Freshwater clams are filter feeders, which means they’ve been snacking on lake dirt and grit. That’s charming, but we don’t want to eat it. You need to give them a good, long bath—like a spa day, but with less cucumber water.
Drop your clams in a big bowl of cold water. Add a hefty pinch of salt or cornmeal (about a tablespoon per quart). The cornmeal is like a laxative for clams—it makes them spit out all the sand. Let them soak for at least 20 minutes, up to an hour. Change the water once or twice if it looks gross (spoiler: it will).
While they’re soaking, give each clam a scrub with a stiff brush. Think of it as giving them a little back rub. If any clams are wide open and don’t close when you tap them, toss 'em. They’re not sleepy—they’re dead. No dead clams allowed at this party.
