Thoroughly clean the pieces by tumbling, heat them and plunge while hot into the following liquid: Ten pounds hydrochloric acid and sufficient sheet zinc to make a saturated solution. In making this solution, when the evolution of gas has ceased, add muriatic, or, preferably, sulphate of ammonia, 1 pound, and let it stand still dissolved. The castings should be so hot that when dipped in this solution and instantly removed, they will immediately dry, leaving the surface crystallized. Next plunge them while hot, but perfectly dry, in a bath of melted zinc, previously skimming the oxide on the surface away, and throwing thereon a small amount of powdered sal-ammoniac.
Another method, given more in detail, and, therefore, more useful, is as follows:
The castings are first to be cleaned and scoured by immersion in a bath of water acidulated with sulphuric acid. The strength of acid required for this purpose will depend somewhat on the nature of the casting and the amount of scale to be removed. It is sufficient for us to say, however, in this connection, that the surface of the casting must be made perfectly clean in order to insure satisfactory results in the subsequent processes. They may be scoured with sand and water or scraped with a tool in order to make sure that every portion of the surface has become clean. After the parts have thus been cleaned they are thrown into cold water in order to prevent oxidation. Pure zinc, covered with a thick layer of sal-ammoniac, is then melted in a bath and the iron is dipped in the preparation. In removing the pieces they are raised slowly to allow of draining, and are then thrown into cold water. The latter is done in order to keep the surface bright. Where the parts are very large it is necessary to heat them somewhat before plunging them into the molten bath of zinc, as otherwise the presence of so large a body of cold metal would chill the zinc and prevent the formation of a satisfactory coating. The object of the coat of sal-ammoniac above the melted zinc is to prevent the waste of zinc and also to act as a flux to the pieces passing through it into the metal. In some cases the sal-ammoniac is mixed with earthy matter or sand, in order to lessen the volatilization of the sal-ammoniac, which becomes quite fluid. In order to give a crystalline appearance to the surface coated by the zinc, sometimes a light coating of tin is given to the article.