Vitreous is either something made of glass or pertaining to glass. In addition it is loosely associated with things that are deemed glasslike by having the external properties of glass. E.g. hardness, high gloss and/or resistance to chemical attack.
All pottery that has been fired in a kiln may be said to be vitrified, and hard-glazed porcelain and porcelain enamel are vitreous substances.
