging. The difference is that the migrated OP4 stamps will have bars that are 3mm wide. The OP4 stamps also do not have the phosphorescent afterglow that is a feature of the W2B tagging.

Once the OP4 tagging has migrated, these stamps are not as harmful as the originals that show plenty of the migratory yellow/green fluorescent tagging. However, I would still avoid keeping them in direct contact with other stamps.

Note also that the migratory tagging is sometimes absorbed by the other ink on the Landscape definitive stamps. If some of the printed areas of your stamps show some glowing under the UV lamp, then there is still a danger of this "tagging" migrating to other stamps.

The migratory tagging is like shall we say a never drying ink. It could eventually disperse throughout the stamp. It could migrate onto and into album pages and other stamps. A mount helps some but if there is an area that the tagging could move through for example a seam or opening it could escape. I ended up buying clear interleaves to put between my album pages to lessen the chances of it contaminating other stamps. This information was well published in the philatelic press at the time. As I recall even storing the stamps in glassine envelopes didn't prevent contamination.

Okay, but addressing the question of fading, does enclosure help to delay

Apparently not. My 594 landscape has been in a glassine envelope since I got it from the Philatelic Bureau in Montreal, back in the 1970s. And there is not a speck of tagging on the stamps (a single PB of 4) or on the glassine, just shiny colourless transparent lines where the tagging used to be.

If the tagging migrated, you should find traces of it on the glassine.

I think that, while some tagging migrated (OP4), others just fade away.
Maybe in a decade or so, album pages of Winnipeg tagged stamps will be indistinguishable from regular ones.