I made a cioppino yesterday, using some little clams that I'd bought it the store. I can't recall the name, but it was something I hadn't heard of before. Not littlenecks, not cherrystone. Anyhow, having read about cleaning grit/sand out of clams in advance, I put them all in a bowl of cold water with a tablespoon or so of cornmeal. Into the fridge for about 2 hrs before cooking. Then rinsed off and threw in for the last 3-4 minutes of cooking.
At least 1/2 had lots of grit/sand in them. Like enough that if we collected it all we could've built a miniature sand castle. It was terribly unpleasant and sort of ruined the dish.
The grit, interestingly, wasn't in the shells, but in the bodies of the clams themselves.
So - what should I do next time?