Vitreous enamels, glass linings, and porcelain enamels are all essentially glass coatings of suitable coeffi cient of expansion fused on metals. Glass in powdered form (as glass frits) is applied to a pickled or otherwise prepared metal surface, then heated in a furnace at a temperature that softens the glass and allows it to bond to the metal. Several coats may be applied. Vitreous enamel coatings are used mostly on steel, but some coatings are also possible on copper, brass, and aluminum.