I have a favorite cup made in China that has developed black spots inside the bottom. I'm worried about lead -- should I stop using it? It isn't crazing but seems to be a failed glaze. I bought 6 of these from Cracker Barrel restaurant a year ago as part of a set with serving dishes signed by Susan Winget -- the Nature Walk series for Cracker Barrel Old Country Store. The add picture function isn't working but I can send pictures to anyone who's interested.