Need help in cleaning a ceramic (Ceran-Schott) cooktop. It's black with a smooth top. Actually, I got involved in reading while boiling water in a stainless steel pot. There was no boil over and the pot just boiled dry. Now there is an ugly black stain on the burner which I have not been able to remove. I've tried various creams, Barkeepers Friend, soap....nothing works. Would be grateful for any help.