When the white lead is removed by manual labour, an operation which ought to be forbidden, the lead sheets are unrolled and struck together, whereupon the greater part of the white lead falls off. To remove the remainder of the white lead the plates are laid one upon the other and struck with a hammer until the white lead is loosened; or the plates are cleaned with metal brushes. The masses of white lead obtained in this way are contaminated by larger or smaller quantities of metallic lead, from which they must be freed by a further mechanical operation, namely grinding. The larger pieces of the white lead, which have a thickness of several millimetres, were picked out and sold separately under the name of flake white. This was formerly a highly prized quality of white lead, its appearance being a guarantee of its purity. The flake white generally found in the market nowadays is not obtained in this manner, but by mixing white lead with a solution of dextrin, forming plates from the paste and drying them slowly in the air. In order to separate the white lead mechanically from the remains of the lead sheets, grooved rollers are used, between which the sheets are passed. In order to guard against dust, the rollers are surrounded by a closed casing, in which there is also a sieve which serves to separate the larger pieces of white lead (flake white) from the fine dust. The arrangement is represented in Fig. 5. The unrolled plates pass through the opening, B, on to an endless leather band, by means of which they are carried between the grooved rollers, D and E; after they have passed through these they go between a second pair of rollers, F and G, which are placed nearer together; they then fall into the drum-shaped sieve, H, out of which they leave the apparatus in the direction of K. The white lead falls through the sieve, is caught in trucks placed at J, and carried away to the mills. [Illustration: FIG. 5.] A mechanical arrangement for the separation of white lead from the unaltered metallic lead, due to Horn, is represented in Figs. 6 and 7. It consists of a drum, in which is the spindle, _b_, provided with the arms, _c_. The teeth, _e_, on the arms reach nearly to the lower curve of the drum, but pass at a somewhat greater distance from the upper portion so that the pieces of lead can fall down again after they are carried up. The material is fed in through the hopper, _t_, the rotation of the arms loosens the white lead from the pieces of metallic lead, and it is carried on by the water which passes through the sieve, 3, and the stop cock, _h_, into the settling tank. The lead pieces which are carried forward have their progress checked by the bridge, _o_, they are collected by the perforated scoop, _n_, are raised above the hopper, _m_, and fall out of the apparatus at _r_. If it is required to continue the treatment of the material in the drum for a longer period it is only necessary to close the hopper, _m_, by the slide, _n_.