Manufacturer and builder 5, 1869
In an article on chrome-orange, in one of our previous numbers, a chemical and a practical method is spoken of for finding the value of chromate of lead. This value depends on the degree of its purity as regards admixtures of diverse white paints, with which almost all chrome colors of the trade are more or less adulterated. The best test is founded on the fact that yellow mixed with blue forms green and white with blue gives not green, but only a blue of a lighter shade of color. Now, when a pure chrome-yellow is prepared from some acid lead salt and bichromate of potassa, in the manner explained on page 52, a shade is obtained that may be kept as a standard of the pure color. The same also, mixed, respectively, with ten, twenty, thirty, forty, sixty, and eighty per cent of sulphate of baryta, gives six adulterated standards of comparison. The paint to be investigated and one of these standards are mixed respectively with an equal quantity of blue. In this way the amount of adulteration is tolerably well ascertained. Suppose, for instance, a light lemon-yellow is to be tested, to find out if it is a pure light chromate of lead prepared from acid solutions, or a darker made from neutral solutions, and afterward mixed with white to give it the ligher shade of color, a trick often practiced by manufacturers so that it may be sold for a pure light chrome-yellow. All that is necessary to be done is to take, say, ten grains of the paint and mix it well with three grains of Prussian blue. This gives a definite shade of green. Now ten grains of one of the standard paints, the one containing, say, twenty per cent of sulphate of baryta, is also mixed with three grains of Prussian blue. If, now, the same shade of green is obtained as in the first instance, the paint in question contains same amount of adulteration as the standard used. If adulterated to a greater degree, it will be green of a much bluer shade. If, however, the green obtained is more yellow than that obtained from the standard the chrome-yellow is better than the standard mixture it is compared with. In the first case, we may add grain after grain of the paint to be tested, until we obtain a shade of green the same as that of the standard color. If twenty grains are required to produce the same green shade of color, we conclude that the paint contains only hald the amount of chromate of lead contained in the standard. The advantage of this method is that slight differences in the shades of yellow are without influence. But when very dark shades are to be tested, the standards should also be made of a similar dark shade of yellow. For the orange and reddish chromes, this method can not be used to the same extent and with the same advantages, since they do not form good greens. However, to a certain degree an estimate may be made which becomes more accurate when a test is made as regards the covering quality; a standard orange paint, prepared in accordance with the instructions given on page 80, being used in making the comparison.