Paint grinders have had, some time or other, a puzzling experience in the grinding of combinations of pure lead and zinc paste paint, when the specification called for certain percentages of pigment and linseed oil, especially where raw oil was required. When the two pigments are mixed with the oil and then run through stone mills very close set in order to obtain good fineness the mills become heated much more rapidly than when either pigment is ground by itself. The only sure remedy to prevent lumps through the material after cooling is to keep it agitated after leaving the mill until it cools.
A sure way to prevent this lump formation is to place in a power mixer the proper percentage by weight of white lead in oil and zinc oxide in oil, both ground to proper fineness and give the material a thorough mixing, then discharging it direct into the containers required. This will cost a trifle more for labor, but if judiciously arranged, it will pay to do it rather than take the risk of having the material rejected.
Assuming that a certain white lead paint to be furnished in paste form is to be composed of the following: Sixty-six per cent. by weight of lead carbonate, 22 per cent of zinc oxide and 12 per cent of raw linseed oil, a mixing of 73 pounds of white lead in oil and 27 pounds of zinc, which, the latter having been ground at the rate of 22 1/2 pounds oil to 77 1/2 pounds dry zinc, will make a composition as per specifications, because there is usually a leeway of 1 per cent on the oil permitted, either way, the usual proviso being, that paste must not contain less than 11 per cent. nor more than 13 per cent. by weight of well settled, pure, raw linseed oil. Whenever a chaser and roller mill is part of the equipment of a paint establishment, it is best policy to make use of it for combinations of lead and zinc, as well as for the grinding of the so-called combination whites. By long practice it has been determined that white paste paints of this combination, be they admixtures of lead or zinc oxide, sublimed lead or lithopone with barytes whiting, gypsum, clay or asbestine or any other inert white mineral are giving best results when manipulated on chaser and roller mills, instead of being forced through stone mills, where often they become overheated and when put up for the trade and kept in stores or warehouses for any length of time, prove on opening by the consumer to be badly settled and caked hard in bottom of containers. While the heavy roller in the chaser makes the material compact, its being put through the mixer and over the rollers loosens it up again without, however, taking away density, thus preventing gumming up in the package.