The processes ordinarily used for covering metals with zinc, tin and lead have not, up to the present, appeared to be applicable to aluminum. When a plate of aluminum, mechanically or chemically cleaned, is immersed in melted tin, zinc or lead, these metals slide over the surface of the aluminum without alloying therewith. In order to fix the above-named metals, it suffices to submit the surface of the aluminum to a vigorous brushing in the metallic bath. For this purpose a steel brush or any other analogous instrument may be used. Under such circumstances the aluminum becomes covered with a regular layer of the melted metal. The success of the operation was due, it appears, not to the want of affinity of the aluminum for the metals in question, but to the immediate formation in contact with the air, of a thin stratum of oxide of aluminum, which friction removes.
(2) To coat aluminum with other metals: Dip the aluminum in a solution of caustic potash or soda, or of hydro-chloric acid, until bubbles of gas make their appearance on its surface, whereupon it is dipped into a solution of corrosive sublimate to amalgamate its surface. After a second dipping into the potash solution until bubbles of gas are evolved, the metal is placed in a solution of a salt of the desired metal -for instance, bluestone for copper, and lunar caustic for silver. A film of the metal is rapidly formed, and is so firmly adherent that, in the case of silver, gold or copper, the plate can be rolled out or polished. When coating with gold or copper, it is best to first apply a layer of silver. When thus treated the aluminum may be soldered with ordinary zinc solder.