According to one expert, with a background in chemistry, "The definition of porcelain and its soft-paste and hard-paste varieties is fraught with misconceptions", and various categories based on the analysis of the ingredients have been proposed instead. Some writers have proposed a "catch-all" category of "hybrid" porcelain, to include bone china and various "variant" bodies made at various times. This includes describing as "hybrid soft-paste porcelain" pieces made using kaolin but apparently not fired at a sufficiently high temperature to become true hard-paste, as with some 18th-century English and Italian pieces.