Just a selection of surprisingly well preserved limpet shells, probably Patella vulgata Linnaeus, from an archaeological excavation of a medieval UK east coast town. They are about 900 years old and were found in a food rubbish pit dug between the 12th and 13th centuries. The shell sculpturing survives but the surfaces have lost their sheen and colour. These are now matt and dusty due to partial denaturing of the shell – the organic lattice of the shell structure has disappeared, leaving just the mineral components.
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Jessica's Nature Blog 
Oh my! Medieval shells! So these were eaten?
I wonder what effect the other foods in the rubbish pit had on the preservation/deterioration of these shells.
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The assumption is that the limpets were eaten but they may have been used as fishing bait. As the hard parts – the calcium carbonate crystal matrix – of the limpet shells were well preserved, it is likely that the other contents of the food rubbish pit, and the surrounding soil itself, did not create an acid burial environment. Acid conditions (particularly wet ones) tend to preserve soft organic matter (like vegetable matter and leather) but dissolve the calcium-based hard parts of bones and shells. Shells and bones may survive acid conditions but, when exposed to the air, they are soft, lose their shape, dry out and distort, and decompose readily.
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