In all colour works operating on a large scale, special apparatus is used, in which are carried out the washing, pressing and drying of the pigments obtained as precipitates. Only pigments prepared in small quantities are filtered through filter paper. The treatment of enamel white and white lead requires the use of apparatus for this purpose in a special degree. We insert a short description here.

The preparation of enamel white takes place most conveniently in tubs provided with a stirring apparatus. When the precipitate of barium sulphate has once settled to the bottom, it is very difficult to again mix it up with water by means of a hand stirrer, an operation which must be often performed in washing. If vessels be used provided with a suitable mechanical stirrer, the precipitates are rapidly and thoroughly washed. We have already stated that vessels with a stirrer capable of being raised out of the liquid were specially suitable for washing white lead. Such an arrangement can with advantage be used in washing all precipitates. Many of the mineral pigments have to be freed from admixed salts by washing. A description follows of an effective washing apparatus with movable stirrer. In the cylindrical vessel (Fig. 21), which may be of any size, is a vertical iron shaft rotating upon a pin in the bottom of the vessel. On this axis is a horizontal wooden crosspiece, the under surface of which is studded with brushes; the disc, which this crosspiece carries, is united by means of two bars with a second, in which is cut a screw moving on the screwed shaft. The shaft is rotated by means of the cog-wheels shown in the illustration. The handle fastened to the upper disc enables the crosspiece carrying the brushes to be raised or lowered. When the handle is held fast, this crosspiece rises or falls according to the direction of rotation of the axis. The pipe shown at the side supplies the water for washing the precipitates.

In the preparation of colours liquids have often to be brought into the precipitating vessel which would attack the iron of the stirring apparatus, so that it is advisable to make the connecting rods between the two discs of such a length that the screws may be above the vessel. All iron parts of the apparatus dipping into the liquid should be protected by asphaltum varnish.

When a specially heavy precipitate is to be washed, such as enamel white, chrome yellow or white lead, the stirrer is raised as high as possible before the commencement of the operation. When the precipitate has formed and settled, the liquid is run off, the water tap opened, and the stirrer slowly brought down to the precipitate; the brushes fastened to the crosspiece stir up first the top of the precipitate, then the next portions, and so on until the whole of the precipitate has been stirred up into the liquid. When this has been accomplished, the stirrer is kept going for some time, so that the water may take up as much of the soluble materials as possible; it is then raised out of the vessel, in which the precipitate again settles.

As a rule, two or three washings of permanent white, in an apparatus of the construction described, are sufficient to render it quite free from acid. Washing must be continued until the wash water leaves blue litmus paper quite unchanged. When dry, precipitated barium sulphate is a very soft powder of great whiteness, which, on account of the fineness of its particles, can be readily ground with binding materials.

Enamel white loses, in a remarkable manner when completely dry, a great portion of its covering power and of its valuable property of being easily ground with oil or size to a homogeneous paste. It is not known whether this alteration is caused by a molecular change of the barium sulphate, as is not altogether improbable. In order to preserve the valuable properties of enamel white it is not, as a rule, completely dried, but is brought into the market in the form of pulp, which is obtained by bringing the washed precipitate into strong linen bags and allowing the water to drop through. This object is more quickly accomplished when the last wash water is drawn off only to such a point that, when the precipitate is again mixed up by means of the stirrer, a thin paste is formed. This paste is run into a centrifugal machine, of which Fig. 22 is an illustration. In the drum, B, provided with an outflow, R, the smaller removable drum, C, with perforated walls, is caused to rotate by gearing, G, to which it is attached by the screw, V. The drum, C, is lined by a tight linen bag.

When the drum is in rapid motion, the thick liquid in the washing vessel is run in. In consequence of the centrifugal force due to the rapid rotation, the whole mass is at once thrown on the sides of the drum, the liquid penetrates the fine openings, is caught in the outer vessel, and runs away to a receiver, in which it is kept until the finest particles of the precipitate, which will penetrate even the closest fabric, have settled. The operation is continued until the excess of water has been separated, when the bag containing the precipitate is lifted out of the drum. Centrifugalised enamel white is a fairly stiff, white paste, which should be packed in sacks lined with oiled paper to prevent drying.

Filter Presses.

In recent years the use of filter presses for separating liquids from precipitates and for washing the latter has become general. A filter press consists, as shown in Fig. 23, of a number of frames, between which are perforated plates and sheets of filter cloth, and which can be pressed tight together by a screw. A powerful pump forces the liquid containing the precipitate into the hollow spaces of the frames (known as chambers), in which the solid body remains, whilst the liquid goes through the filter cloth. By afterwards pumping fresh water through, the solid remaining in the chambers is soon completely washed.