Quote:
Ya I think it is harder paper than the other. I get what you are saying.
Actually, no it is not...

paper remains similar, (some is blued)
What you are experiencing, is the stamp still adhered to part of the envelope, or paper when originally used.
Placed in cold water, the paper on the back should float off within 10 minutes. (along with the moisture of the tongue of the person whom licked it, 140 years ago)