Someday somebody with access to a mass spectrometer may tell us what that is. I suspect silica with some impurities, considering how resistant is to acid.2nd. Even with the sanigloss over the years and in the country I do have a yellowish ring in the bowl where the water sits
and I'm carefull on using something too abrasive to remove the gloss...... Any suggestion?????
AFAIK, nothing short of Pumie Scouring Stick can make a dent in it, and even that doesn't take it right off. I was less vigorous than I could have been. It is a definite violation of the no-abrasives thing. Could be worth considering if you are otherwise considering replacing the toilet. In theory I could replace the bowl on my Vespin, but that is a lot of work.