The methods of doing this most economically depends upon the arrangement of the building in which the ready-mixed paints are made and canned or packed for the trade. Naturally, the best arrangement is to have a building three or four stories high, mixing the pigment with the oil on the top floor, in apparatus discharging directly into the hoppers of the mills on the floor below and chuting the ground base into liquid paint mixers on a floor below the milling or grinding room in order to do away with handling the material several times, which entails loss of time and loss by waste. This method is, however, not always feasible, unless in a factory where large quantities of standard paints are being made on certain formulas. Where paints are being manufactured on special formulas of diversified nature it is more convenient to have the various pigment bases ground and stored in convenient tanks separately, so that the required quantity from each can be drawn and placed into the liquid paint mixer and compounded with whatever other bases constitute the formula. Thus, one tank would contain lead carbonate in soft paste form, another basic lead sulphate, another French zinc, while others would contain American zinc, zinc lead, lithopone, whiting or some kind of inert material that would be mixed for either pure white or tinted with the required colors in oil and thinned to the consistency suitable for application with the brush or for dipping purposes. In other paint factories again, the bases are conveyed in suitable trucks to the mixers, but no matter how disposed of, they are always ground rather soft, so as to mix readily and, in the case of liquid paints, it is immaterial what percentage of oil is being used in grinding the base, because it can be readily adjusted in the final manipulation so long as the exact proportion of pigment and oil in the base is known. When it comes to the bases for dipping paints or flat finishes it is different and more care is required, as an excess of oil will give trouble because in one case it will retard drying, in the other it will produce flashing. In grinding these bases for ready-mixed paints the mills best adapted are those with esopus stones, because the pigments used are not refractory and buhr stones do not take a good hold on soft material, as they polish too readily. There is a great advantage when such mills are provided with pans around the running stone, which keeps the soft material from slinging all over the frame and floor, thus preventing waste and untidiness about the mill room.
We might say right here that in the grinding of white lead and white paints generally the men operating mixers and mills should be closely supervised in order to observe strict attention to having their own persons, as well as the apparatus as clean as possible, allowing no accumulation of waste, and before shutting down for the night or holidays to scrape down chutes and sides of mixers, scrapers and rims of mills, so that crust may not form over night or during shutdown and find its way into the product when starting up again.