An essay on the art of painting on glass. From the German by Emmanuel Otto Fromberg

Author:Emmanuel Otto Fromberg
Language: eng
Format: epub, pdf

1 atom of oxide of zinc 50332

1 „ „ acid 501-16

1 „ „ sulphate of zinc 1004-48

The quantities of the sulphate of zinc and the sulphate of iron, each of which contains 1 atom of oxide, are in the proportion of 2 to 5.

When the oxides are precipitated by means of a fixed alkali any excess of the precipitating substance must be avoided, to prevent the oxide of zinc from being dissolved again. Just so much, however, is added as will produce a complete precipitation. The product is filtered, washed, and dried. Lastly, it is submitted to a red heat, in order to drive off the water which it contains. If the solutions of zinc and iron, after they have been poured together, are treated with an alkaline carbonate, the precipitate must in this case be heated to drive off the carbonic acid. The calcining is intended moreover to give stability to the combination of the two oxides. If it is required to colour a fused pigment by chemical combination or simple mixture, the object must be to obtain the colouring oxide in a state of the finest possible division. By this means, in the former case the combination takes place more easily, and in the latter the mixture is more intimate. Now calcining is always opposed to the direction we have given, for oxides condense by being calcined. This is also the case with the kind of pigments we have just been speaking of. On this account it answers to use the means which we recommended for obtaining the flesh-red; for they not only admit of compliance with the conditions which require calcining, but of the production of the colouring matter in a state of the finest division. We will now speak of the mixture of sulphate of potash.

The precipitate from the two oxides contained in the carbonates of zinc and iron is mixed with a concentrated solution of this salt; the liquor is evaporated, and the precipitate, when dry, is calcined at a dull red heat. It is then softened with water, placed in a filter which separates the zincate of iron from the solution of sulphate of potash: this precipitate is carefully washed and then dried.

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