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When you get your fossils home

Most fossils need little attention other than a wash when you get them home, but fossils made of Pyrite or Fools Gold are different.

 

Fossils made from Iron Pyrites (FeS2) commonly called Fools Gold are susceptible to what is known as Pyrite disease. Don't worry it's not catching; however it can ruin your prize fossils and is a major concern to major museum collections around the world. Many fossils can be wholly or partly preserved in Pyrite; from vertebrate bones to tiny snails and ammonites.


This white/yellow powder was an ammonite fossil, 3 years ago my wife popped it into a glass, open topped vase with beach pebbles in it. The humidity in the room was enough to turn it to iron sulphate. A 200 million year old fossil reduced to powder in just 3 years!

Echioceras raricostatum starting to oxidise, a dull power is forming on the surface.

 



Silica gel bag from a shoe box.

So what is the problem? Well it's all to do with the chemical reactions between the pyrite and the air, the fossil is oxidizing, like when a nail rusts. Leave your fossil in a draw for several years and go back one day to find a pile of yellow white powder. One school of thought is that it is caused by sulfur eating bacteria, but I tend to think it's the former not the latter.

47% Iron and 53% Sulphur, the mineral is a compound of elements. What's happening to the fossil is the mineral is oxidizing due to contact with water in air. It turns into ferrous sulphate then ferric sulphate and sulphuric acid. High humidity is the problem. So store your fossils in low humidity and you should not have a problem.

Different fossils vary in how susceptible they are, for example in the Lower Jurassic ammonite faunas persevered in pyrite at Charmouth species vary. One which is very susceptible is Echioceras raricostatum. Others from different layers are not so prone to decay.

So what can you do? Many things have been tried over the years, with varying success.

I do not advocate coating the fossils with varnish or a similar hard clear layer, this spoils the fossil and looks horrible and the decay continues anyway. Rather I clean the fossil with water and dry it very slowly and thoroughly. Next I use Savlon to put a layer of protection on the fossil. Rubbed in well with your fingers it does not show. This may have first been used as a anti-bacherial layer but I think it just adds a barrier to the air. I store the fossil in a sealed plastic container with silica gel granules, either still in the bags they came in. (Find them in your shoe box or with electrical equipment packaging) or removed from the bags and sprinkled into the container. The gel soaks up any moisture in the box and thus reduces the humidity. Dry the bag gently on a radiator or warm, switched off oven to ensure it soaks water when you use it.

Place the fossils on display where you can see them and keep a weather eye. If they still start to decay, bush it off with a brass wire brush and reapply the Savlon, store as before with silica gel.

There are other methods including the use of chemicals such as ammonia and Ethanolamine Thioglycollate.


This specimen of ammonites and snails was immersed in Glycerin for 6 years, now it's in a box with silica gel.
This specimen seems very resilient to the problem. I have had it for 30 years kept in various states of undress, no problems.