Metal bath tubs (made from stamped steel or cast iron) have a hard baked, kiln fired enamel finish. This is a traditional and popular type of soaker tub that is durable and features basic designs. This fabrication is also used for many bathroom sinks. The enamel can accidentally chip which is repairable. Enamel tubs wear out when the top surface of the enamel is worn away from use and cleaning. Once the top surface is worn, the more porous substrate is revealed which stains and breaks down more readily. The tubs generally wear out on the bottom surface first. Sometimes metal tubs get referred to as porcelain. This is a bit of a misnomer as porcelain is a ceramic material, much like china dishes. No tubs are actually made of this material, but some sinks are and most toilets bowls and tanks are.