How are postage stamps voided?


How do the UK postal service know if a stamp has been used before?, (Sometimes they do not have a postal mark across them).

I think that it used to have something to do with phosphorous dots... um....

Thank you

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They are marked with special UV sensitive inks

FriendOfHumanity (author)  steveastrouk7 years ago
 Thank for the reply.
Do you mean that as the envelopes are scanned, any with uv ink on are rejected?

Yep. If they detect them, the letter would be delivered, but with the Royal Mail's own little fine attached.
Interesting. I've started seeing apparently-uncancelled stamps in the US mail; I'll need to investigate. (I know someone who would know the answer.)
caarntedd7 years ago
I don't collect stamps, but don't some collectors want stamps with a visible postmark?
lemonie7 years ago
These things go through machines, if you tried a re-use the recipient would be charged £1.50.