Cleanse the surface of the iron perfectly by the joint action of dilute acid and friction, plunge it into a bath or melted zinc covered with sal-ammoniac, and stir it about till it be alloyed superficially with this metal. When the metal thus prepared is exposed to humidity, the zinc oxidizes slowly by a galvanic action, and protects the iron within from rust; whereby the outer surface remains for a long time perfectly white, in circumstances under which iron tinned in the usual way would be corroded with rust.