A black varnish for zinc is made by the dissolution equal parts of blue vitriol and chlorate of potash in 36 times their volume of warm water. The solution is then permitted to cool. Should there be iron in the blue vitriol it is precipitated as a hydrated oxide and its removal can be made by filtration. Next, plunge the zinc castings in this solution for a second or so, when they will become very black. Then rinse them off in water and dry them. This black coating will adhere to the article even before it is completely dried. The solution should be applied a second time should copper-colored spots make their appearance during the operation. If you rub it, a glittering appearance, something like indigo, will be acquired by the coating. This, however, will disappear on the application of a few drops of Linseed Oil Varnish, which is composed by boiling together 5 parts of Japanese wax and 1 part of yellow soap in 21 parts of water, until the dissolution of the soap. The zinc, after being treated with this varnish, has a deep black color and gloss.