This is a color generally used in the glazing of earthenware, glass, porcelain, and enamels. It is one of the most ancient as well as the most beautiful of colors and has rather a scarlet tinge. Chemically it is composed of oxyds of tin, oxyd of gold, and water ; and according to slight variations in the amount of either metal various shades of color can be obtained. Various methods have been proposed for its preparation, the best process for obtaining,it in a pure state being to take 310 grains of fine gold dissolved in 1550 grains of aqua regia, consisting of one part of commercial nitric, and four parts of commercial hydrochloric acid ; the solution is evaporated to dryness in a water bath, the residue dissolved in water, filtered and diluted with 20 or 30 ounces of water and placed in contact with granulated tin, the purple precipitate being the desired compound. When freshly precipitated it dissolves in ammonia, but by exposure to the light the solution gradually decomposes, becoming gradually blue and then colorless, but when fused with a glaze on porcelain it is a most durable color. The richness of its tints is evidently due to the presence of the gold, which, causing it to be very expensive, has often been endeavored to be replaced by another metal; and often in experimenting, much richer hues have been observed during certain stages in the oxydation of copper, these however are only evanescent, the color quickly changing to the dead black of cupreous oxyd. We believe that some French chemists are now engaged in searching after a good scarlet or purple for porcelain, from copper, and we hope that they may be successful, as it will greatly aid the progress of the art of decorating the works of the potter's wheel.
This article was originally published with the title "The Purple of Cassius" in Scientific American 13, 14, 107 (December 1857)