Zinc, when cast into plates or ingots, is a brittle metal, easily broken by blows from a hammer. In this state it is evidently somewhat porous, as its specific gravity is only 6.8, while that of rolled zinc rises as high as 7.2. Zinc, when heated to 212° Fah., or over, becomes malleable and ductile, and when rolled into sheets it becomes exceedingly tough and does not regain its brittle character on cooling. Hence, sheet zinc has come into very extensive use in the arts.
Zinc becomes exceedingly brittle when heated to nearly its melting point. To reduce it to powder, therefore, the best plan is to pour melted zinc into a dry and warm cast-iron mortar, and as soon as it shows signs of solidifying pound it with the pestle. In this way it may be reduced to a very fine powder.
Professor Bottger prepares a black coating for zinc by dissolving two parts nitrate of copper and three parts crystallized chloride of copper in sixty-four parts of water, and adding eight parts of nitric acid. This, however, is quite expensive; and in some places the copper salts are very difficult to obtain. On this account Puscher preimres black paint or varnish with the following simple ingredients: Equal parts of chlorate of potash and blue vitriol are dissolved in thirty-six times as much warm water, and the solution left to cool. If the sulphate of copper used contains iron, it is precipitated as a hydrated oxide, and can be removed by decantation or filtration. The zinc castings are then immersed for a few seconds in the solution until quite black, rinsed off with water, and dried. Even before it is dry, the black coating adheres to the object so that it may be wiped dry with a cloth. A more economical method, since a much smaller quantity of the salt solution is required, is to apply it repeatedly with a sponge. If copper-colored spots appear during the operation, the solution is applied to them a second time, and after a while they turn black. As soon as the object becomes equally black all over, it is washed with water and dried. On rubbing, the coating acquires a glittering appearance like indigo, which disappears on applying a few drops of linseed-oil varnish or " wax milk," and the zinc has then a deep black color and gloss.
By means of the following simple process, recommended by Bottger, a brilliant coating of zinc may be deposited on brass or copper: Boil a large excess of so-called zinc-dust, some time, with a concentrated solution of caustic soda, or potash, and place the copper or brass articles to be coated in the boiling liquid. By continuing the heating, after a few minutes a beautiful mirror-like film of zinc will form upon them by the deposition of the alkaline solution, in consequence of their electro-negative character in combination with the zinc. It is suggested that the process is applicable to the preparation of disks for dry-piles, and also for forming a layer of tombac, by heating a copper article thus coated, carefully, to about 248 to 284 deg. (best under olive-oil), when the will unite with the copper support to form a gold-tinted tombac, and the article need only be quickly cooled in water, or some other suitable liquid, as soon as the desired color is apparent.
A chemical process for covering zinc with colored coatings has lately been described by Dr. L. Stille. The articles of zinc are first brightened by scouring with quartz sand, moistened with dilute muriatic acid, putting them quickly in water, and then carefully wiping them dry with white blotting-paper. To insure success, however, it is necessary to employ zinc as free as possible from lead, and to have it as bright as a mirror. When these conditions are fulfilled, the metal may be coated with a variety of beautiful colors by immersion in a solution of alkaline tartrate of copper for a shorter or longer interval of time, depending on the color that is desired.
Clean the surface carefully with fine sand or emery and sulphuric acid, and immerse for an instant in a solution of sulphate of nickel and ammonia, - 4 parts in 40 parts of water acidulated with 1 part of sulphuric acid. Wash and dry. When burnished, this takes a fine bronze color.