Aluminum powders have been made in a series of stamp or hammer mills to which is fed metallic aluminum in the form of small pieces of sheet or foil. Sometimes the mills are charged with aluminum splatter, formed by directing a coarse spray of molten aluminum against a cool surface. Along with the metal, a lubricant usually stearic acid is also introduced. The function of the lubricant is to keep the particles separated and prevent the welding together again during the hammering operation. The stamp mills are completely enclosed in order to confine the fine powder which is formed. The first mills in the series are heavy ones, which serve to break-up the charge. As the particles become smaller, lighter stamp mills are used until the desired particle size has been obtained. The discharge from the last mill is screened and the oversize, coarse particles are returned to the mills for further reduction. This process is being displaced by the safer and more efficient ball milling process described below.
The screened and classified powder are finally polished in large drums containing bristle brushed mounted on the axis of rotation. The inner surfaces of the drums are irregular or ribbed and the ends of the bristle brushed contact the inner wall of the drum. The powder is charged into the drum, additional lubricant is added and the particles are polished by the action of the brushes. The lubricant coat the surfaces, which are gradually smoothed and prevent their welding. It is usually necessary to age the polished powder for several weeks to produced pigments which will leaf properly. Other more recent methods of manufacture are represented by the Hall paste process. Finely divided metal is fed into ball mills as sufficient mineral spirits to form a paste. When the pigment has been excess mineral spirits removed to leave a paste containing from 60 to 73.5 percent by weight of metal. The Hametag process is another modification of the older ball milling process in which the aluminum metal, in a finely divided condition, is charged with a lubricant into the end of a ball mill through which an inert gas circulated. The metal particles are reduced in size and carried over with the flow of gas to a separator which remove over size particles for return to the mill. Particles of suitable fineness go to the polisher through which they pass continuously and are discharged as finished pigment.