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pigments, while it may be used for all colours that require grinding and levigating.
The mill requires about 2 horse-power to drive it; its output will vary according to the character of the material which is being dealt with.
DRYING OF PIGMENTS.
After a colour has been prepared for use as a pigment by the process of levigation, as just described, or by that of precipitation, described below, and also by other processes, it is in a wet condition, probably containing from 25 to 50 per cent. of water, according to its nature. If required in what is known as the pulp state, in which condition it is used by paper-makers and stainers, no further treatment is necessary; but, if required to be used in the preparation of paint, it is absolutely necessary that it be dried, otherwise it will not mix with the oil used in the manufacture of the paint.
The drying of pigments is carried on in what are called “ drying-stoves;" these are usually nothing more than brick chambers with solid walls on three sides, and a door on the other, covered with a roof; round the bottom of three sides runs a horizontal flue belonging to a furnace which can be fed from the outside. The wet colours are usually placed in shallow, flat, earthenware pans, which are placed in piles one above another, and then left in until they are dry. This is by no means a satisfactory method, the piling of the pans, one above another, and the absence of any system of ventilation beyond accidental cracks in the door and walls, tend to keep the atmosphere of the stove saturated with steam, and to check the drying operation.
A better plan is shown in Fig. 31; it consists of a brick chamber built of any convenient size; as before, the flue, F, of a furnace runs round the bottom; the sides of the flue are built of brick, the top of flagstone, and the fireplace, E, is placed outside the chamber. Instead of such a flue, steam pipes may be used for heating it. Above the flue or steam pipes, is a staging, s, forming a false floor, on which is erected a framework, C, C, C, O, of iron or wood forming skeleton shelves on which the pans of wet colour are placed. These shelves support the pans a small distance apart from one another, and so allow free egress for the water-vapour which comes from the colour. A constant current of warm air, generated by a fan or air propeller, is continually flowing over the pans of colour and out through the ventilator, V, in the roof of the stove, thereby carrying off the water vapour
as fast as it is given off from the wet colour. It should be borne in mind that the colour, just as it comes from the filters or
Fig. 31.-Drying stove for pigments. presses, may contain from 25 to 50 per cent. of water; if, by any means, this water is prevented from escaping from the colour,
then the drying is retarded; or if it is prevented from readily escaping from the stove, it is liable to condense on the inside of the roof, and to fall down in drops on to the colour below. In some cases, e.g.,. chrome-yellows, these drops are apt to produce spots on, and discolouration of, the pigment which is being dried. The mc. e freely the water vapour can escape into the atmosphere the less chance there is of such mishaps occurring. D is a door for filling the stove, and G, G skylights.
In dealing with barytes and china clay, special forms of drying stoves have been described.
Some pigments, like the two just mentioned, the oxide reds, burnt umbers, burnt siennas, ultramarinė, Guignet's green, are
Fig. 32.-Drying stove for pigments. capable of standing a high temperature without being altered in shade; these may be dried in a stove heated to a high temperature, in which case the drying is done quickly. On the other hand, certain colours, such as the chromes, Prussian blue, emerald green, &c., must be dried slowly; for such colours the stove shown in Fig. 32 would be very useful. The two sides not represented in the drawing are of brick, and support the roof.
Stretching from side to side are a number of iron shelves just far enough apart to take an earthenware pan and leave a little space between it and the shelf above. These shelves do not stretch completely from back to front, but, as shown in the drawing, they are arranged to come alternately flush with the front and back, the side of the shelf nearest the front and back . ' each shelf being turned up to form a flange. The front and back of the stove are made of a number of iron plates, which form a series of doors to the shelves, the top of the plates being bent over to catch on the flange of the shelf above, as shown in the drawing; it is not necessary that the doors should fit air-tight. A fan at the top of the stove creates a current of air through it, a chamber at the bottom is kept hot by steam pipes, or flue from a furnuce; through this chamber passes all the air that is allowed co go into the stove; this hot air passing over and under the colours dries them, and, being hot, absorbs and carries away the water vapour liberated from the wet colours. This stove is effective and economical, and is so constructed that the pans of colour can be readily removed and the shelves quickly refilled.
A drying stove has been constructed in the following manner. A cylindrical vessel was constructed of iron plates of any convenient size. This was divided into three chambers by two perforated iron plates; in the central chamber, which is the largest, is placed the material to be dried; the bottom chamber is kept hot by means of steam pipes, and is provided with an opening to admit air. The upper chamber is fitted with an exhaust fan, so arranged as to draw the air out of the central chamber; the perforations in the plate dividing the central from the top chamber are larger than those in the plate dividing the bottom from the central chamber, the consequence being that the air is drawn away from the central chamber faster than it enters from the bottom hot air chamber, so that a partial vacuum is created in the central chamber which is beneficial to effective drying.
Messrs. W. J. Fraser & Co., Engineers, of 98 Commercial Road, London, E., make a drying machine (nominally for white lead, but which may be used for other pigments) which is shown in Fig. 33.
The construction of the machine will be seen from the drawing. It consists essentially of a strong iron box, rather narrow in proportion to its length and height, this box being supported on girders. In this casing is a number of rollers opposite one another at each end of the casing. Between each pair of rollers is arranged an endless steel travelling-band
carried by chains; in the drawing a portion of one side has been cut away, so as to show these travelling-bands. The
number of these bands can be varied; the larger the number the greater will be the drying capacity of the machine; although an increased number of bands will not materially increase the