Going a bit off topic, to comment on what Rod said earlier
Sometimes on early Great Britain stamps, the mucilage is so thick and brown, you have to gently rub betwixt thumb and digit, to remove.
This is true - perhaps even more so - of older French stamps. Their glue is so gloopy and horrible that I find if an earlier collector has left the glue or bits of envelope on the back of the stamp the glue actually stains the paper.
Washing the stamps can reverse this to some extent, or if you're lucky completely, but the soaking water usually needs changing after a few minutes, having turned an unpleasant orangey-brown.