This object was found on a beach in Cornwall about ten years ago. It was lying on the surface, below the tide line, so I am assuming it was washed in with the tide. It is 13.5 cm wide, 8.0 cm deep and quite uniformly 1.0 cm thick. It is very light for its size and utterly inflexible. When I found it the colour was a much brighter pink but it has been sat in my windowsill for ten years.
One edge of it appears to be broken off a much larger piece, and the "straight edge" has a very machined feel to it.
I have two guesses, both of which I can find fault with.
1. From the even curvature it looks to be part of the skull of something, a whale of some type perhaps. But the colour makes me suspect not since I don't think bone is normally coloured like this.
2. Part of a shell, a conch or clam shell or something like that. The thickness, however, makes me wonder how long it would take for a shell to grow that thick, and do shells grow that thick in British waters?
The thickness seems too perfect, I think it is a piece of pottery.
The colour, the size, the thickness, the rigidity and uniformity of this suggests to me it is part of a broken fishermans' bouy - the sort of thing that fishermen use to mark a line of lobster pots. It seems to have bleached over the years you have had it - you say it was brighter pink ten years ago - Maybe orange even? I know some of these bouys do loose their bright colour after years of exposure outside - sunlight does nasty things to some rubbers and plastics.
The broken edge also suggests a machined edge. Maybe this is the join of the two halves of the sphere which make up these floats/bouys.
I am always picking up interesting bits of fishing gear like this - its almost as interesing as the natural stuff we find washed up.
Thank you both for your interesting new perspectives. I had not considered pottery, at all. It just doesn't feel like pottery and is very light for its size. A few of the folks I have shown it to have scoffed at the idea of it being natural but none of them have suggested pottery, either. The profile inside where it is broken also doesn't look like I would expect pottery to look, but there are many types of pottery and a hand thrown pot will look very dfferent to machine mass produced platewear so I am not discounting this. Thank you for suggesting it.
Funnily enough I had considered plastic. I couldn't help thinking it looked like a bit out of a motorcycle crash helmet, but the colour, and the cross section (what it looks like inside) eventually made me reject that idea. It was, incidentally, a very bright pink, sea shell pink, not marker buoy orange, but that fact alone does not competely discount Andy's very helpful suggestion.
I think I'll take it along to my local museum. They won't be able to identify it, but they might be able to tell me it's not pottery. After that, I might try the local fish dock and see if one of the fishermen recognises it, but fishermen are known for tall tales so they'll probably suggest the Kraken's soup bowl or something equally as bizarre.
Thank you both, very much.
I think the bouy idea is a good one, if not a fishermans bouy perhaps a float from an old toilet cistern. Anyway Plastics melt and pottery/shell does not. If you don't mind a bit of destructive testing then heat a needle red hot (hold it in pliers) and push it into the object. If it is plastic then it will melt it and go in and if it resits it it is not plastic. I also tend to use the very old fashion 'tap it on your front teeth and see what it feels like'; this is obviously extremely dangerous if you don't know if it is poisonous, tapping it on a drinking glass can tell you a simillar thing. Pottery will make a different sound to plastic, even hard plastic.
I was doing some soldering when you're email came in so I touched it with the soldering iron and it melts. So it isn't the British Record biggest clam shell ever, nor is it the braincase of a prehistoric monster dolphin. I did some maths, and assuming that curve is part of a circle it has a diameter of 18 cm, so that would have to be one enormous toilet cistern, but a float or buoy of some sort now seems the most likely answer.
Although I am grateful for your help there is also a sense in which I wish I hadn't asked. I quite liked having a piece of a giant shell on my windowsill but a nondescript bit of plastic junk doesn't quite have the same cache. I guess I'll just have to go and do a bit more browsing along the foreshore and find another bit of flotsam to ponder over.