The actual chemical composition of the structure in question eg, purity or otherwise of the iron is an interesting line of inquiry, as mentioned above. One tends to concentrate on the presence of water, maybe because it's a more obvious factor.
It is interesting that the level of abundance of rust shells seems to be related to the degree of accessabilty of a given area of a mine. I was unaware that they could so easily be crushed or broken. Will take extra care on my next trip to Cwmorthin which is due in May.
I would be interested to know if there are any other threads on the forum which mentions rust shells.

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Been racking my memory and as far as I can recall, shells tend not to form on cast iron (e.g. tram wheels), mainly (only?) on wrought iron. Differentiating steel from wrought iron underground can often be problematic but as far as I can recall, shells are uncomon - or possibly absent - on steel. Bearing in mind that the replacement of wrought iron rails by steel rails commenced in earnest in the early 1860s and was largely completed in the 1880s, a proportion of the rails encountered underground will be steel so a close examination of rails of more-or-less known date in relatively untrodden portions of mines might provide useful confirmation whether shells are restricted to / more common on wrought iron.

If I remember rightly, the upper workings of Cwmorthin, eg 4th floor, are the older part of the mine. Is it likely then, that iron work used in this part of the mine would probably consist mostly, if not exclusively of wrought iron and therefore promote the growth of rust shells?

I also don't remember having seen rust shells growing on the metal wheels of slate trolleys in Cwmorthin.

All very interesting!

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Hmm, a complex subject no doubt. The composition of the original iron probably plays a part in so much as the ability of the iron to dissolve in the first place is going to be a function of original composition and water composition, particularly the acidity (pH) and availability of oxygen.

Having said that these largely appear to be depositional features, effectively the ferruginous equivalent of a stalactite. Once iron gets into solution its ability to stay in solution is largely a function of oxygen availability. Increase the oxygen in the water and it will dump out as iron oxide very quickly - most of you will have noted this at a spring issuing from the ground or groundwater entering a mine through a joint or fracture. In the case of rust shells it's probably more of a local phenomenon; moisture on the iron causing local Fe dissolution and then the iron reprecipitating as iron oxide on the external or more oxygenated outer part of the water droplet or drip ?

I've seen a lot of these 'growths' on iron objects in Cornwall, simple ones are very common. In some places there's probably a seasonal variation in conditions going on which will cause some sort of textural variation or banding no doubt.

Some food for thought hopefully.

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I don't think the type of iron has much influence on formation, I'm pretty sure I've seen them on cast iron in the past and when wrought iron corrodes, what you normally get is a "wood-grain" effect caused by the slag inclusions. Imageo is most likely right with his suggested formation process, as there must be something causing the oxide to come into and out of solution to form these.

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the hand pump in Moel Fferna is a wash with them.
[photo]Personal-Album-6117-Image-60877[/photo]

on pipes and rails to,i have some more shots if you need them .

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