Fig. 8.
Hard White Lead presents a shining mass, broken with difficulty. This appearance is a guarantee against adulteration with barytes; white lead, which contains this adulterant, does not give a smooth, but an uneven earthy fracture. Hard white lead is rather difficult to grind, and requires very careful treatment to be brought into that state of fine division in which it is usable.
The dry white lead, after powdering under the edge-runners, must pass through a sieve, which retains the particles of metallic lead, before it is subjected to wet grinding. The mills used for wet grinding differ little, or not at all, from the ordinary pattern.
Fig. 9.
A mill designed by Richter of Königsee, in Thuringia, for wet grinding, is constructed as follows (Figs. [8] and [9]). The shaft, F E, is driven by a water-wheel and the necessary gearing at 60 revolutions per minute; the bevelled cog-wheel G, by means of the cog-wheels H and J, drives the shafts K and L, which, by means of the bevelled cog-wheels a and b, communicate to the runner stones of the mills, M, N and O, a speed of 140 revolutions per minute. C D is the framework for the support of the storied arrangement of mills, f and g are bearings with cast-iron cups to receive the escaping oil, i is a clamp fastening the axle d to k. A steel cup in k rests upon the hardened top of the spindle, l; the bed stone rests upon the beam n (80 millimetres thick) and is prevented from lateral movement by the surrounding oo; p is a wooden support in the opening in the bed stone, carrying the stuffing box, q, which prevents the white lead from running out, and connects the spindle, l, with the bed stone. The runner stone, r, is surrounded by the box, s, cemented with white clay inside and out and fastened to the bed stone. The lever, t, turning about w, moved by the screw, u, and handle, v, regulates the position of the runner. The diameter of the stones is 95 centimetres.
Thirty kilogrammes of white lead, mixed with water, are fed into the top mill of the series shown on the right; the mixture flows by the spouts, e, into the lower mills, and is received in vessels, from which it is emptied into the holder, P. From P it goes through the 3 mills on the left. After passing through 6 mills, white lead without barytes is ready for use. If the white lead contains an admixture of barytes, it is put through the 3 mills on the left a second time. In 24 hours 900 to 1,200 kilogrammes (18 to 24 cwt.) of white lead are ground.
In dry grinding, mills are used arranged so that the formation of dust is avoided, a matter which is of particular importance. The usual size of these mills has a diameter of 90 to 95 centimetres for both runner and bed stone. The grinding surfaces of both stones have radial grooves.
