Stabilising pyrite: dry, seal, coat — and what not to use

Pyrite oxidation needs oxygen and water. Below about 30 % relative humidity it stops; between 30 and 60 % it creeps; above 60 % it runs. Every treatment is therefore a way of keeping water away, and the measure of success is a stable RH reading inside the box, not the appearance of the specimen.

The reliable method is a sealed micro-climate: an airtight polypropylene or polyethylene box, the specimen on inert foam, a sachet of indicating silica gel conditioned dry, and a small RH indicator card. Change or re-dry the gel when the indicator shifts. For valuable pieces, an oxygen scavenger (Ageless, RP System) in a heat-sealed barrier-film bag removes the other reactant as well.

Consolidating with Paraloid B-72 in acetone (3–5 %, brushed or dipped after thorough drying) slows moisture uptake and holds a cracked surface together. It does not seal a specimen: acrylic films pass water vapour. Do not use it on a specimen that is still damp or still actively efflorescing — it traps the reaction underneath.

Do not use polyvinyl acetate (Vinac, PVA emulsions) in humid conditions: the film is water-permeable, hydrolyses to acetic acid, and the acetate feeds the very reaction it was meant to stop. Do not use varnishes, lacquers or oils, which are irreversible and also permeable. Ammonia-vapour and ethanolamine thioglycollate treatments neutralise existing acid and belong to conservators; they are not a substitute for dry storage.

Pyritised ammonites from the Lias, Devonian goniatites from Morocco and the pyritised material of the Pierre Shale and Oxford Clay are the classic risks; brassy, bright pyrite is usually stable, dull grey-black marcasite-rich material usually is not. Record the condition on inspection and photograph it, so a change is visible rather than remembered.