The ground products, however, are not entirely homogeneous, always containing, in addition to the very finest particles, those of a coarser nature which must be removed by sifting. Sifting machines are essentially sieves through which the colour is passed. The sieves are made of wire gauze or bolting-cloth, stretched on prismatic frames which are rotated (centrifugal sieves), or superposed on the flat and reciprocated. In centrifugal sieves, the material is projected against the sieve, and the whole apparatus is in a state of vibration, or else beaters are provided to keep the fine orifices in the sieve from choking up. Nowadays there are numerous types of sifting devices, none of which, however, can be considered as the best for all purposes, since each type of earth colour behaves differently and requires special treatment. The proportion of moisture in the material, also, has an important influence on the method of treatment required. [Illustration: FIG. 21.] A typical flat sifting-machine, with eccentric jig motion, is illustrated in Fig. 21. The machine is fed through a hopper provided with feed rollers, the rate of feed being adjustable. The screened product is discharged through a shoot at one side of the machine, and the residue at the opposite side, into boxes, etc., placed underneath. For materials that give off a large amount of dust, the machine can be enclosed in a dust-proof casing, in which event the product and residue are delivered into drawers. The machine is easily cleaned and the sieves quickly changed, and is well adapted for dealing with a succession of different materials. The hopper can be fitted with a pair of adjustable crushing rollers. [Illustration: FIG. 22.] Fig. 22 is a drum sifter, which is fed by means of a hopper and worm; and the drum can be covered with wire or silk gauze. The sifted product falls into a worm conveyor in the bottom of the casing and is discharged at the side. This may be replaced by a series of mouths for discharging direct into bags, or the machine can be adapted to deliver into an elevator, worm conveyor or other means of transport to a distance. The screenings are discharged through a shoot at the back of the machine, and can be handled in various ways. A beater is provided to clear the drum and increase the output. [Illustration: FIG. 23.] Fig. 23 illustrates a centrifugal sifting-machine for producing very fine powder in large quantities without any escape of dust. It contains a screening drum, the frames of which are detachable and facilitate changing the sieves. A beater revolving inside the drum projects the powder against the sieves, such portions as pass through being taken up and discharged by a worm conveyor; this, however, can be replaced by a bagging device, etc. CALCINING