Egyptian Blue

A beautiful and permanent blue pigment, generally known as Egyptian blue, has been studied by many chemists. It is found on objects of Egyptian origin from the time of the fourth Dynasty onwards, and was in use during the time of the Roman Empire. Specimens have been found in Pompeii and other Roman sites; also in Britain, at Wroxeter, in Syria, and in Crete. Our knowledge of this pigment has become more exact in consequence of the researches¹ of Professor A. P. Laurie, who has re-determined its chemical composition as well as its physical properties, and has, moreover, ascertained the precise conditions under which it is produced. In one of Professor Laurie's trials he took 180 parts of fine sand, 48 copper carbonate, 36 calcium carbonate, and 20 parts of 'fusion mixture.' A few grams were submitted to definite temperatures in an electric furnace for some hours, the mass being cooled, re-ground, and re-heated. Finally it was found that the optimum temperature for the production of the crystalline blue was somewhere about 830° - 850° C. The crystals, which are transparent and dichroic, and have a density of 3, consist of a double silicate of calcium and copper, represented by the formula CuO,CaO,4SiO2, but generally containing about 2 percent of potash and soda, as substituents for a part of the copper and calcium oxides; the actual percentage of copper oxide present in the pure blue is therefore usually somewhat lower than the theoretical.

It should be added that the interest attached to the subject of Egyptian blue is rather academic than practical, for this pigment is not now made except on a small laboratory scale, and has not found for many-centuries a place on the palette of the artist.

1	Proceedings of the Royal Society, A. (1914), vol. 89, pp. 418-429.