Paris Green, or emerald green, as it is sometimes designated in oil color lists for the general trade, is an aceto-arsenite of copper, consisting of about 32 per cent of copper oxide, 58 per cent of arsenious oxide, 7 per cent of acetic anhydride, 2 per cent of sulphur trioxide and 1 per cent of water. It is a very fine type of blue-ish green that cannot be imitated with what we know as chrome green, although the latter has made the former almost obsolete in the general line of painting, because of its decidedly poisonous properties. Paris green in oil is still employed to a small extent as a finishing coat over light shades of chrome green, because of its richer effect. The color grinder can test the color or shade and the tinting power of Paris green in the usual way, and a quick test for purity can be had by dissolving a small portion of the pigment in a test tube with strong ammonia. It should dissolve with a deep blue color and show no precipitate. Pure Paris green, being of a rather heavy specific gravity, will not require over eighteen pounds of linseed oil to eighty-two pounds dry pigment to produce 100 pounds of marketable paste. On account of the very poisonous character of the dust arising in mixing the dry powder with the oil, the operator should be cautioned to be very careful in doing the work. The grinding is best done in steel mills, because stone mills are apt to become overheated and will in this case destroy the richness of the green, making it dull and milky. If steel mills are not part of the equipment then the pigment may be ground on mills, the grinding stones of which are freshly dressed.
Verdigris in oil is also to be found in commercial oil color lists, and it is best for the color grinder to select the material known as French distilled verdigris, which is made by dissolving oxide of copper in acetic acid, the pigment consisting of about 43 per cent of copper oxide, 29 per cent of acetic anhydride and 28 per cent of water, when chemically pure. It is a rather transparent deep, greenish blue pigment, rather fugitive in water, but more permanent in oil, if dried well before mixing and grinding.
There is not much demand for it by painters, but it is used in large quantities by the United States Light House Service for the prevention of marine growth, as the action of the salt water on paint made from this pigment gradually dissolves the cuprous salt, so that animal life or marine vegetation cannot retain a hold upon the surface thus painted. Pure verdigris of the type referred to will require twenty-eight to thirty pounds of raw or refined linseed oil to from seventy to seventy-two pounds of the dry pigment to make 100 pounds of paste color in oil. It requires very careful grinding, as it must be impalpably fine to give satisfaction to the user, especially if the latter be a decorator, who requires it for a glaze. The testing of the dry pigment is very important, as the government service does not allow over 3 per cent of insoluble matter, which must be considered rather liberal. To test for insoluble matter, weigh out a certain quantity of the pigment, treat it with dilute hydrochloric acid until a solution is effected and if there is any insoluble matter, collect it on a filter, dry and weigh it. Should there be any effervescence noted when treated with acid, it indicates the presence of carbonates of copper or calcium or both. If the color grinder has any cause for suspecting verdigris of being sophisticated, it is necessary to submit a specimen to an expert paint chemist. The writer has had an experience where a lot of verdigris during the grinding in oil became black and after standing awhile, turned into a solid black mass, similar to cinder. The oil was pure, but the pigment contained free acetic acid and sulphur. The same precautions on mixing verdigris are necessary, as those referred to on Paris green. Neither of these two colors should be mixed with pigments that contain sulphur, owing to the formation of the black sulphide of copper. Such pigments are ultramarine blue or green, cadmium yellow, lithopone, etc., and any apparatus for mixing or milling must be thoroughly cleaned when any of the sulphur compounds are followed by the greens in question.