11Blue was first manufactured at Alexandria, and afterwards by Vestorius at Puzzuoli. The method of making it, and the nature of the ingredients, merit our attention. Sand is ground with flowers of sulphur, till the mixture is as fine as flour, to which coarse filings of Cyprian copper are added, so as to make a paste when moistened with water; this is rolled into balls with the hand, and dried. The balls are then put into an earthen vessel, and that is placed in a furnace. Thus the copper and sand heating together by the intensity of the fire, impart to each other their different qualities, and thereby acquire their blue colour.
2Burnt yellow, which is much used in stuccos, is thus made. A lump of good yellow earth is heated red hot; it is then quenched in vinegar, by which it acquires a purple colour.