Pyrite decay

Pyrite (iron sulphide) and its unstable cousin marcasite oxidise when relative humidity climbs above roughly 60 %. The reaction produces iron sulphates and sulphuric acid: the specimen swells, cracks, grows a pale yellow-white or grey efflorescence, and the acid eats the label, the box and any neighbouring shells. Collectors call the result “pyrite disease”, but it is chemistry, not infection, and it does not spread between specimens except by the acid vapour and humidity they share.

Prevention is storage below about 45 % relative humidity. A sealed box with indicating silica gel, replaced when it changes colour, is the cheapest reliable measure. Do not seal an already-damp specimen: dry it slowly first. Keep pyritised material away from shells and bone, whose calcium carbonate is attacked by the acid.

Active decay can be halted but not reversed. Brush off loose efflorescence with a dry brush outside the cabinet, neutralise with an ammonia-vapour treatment or a conservator-applied ethanolamine thioglycollate solution, then consolidate with Paraloid B-72 in acetone once completely dry. Record the treatment on the specimen record: a later owner needs to know why the surface looks the way it does.

Inspect at-risk specimens twice a year. Stratum's Condition card carries a pyrite-risk flag and a last-inspected date so the reminder is in the record, not in your memory.