EMERALD GREEN

Basic copper arsenate, the most poisonous and dangerous of all pigments, therefore to conceal this fact it appears under various fantastic names. The color is luminous by itself, bluish or yellowish green, highly permanent and would be very useful except that it is incompatible with sulphur colors such as cadmium yellow, vermilion and ultramarine. It is also doubtful with white lead. On mural painting of the Romanesque period light copper greens have stood up well through time, but was most likely natural mineral greens as emerald green was not known in those days. Malachite was its predecessor being basic copper carbonate, but as an oil color, emerald green requires only small amounts of oil: no more than 30%, and dries well. With sulphur colors it turns black. When verdigris and other blue and green copper colors were used by the old masters because of a lack of other pigments, they were well aware of the dangerous incompatibility and used coats of varnish between opposing layers to prevent blackening. All copper colors are easily recognizable as they turn ammonia a deep blue color. Emerald turns black when heated and smells of garlic. Potassium hydroxide discolors emerald to an ochre color, and in weak sulfuric acid it dissolves, turning the solution blue. The copper colors of the old masters look under the microscope like coarse glass splinters as compared with modern colors which have a mud-like character.