In this case it is therefore necessary to provide means for retaining the solid matter in the drum, and allow only the water to escape, with which object the drum is lined with a bag of closely woven fabric, open at the top and fitting snugly against the inner surface of the drum. When the drum is first started, the ejected liquid is milky, no fabric being sufficiently close to retain all the extremely fine solid particles present. In a very short time, however, the liquid will begin to run away perfectly clear, this occurring as soon as the pores in the fabric have become so far obstructed by the projected solids as to allow water alone to pass through. The milky water is then returned to the feed tank and run slowly into the machine. The water is very quickly expelled, and the colour remains in the drum as a stiff paste, of sufficient consistence to be moulded into lumps of any desired shape. The use of the hydro-extractor may be particularly recommended when ample motive power is available and accelerated draining is desirable. [Illustration: FIG. 17.] Fig. 17 illustrates a modern type of hydro-extractor with bottom discharge and suspended drum, the shaft of which is coupled directly to an electro-motor. _Filter-presses._--Whereas the hydro-extractor is only used in particular cases for the purpose of the earth-colour manufacturer, the filter-press enjoys more extensive application. Every filter-press is composed of a number of closely fitting press frames, held together by the pressure of a screw. These frames, when assembled, form chambers provided with inlet and outlet openings. Suitably shaped and stitched filter-cloths are secured inside the chambers, and the sludge to be filtered is run into the press from a high-level tank. The water passes through the filter-cloths and runs off, whilst the colour earth gradually fills the chambers. When draining is completed, the press is taken apart and emptied. In this way the earths are obtained in the form of more or less dry cakes, which are then put through further treatment or dried. [Illustration: FIG. 18.] Fig. 18 shows a Dehne filter-press suitable for the earth-colour manufacturer. Wood internal fittings are often used, because wood does not affect the shade of the colours; but, wherever the nature of the materials admits, iron presses are to be preferred on account of their greater durability and the certainty of the joints continuing tight. The finer the grain of the levigated colour, the more difficult the expulsion of the water; but as a rule, a pressure of 115–195 inches, water-gauge, will be sufficient.