I recently picked up an unmarked 5qt dutch oven with a weird coating on the inside of the lid. It's impervious to heat (Weber gas grill cranked on high, 2 hours at 900+ degrees on an IR thermometer). 48 hours in an electro tank does nothing. 2 days in a lye bath (easy-off method) didn't even make the rag turn black. Wire brush on a 10000 rpm angle grinder seemed to wear the brush down but did nothing to the coating. 100 grit silicon carbide sandpaper just makes the finish dull. Cracks in the middle were there when I got it, so I want the stuff off since I have no idea what it might do if it leaches into food. I've never seen an enameled lid with a coating that wasn't glossy or with a coating on the inside but not the outside which makes me think the coating wasn't a factory job. Any idea what this stuff is?