Bronzing Iron.

Practical Magazine 23, 1876

(Chemistry applied to the Arts, Manufactures, &c.)

The old processes, which are still used almost exclusively, are founded on the formation of an artificial layer of rust, and necessitate a long manipulation. A bronzed piece of machinery lately submitted to the Military Technical Committee at Vienna showed an appearance quite different from that generally observed on bronzed articles. Mr. HESS states that it was black, with a graphitic gloss on the surface, which was very hard, so as to bear the wire-brush. By analysis it was found that this coating was formed of ferroso-ferric oxide. The formation of this compound may take place in different ways. The piece may be painted with linseed oil, and then subjected to the action of heat, and thus a skin of magnetic oxide and carbon is produced. With articles which will not bear heating the coat is obtained by using Thirault's method, which consists in depositing on the iron a uniform coating of hydrated ferric oxide, and transforming this coating into ferroso-ferric oxide by dipping it in water heated to from 80° to 100°. It results from the researches of Mr. Hess that the bronzing is still more rapidly produced by putting the article for some time into an acidulated solution of ferrous chloride, by which a black pellicle of magnetic oxide is produced, which is made to adhere firmly to the surface of the piece by steeping it in hot water. After drying the piece may be painted with linseed oil or wax.

In the latter case, the magnetic coating is formed in consequence of the reductive action upon the ferric oxide and its salts:
4Fe₂O₃ + Fe = 3Fe₃O₄

The advantages of these new bronzing processes are, greater despatch and a better adhesion of the coating as compared with the old methods. Experiments on a small scale have shown that under certain unfavourable circumstances the iron bronzed by the new process may take to rust slightly, but this could be removed easily with the wire-brush, without injuring the bronzing. Never was there any rust found under the bronze coating which would cause it to scale off, as it frequently happens with the old process.

Mr. Hess further recommends the use of sulphide of copper for giving a very fine, brilliant, blue-black coating. The articles, having been well cleaned, are suspended for a few minutes in a solution of sulphate of copper, so as to cover them with a film of copper. The pieces are then washed in water and immersed for a few minutes in a solution of hyposulphite of soda slightly acidulated with hydrochloric acid. A coating of copper is thus produced which completely resists the action of air and water. The article is at once washed in water, dried with a cloth or filtering paper, and polished with a burnishing stick. The coating is beautifully steel-blue, adheres closely to the iron, stands the scratching-brush, and is proof against rust.

- Deutsche Industrie Zeitung.