When zinc oxides are to be ground in oil in paste form for the trade it is essential for good results that the dry zinc be stored in a dry place, because green or moist zinc oxide will neither mix well with oil nor grind smoothly, and when packed into containers in such condition does not keep or open up well when about to be used. Not only is it liable to open up with a crust on top, but also with lumps throughout the package. But when stored for too long a time dry zinc oxide is apt to become more and more transparent, especially in open containers where the air has free access to the pigment. In such case the zinc oxide, or at least a portion of it, undergoes a chemical change, turning into zinc carbonate, and it is well known to experts that carbonate of zinc is transparent and lacks the opacity or hiding power of zinc oxide. In large establishments, where large quantities of zinc whites are ground in oil for the trade, the best apparatus for the purpose is composed of a chaser of good power, with a bed pan eight feet in diameter and a 5,000-pound roller with well-adjusted scraper and gate opening into a mixer, while the mixer discharges zinc and oil onto the stone rollers of a suitable roller mill, that must also be carefully adjusted to give the paste a good finish and appearance, no matter how stiffly the material may be mixed. When ground on a stone mill the resulting product is either too soft or becomes too ropey on cooling. The advantage of the apparatus referred to will be seen when it is considered that the chaser exercises a condensing action on the pigment, thus requiring a lesser percentage of oil to form the paste than would be necessary in an ordinary mixer and stone mill. The material exhibits greater opacity and, as a great portion of zinc in oil is wanted for interior flat work by decorators, or at least was in use for such purposes before the advent of the so-called modern flat wall finishes, the zinc so ground was more easily flatted without drawing the oil. To illustrate why this is possible we will say that when a package containing 300 pounds of dry zinc oxide is emptied into a chaser of the type referred to and the crushing wheel run over the pigment for, say, thirty minutes, and the zinc returned to the original package, the container will be little more than half filled with the 300 pounds taken from the chaser. In other words, the zinc has been condensed to a little more than one-half of its original bulk.
It is not necessary, however, nor would it be beneficial, to go to such extremes when grinding zinc in oil, because when done properly a portion of such density is imparted during the manipulation. The dry zinc is placed in the chaser with sufficient of the oil to lubricate the material while being mixed, and the weight of the crushing wheel gives enough of the condensing action to make it possible to get along with a smaller percentage of oil than is required in the ordinary process of grinding through a stone mill. Here an average of 18 per cent. of oil to 82 per cent of pigment is required to have the paste go through the mill, while with chaser and roller mill the average will run 15 per cent of oil to 85 per cent pigment, all other features being equal. Furthermore, the daily production of the latter apparatus is at least double, and at times even threefold, as compared with that of a thirty-six-inch diameter stone mill. Of course, when the zinc oxide in oil is to serve as part of the base for ready-mixed paint it can be mixed in a rather soft form, and, in that case, run through a rather open stone mill and the output increased to a very large extent. Zinc white for the trade is usually ground in refined or bleached linseed oil and is in very little demand at present in poppyseed oil. Nor is raw linseed oil used for mixing with zinc for the trade unless so specified. Great care is necessary to have the oil as free as possible from moisture, as it tends to harden it in the package. Nor must zinc oxide be covered with water to keep the top of the material from forming a skin, because it acts as a hardener. When containers are not airtight, as is the case with the so-called wire rim or wire top pails or kettles, a good method to keep zinc contained in them from forming a skin or crust on top is to cover the material over with disks of strong parchment paper of the diameter of pail before adjusting the lid. When zinc oxide is ground in oil for stock and stored in bulk in large containers, the top of the paste should be covered with a little of the oil, in which it has been ground, which can be removed and used again in future grindings, when the paste is to be canned or otherwise disposed of.