Stamp Tagging: Why? How?

One thing I have never completely understood about stamps is tagging. I know that some stamps have a coatings of various inks or chemicals that allow them to be "seen" under certain lights.

But what I want to know is WHY? I assume that modern mail sorting machines scan mail for the tagging. But what good is it if you have a tagged .10 stamp on an envelope? How does the machine know that tagged stamp is the right value for that envelope? Do envelopes without tagged stamps get kicked off the mailstream for hand servicing?

Answers... I want ANSWERS!!

I believe the tagging is there so the canceling machine can detect the stamp and then orient the envelope properly so it can be cancelled. I don't think current mail sorting machines check for proper postage. I'm sure they could build a machine that would, but I think it would be had to justify the cost as I imagine the amount of improperly posted letters is relatively small and it would probably take decades for the USPS to recoupe the cost of the new machinery from the additional postage due revenue.

That is correct. In the US, the purpose of the tagging is for letting the machine know there is something luminescing at the proper wavelength window on the envelope. It does not determine whether it is actually a valid stamp or if the postage is correct. The fraud filter is the postal employees who handle the mail and also the conscience of the mailer.

There was an attempt to use different taggant for different classes of mail service -- the early tagged airmail stamps used taggant that luminesced red, compared to the usual green luminescence. As far as I know, this concept was not further developed, and later airmail stamps used the same green luminescence. I do not know the exact reason.

OK, so NOW we get into the topic of another thread here on the forum, mail passing through the system with Non-postage. Years ago I mailed an envelope to myself with a label from a self-stick booklet. Sure enough, it showed up just fine. I did it again, with the same result. My thinking was that the label was also tagged.

https://goscf.com/t/28534