Wikipedia
Vitreous china
Vitreous china is an enamel coating that is applied to ceramics, particularly porcelain, after they've been fired, though the name can also refer to the finished piece as a whole. The coating makes the porcelain tougher, denser, and shinier, and it is a common choice for things like toilets and sink basins.
Usage examples of "vitreous china".
He turned the lid over to reveal a blue plastic package, sealed finmly air- and watertight, taped to the underside of the vitreous china.