The lid on that little round box was a super-tight fit; it got stuck while I was trimming it, and I couldn't get it off while the clay was unfired without risking breaking it. I poked a tiny pin-hole in the lid to allow air to escape as it cooked, but apparently it was too small because the lid popped off in the kiln, taking a little chip out of that one spot but not damaging anything else. I'll glaze it eventually, and use it for things that don't require a perfect air-tight seal. :)