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pauledevans
I’ve been to Charmouth half a dozen times, but only to the west beach and always to be disappointed… though I think this was largely due to my lack of experience at the time.

Anyway, we were camping down just north of Charmouth a couple of weeks back, and after reading up about tactics, we (my daughter and I) took to the east beach. A cross section of our pyritised ammonites in one image. These we found be walking the sandy bit of the beach looking for areas where a lot of pyrite was being washed out, and then picked through the gravel.

…and something a bit different in the other. I was scanning over a large clay slip, roughly a kilometre down the beach towards Golden Cap, and spotted a 2” square patch of that ammonite segment exposed. Scraped it out by hand, and cleaned it down in the sea.

I was elated! A4F24225-E065-43DA-B757-AD2B716FA7EE.jpeg  26566B74-3CBE-4F2C-B6FC-4360A04D7BE3.jpeg
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Barrow Museum
I see a nice assortment of ammonites from the lower part of the Charmouth Mudstone Formation.  Quite a few are Echioceras and what looks like Promicroceras.  The rest I'd need to see from more angles to express an opinion on their identity.

The second and larger piece is probably from higher in the Charmouth Mudstone Formation and can be identified as Lytoceras (probably L. fimbriatum).  This is an interesting ammonoid genus, as it occurs almost unchanged from the Lower Jurassic up to the Upper Cretaceous and is thought to have been the root stock from which most if not all of the more familiar Jurassic and Cretaceous ammonite families evolved.

I expect you know that the pyrite from this locality is notoriously unstable, and to prevent it oxidising and disappearing into a pile of sulphurous dust, you need to wash the fossils for a long time in fresh water to get rid of the salt, then dry thoroughly and coat (or better, soak) in a dilute varnish to keep them from the atmosphere and store in an environment that does not suffer extremes of temperature or humidity.  The best coating is Paraloid B72, dissolved in Acetone.  Even then, some will probably break apart after maybe a few years.
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s.c.
Good preservation advice from Barrow. My tip for washing/soaking out the salinity from pyrite fossils is to put them in a sock/stocking/mesh bag and place them in the loo cistern for a week or so. That way you get many changes of clean water effortlessly and without wasting water (in fact you save some 😀). 
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