Kath
It was something I read during a google search which I can no longer find but I did find the following.
"Under damp conditions pyrites will decompose to a yellowish powder or spicular green vitriol, and therefore pyritised fossils have to be treated with a special damp-resisting preparation to prevent disintegration."
Thinking about it the copperas was made from decomposing of the pyrites for a year or more them adding scrap iron, so presumably the break down of the damp pyrites gives iron sulphates and sulphuric and sulphurous acid. The addition of iron reacts with the acids to increase the yield of copperas or iron sulphate.
So the damp pyrites may form an acid in which in turn can decompose cloth wood paper etc.
There going to be a chemist out there who will tell me im wrong?