Drying-tunnels are specially adapted where large amounts of material have to be dried. The tunnels are built of brick and provided with a rail track on which the trucks carrying a series of trays laden with colour are run. As the trucks move slowly forward, they are met by a current of hot air which dries the charge. The tunnel is kept filled with laden trucks, each fresh one introduced pushing a finished one out at the further end.
In many cases, drying troughs are also useful. These are long, semicircular, jacketed troughs of boiler plate, hot air or steam being passed through the jacket space. A worm conveyor keeps the contents moved forward, turned over and mixed to facilitate drying.
Mention may finally be made of vacuum drying-cupboards, which are heated, air-tight chambers, for the material, in which the air is partially exhausted, thus increasing the rate of evaporation of the water and causing the materials to dry quickly at a much lower temperature than otherwise.
Crushing and Sifting
The distributing and covering power of the earth colours depends—apart from their special properties—on the fineness of their particles. For this reason, all the means adopted for the purpose of pulverisation are of particular interest. The most important crushing and powdering devices have already been described, and may be referred to, all that needs mention in addition being the fact that stone mills also are used for fine grinding.
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.
Fig. 21.