To Obtain Different Shades of Chrome-Yellow.

Manufacturer and builder 2, 1869

A great variety of shades, from yellow to orange and even red, may be obtained from the combination of chromic acid and oxide of lead, according to the conditions under which the combination takes place. The main conditions are, that when the two solutions, containing the chrome-salt and lead-salt respectively, are acid, a yellow is obtained; when the solutions are neutral, the precipitate is orange, and when the solutions are basic - which means when they contain an excess of potash and oxide of lead respectively - the precipitate will be red.

Pale Sulphur-Yellow.

When the lead is dissolved in a n excess of acetic acid, or nitric acid, so as to obtain an acid acetate, or nitrate of lead, and such a solution is mixed with a solution of acid bichromate of potassa, a light-colored yellow is obtained. For a pale sulphur-yellow, the best proportions are: Sixty-five pounds of lead-sugar, or eighty-eight pounds of nitrate of lead, dissolved in hot water, and clarified by settling. This solution is then diluted with pure water to such an extent that for every pound of the lead-salt at least fifty gallons of water are added.  Then thirteen pounds of bichromate of potassa are dissolved in boiling water, and diluted with about forty gallons of water, and two pounds of common sulphuric acid are added under agitation. This acid combines with the potash and seta  the chromic acid free; and when, now, this last mixture is slowly poured into the lead solution, under strong agitation, it becomes very opaque, and of a very light yellow color. This soon subsides as a heavy precipitate, which, after washing three or four times with fresh water, is placed on a filter-rack and dried. This precipitate is, strictly speaking, a mixture of chromate of lead and sulphate of lead.

Dark Lemon-Yellow.

When, in the above-described operation, the sulphuric acid is used n smaller quantities, and the bichromate of potash in larger, the precipitate will not have the lemon or sulphur-yellow color, but is tinged darker. For every pound less of sulphuric acid that is taken, the thirteen pounds of potash is increased with three pounds, and for every increase of the latter, the shade will be darker.

To obtain a dark lemon-yellow, twenty-five pounds of bichromate of potash is dissolved, as above mentioned, and only four pounds of sulphuric acid added. In this way the precipitate contains only half the quantity of sulphate of lead, as is, as a yellow, preferred as well for its fine color as its better body. It covers better than the pale sulphur-yellow, but is not so well adapted to make, when mixed with Prussian blue, pure shades of green, as is the case with the pale yellow.

By diminishing the sulphuric acid still more, the shade obtained is darker, and when left out entirely it is slightly orange; principally when the lead solution used is only slightly acid. As soon as these are neutral, or even alkaline, not yellow but orange is obtained.

It must be observed that none of the shades mentioned, when once obtained, can be brought from a darker to a paler one; but that, by means of an alkali, the reverse may be done, and a pale yellow may be transformed into a darker yellow, or even orange. The wors color, avoided by manufacturers, is a dirty leather color, which can not be changed into a yellow, but only into a dark orange.

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