For this reason, if the color grinder has not any special object in view, he can, instead of carrying a stock of extended chrome greens, minimize it by purchasing chemically pure green of such shades as he needs and mix such portions by weight of these and fine, floated barytes with the oil and grind it, thus always being fairly certain of a uniform product so long as the chemically pure green is up to standard, which he can determine in the usual way, testing for color, tone and tinting strength. The constituents of chrome green being Prussian blue and chrome yellow, the mills on which they are to be ground should be run at moderate speed and soft or esopus mill stones are best for grinding these greens, the diameter of the mill should be commensurate to the quantity of the batch or to suit the demand. Twenty-four-inch mills are probably best, but thirty-inch mills are not too large if run at a speed of not over thirty-six or forty revolutions per minute. When a batch of chrome green is being mixed, it should be so arranged that the mixing of oil and pigment is complete before machinery is stopped for a time, because imperfect mixings have been known to take fire when dry green was lying on some linseed oil. As to the percentage of oil required for mixing and grinding these greens for the trade, we will give the following figures that are subject to some variations, according to the gravity of the chemically pure green and the nature of the barytes. Chemically pure green that contains more lead sulphate than others, will require less oil for grinding and the deeper the shade of green, the greater the oil absorption. Assuming that a commercial chrome green is wanted, that is to consist of 25 per cent pure color in the pigment and is to be of a medium shade, it will require twenty-one pounds chemically pure chrome green, medium, dry; sixty-three pounds floated barytes and sixteen pounds raw linseed oil to produce one hundred pounds paste. To turn out one hundred pounds of paste of a 20 per cent green of similar shade, will require seventeen pounds chemically pure chrome green, medium, dry; sixty-eight pounds floated barytes and fifteen pounds raw linseed oil, but the former should turn out a trifle stouter than the latter. There are chrome greens in oil on the market that contain only 10 per cent pure color in the pigment and some even less. A medium shade of this type would consist of about nine pounds chemically pure chrome green, medium dry; eighty-one pounds barytes and ten pounds of oil, not always necessarily pure linseed oil. A reputable color grinder will hardly ever place his name on the label of goods of this type.
The proportions of pigment and oil here given, as above noted, are for greens of medium shade, but for greens of extra deep shade these percentages will differ to quite an extent. For instance, a 25 per cent green of extra deep shade would be made up of twenty pounds dry pigment, sixty pounds barytes and twenty pounds oil for one hundred pounds paste, while a 10 per cent green of that shade would require eight and three-fourths pounds dry pigment, seventy-eight and one-fourth pounds barytes and thirteen pounds oil for one hundred pounds paste. Chrome greens of the latter type were extensively sold to wagon and implement manufacturers, dry and in paste form. Since the consolidation of the agricultural implement and wagon making interests, very little green of that composition is ever sold any longer in paste form to those trades, as they purchase their requirements in the color line in the dry form and do their own grinding. Chemically pure chrome greens are manufactured by color makers in two forms, the nitrate of lead greens and the acetate of lead (sugar of lead) greens. The last named are usually stronger in staining power and of yellowish undertone, while the former, which are higher in price and believed to be more permanent, are of the blueish type. These seem to be rapidly going out of the market and are used only for special brands in oil on account of the cost. The difference is in the chrome yellow used for the mixing with the Prussian blue, one being made with nitrate of lead yellow, the other with acetate of lead yellow, that made with the latter being lowest in cost of production. To grind chemically pure green in oil in paste form requires on the average for the medium shade seventy pounds dry color and thirty pounds linseed oil, for light shades seventy-two pounds dry color and twenty-eight pounds oil, for deep shade sixty-eight pounds color and thirty-two pounds oil, and for extra deep shade sixty-four to sixty-six pounds color and thirty-four to thirty-six pounds oil to form a paste of good consistency, that will break up readily on thinning for use. But as stated above, the demand for green of this character by the general trade is limited and not many jobbers or dealers will carry it in stock. However, the color grinder who makes a full line of paints in liquid form, also will find the chemically pure greens in paste form of decided advantage for many purposes where the heavy gravity of barytes is objectionable, as he can use such paste green as a base for stains or in composite greens. We shall speak of this in detail under the caption of paint making later on.