Madder versus Cochineal

The testing of the madder colours is so important that I introduce here a few additional experiments selected from my own note-books. The washes of the moist-colour paints were, as far as possible, of the same depth of tone, and they were all exposed together in a glazed frame to one year's sunshine:

In contrast to the above results with madder carmine, the following experiment with the ordinary carmine (prepared from cochineal) is instructive. On a sheet of Whatman paper, a space of 10 inches in length by 4 inches in width was covered with a uniform wash of the moist paint, having a depth of tint about equal to that of the petals of the old China rose. This coloured strip was then subjected to summer sunshine in such a way that successive single inches of its length received the light (during the same hours of similarly bright days) for periods of 2, 4, 8, 12, 20, 26, 30, 40, and 100 hours, one single inch at one end being, however, protected completely from all access of light. The exposure of 100 hours sufficed to bleach the last breadth completely, but had the rate of fading been in a simple arithmetical progression, a much shorter exposure would have sufficed. In fact, the bleaching action was far more energetic during the first period of two hours than during the second, about 20 percent of the original colour having been destroyed during these two first hours, while during the second equal period the loss of depth did not exceed one-tenth of this amount. Moreover, it was noticed that the change of hue consequent upon the first exposure was different in kind to that which occurred subsequently.