I finally found the bottle in the back of a shelf!!
Pfaltzgraf stoneware and crystal cleaner.
Can't recall where it was purchased.
Made specifically to remove the gray marks from dishes (someone once told me these are actually a chemical reaction).
Corning Ware may sell something similar.
I would by Barkeepers Friend and try that on there. It is in a gold colored can and is a powder. I bought a slow cooker that had a white removable crock and it specifically told me to use that to clean any marks from the dish. It is not abrasive so it shouldn't harm your dishes.
How do I remove ceramic marring, black-gray marks, on light-colored surfaces, usually plates?
Try rubbing with Glue Be Gone on a soft paper towel. Glue Be Gone is used to remove sticky residue from glass and plastic. But I have also used it to remove expiration dates and other data on the bottom of glass or plastic containers. It only costs $1.00 at dollar stores, so I buy several bottles at a time to ensure it's available when I need it.
Add your voice to the conversation. Click here to answer this question.