This is a free ebook by Fred Clouter and it talks about Pyrite and problems that can arise during storing or lack thereof. Below is an excerpt taken from the ebook:

On Wednesday the 26th of April 1882, the Queenborough Chemical and Copperas Works were auctioned off, heralding the demise of the copperas industry on the Isle of Sheppey. Green copperas was used to make sulphuric acid or vitriol, chemical manures and dye stuff.

Copperas as you will no doubt have gathered is just another word for pyrite (iron disulphide, FeS2). The form found on Sheppey is marcasite (iron disulphide, FeS2) and is dull green in colour when fresh, quickly deteriorating to a rusty brown when exposed on the beach for some time. Chemically identical, pyrite and mascasite are very different in behaviour.

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The normal gold coloured pyrite has dense molecules and tends to be more stable than the more open molecules of the marcasite stones. Fossils preserved either as pyrite casts, or containing pyrite within bone are prone to pyrite decay. Many different methods have been tried by collectors t o preserve pyrite specimens over the years, all with very little long term success.

There is nothing more depressing to the fossil collecor, or the museum curator who, when inspecting prized or unique specimen finds a little heap of whitish dust, an eroded data label and a discoloured box; even wooden cabinets can be severely damaged. It may be a few months, or a few years of exposure to the air, but the inevitable decomposition will take place.

The chief oxidisation products are sulphuric acid and various hydrated sulphates, mainly iron. The acid will also destroy associated shell and bone material. It is now generally accepted that the decaying process is caused by a form of oxidisation and is triggered by exposure to humidity in the atmosphere. It seems that the fossils absorb moisture from the air which reacts with the pyrite and the air. In tests under humid conditions the reactions can be catastrophic. However, if the water vapour is removed the reactions are slowed down and can eventually stop. The more compact forms of pyrite do not absorb moisture so readily and may only evidence decay by surface tarnishing. Various methods have been tried over the years, both by museums and individuals to stop the decay.

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Most have been unsuccessful. I do not believe that there is a method that can guarantee complete success but I do think with effort the process can be slowed down.

In the following paragraphs I shall attempt to describe some of the methods that I have tried with varying degrees of success.