Great Britain Philatelic Society

I have come across a penny stationery envelope that appears to be printed in black. I have never seen this before. Could anyone shed any light please?https://www.gbps.org.uk/boards/download ... ew&id=1410

I can remember wondering about one of those ... sadly the answer is simply that the pink ink has a tendency to oxidise to black sometimes. (Actually, come to think of it, it was probably more the case that it had a tendency to get sulphurised in the highly polluted Victorian city air! But you do see a number of affected examples -- not always all the way like this one.)