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The character of the filter cloth should be adjusted whenever possible to that of the pigment to be filtered ; if of open texture it will filter rapidly, but for fine precipitates, such as blanc fixe, Prussian blue, chrome.yellows, &c., it is not suitable; it can be used for coarse precipitates, like oxide of iron, Derby-red, lakes, &c. Fine filter cloths filter slowly, but the filtration is thorough; such cloths must necessarily be used for the fine pigments named above.

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The modern filter press, such as shown in Fig. 38, has not yet been generally adopted in colour shops, although its use would greatly facilitate the filtration of pigments. Its cost may exclude it from small colour works, but in large works, where quantities of pigments are turned out, its use will be found to save both time and labour.

Fig. 38 shows the form of press specially built for the use of colour makers by A. L. G. Dehne, of Halle (Germany), one of the largest makers of these presses, and supplied by Messrs. Follows & Bate, Engineers, of Gorton, near Manchester. The screw at the end of the press is made rotary, so that it is easily removed out of the way while the press is being discharged or fitted up. The chambers are made on a self-emptying principle. The working pressure is 110 to 140 lbs. pressure. These presses are made with from four to as many as fifty chambers, to suit the requirements of large or small makers of colours. The machines

are also made with small or large chambers. They are made with or without an arrangement for washing the cakes of solid matter after the filtering is done; this washing arrangement is a valuable one, being useful in making pigments, such as Prussian and Chinese blues, which are very fine and light, and yet need thorough washing to preserve the purity of the tints; the chromes also retain their tints much better if washed quickly. In these cases a washing filter press is desirable; after being made the colours are passed through the press, and when ali the liquor has drained off, the washing water is run through the press, this operation being done in a very effectual manner; it not only takes less time but less water than by the old system.

It consists essentially of a number of flat chambers, formed of iron frames covered with filtering felt, placed side by side on a suitable iron frame and pressed against one another by means of a screw or hydraulic press. The material to be filtered is pumped into the chambers, the liquor flows through the felt, while the pigment or solid matter is retained within the chamber. When it is considered that enough has been sent through the filter press, it is taken apart, the cakes of pigment removed from the chambers, and sent forward to the drying stove.

The efficiency of these filter presses depends upon the workmanship given to the machine by the makers; every part must fit well, otherwise the filtration is imperfect. The quality of the filter cloths also has some influence. Generally, these presses require finer cloths than the simple filter described above, and, of course, fine precipitates will require finer filter cloths than coarse precipitates. These points can only be alluded to here in a general way; each colour maker must find out what quality of filter cloth will best suit the press he is using and the pigment he is filtering; and in a similar way, the best conditions of pumping and the pressure used must be found out.

Another method of filtering is by the vacuum filter press, one form of which is shown in Fig. 39. This consists of a hemi. spherical vessel of copper divided into two parts by a perforated plate. The top part is open; the bottom part is closed; but has two openings, one of which serves as the outlet for the liquor which runs into the lower part, while the other is connected with a vacuum pump. The mode of using is comparatively simple. The perforated plate is covered over with a sheet of filter cloth, the material to be filtered is thrown on to it and the vacuum pump, set at work; the liquor runs through, leaving the

solid matter on the top of the filter. This method of working is very simple and efficient, quicker than the first method described, but not so quick as the filter press. Other forms of these vacuum filters have been made. One form consists of a conical vessel fitted on the top of a strong iron box; between the two is a per

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forated plate or a sheet of wire gauze, on the top of which is placed a sheet of filter cloth; the material to be filtered is thrown into the cone, the air in the iron box is pumped out by a vacuum pump, the liquor is forced or rather pulled through, leaving the solid matter in the conical vessel.

GRINDING.

To fully develop the beauty of a pigment, and make it best adapted for conversion into paint, it is necessary to grind the materials very fine. Formerly this was done by hand on a slab of marble, with a muller. This is necessarily a slow and tedious process, only applicable to turning out small quantities of pigment or paint, and which is not admissible where large quantities are required, as in these days of ready-mixed paints. Grinding mills are, therefore, a necessary part of the plant of a modern colour-works.

Grinding is peculiarly a mechanical operation, it is rarely that chemical action plays any part in it; the only examples are, perhaps, oxalate of iron yellow, Naples yellow, and mercury iodide scarlet, which are affected by iron and must be ground in stone mills. These pigments are only used by artists, and in small quantities. As a rule, the manufacturer of house paints need not trouble himself as to the material his mills are made of

so long as they are efficient, although those with granite or stone grinding surfaces are to be preferred to those which have them of iron, as being least likely to affect the tints of the pigments ground in them.

The grinding mills in use are constructed on one of three principles, and, therefore, may be divided into three groups :

1st. Edge-runner mills. 2nd. Flat-stone mills. 3rd. Roller mills.

The first two kinds have been used for grinding purposes for centuries, so that nothing is known as to who invented them, or as to where they were first produced. Probably they were at first very simple in construction, and were afterwards modified so as to increase their grinding efficiency. The third group is of modern invention, having been introduced during the last forty years; but even the name of its inventor is not definitely known.

1st. Edge-runner Mills.—This form of grinding mill has been in use for centuries for grinding corn, stones, and other materials. The essential principle of these mills is that of a circular stone or runner, set edgeways, and running in a circular basin-shaped trough or hopper and bed, on which the material to be ground is placed, and the stone rolling over it crushes it to powder. Many forms of this edge-runner mill are made, and it is also made in a great variety of sizes, suitable for the various materials usually ground in such mills. In some forms the trough and bed is made to revolve, while the runners are made to turn on fixed centres or axes by the friction of the bed on their edges ; in other forms the bed is fixed, and the runners rotate on a movable central shaft. In this case the runner has two motions—one on its own axis, by friction with the bed, and another round the axis of the bed. In some mills the driving motion is applied from above, in others from below. Besides the arrangements for driving the runners, there is also a number of scrapers provided to keep the edge of the runners from becoming covered with a cake of the ground colour;

other scrapers are fitted to the bed of the mill to keep the material under the runners, and prevent it from getting into the corners, where it would not be ground. Usually, also, the central shaft is provided with springs, so that should an exceptionally hard piece of material pass under the runners the latter will tilt a little, and allow the piece to pass without doing any damage.

In Fig. 40 is shown a section of Messrs. Follows & Bate's

newest form of edge-runner mill. In this the bed is made of granite, 4 feet 8 inches in outside diameter; the runners are also made of granite, and are 30 inches in diameter and 8 inches wide. The hopper is made of hard wood, the bearings of the runners of lignum vitæ, and scrapers (not shown in the section) also of the same hard material; thus the pigment or other material being ground does not come in contact with any metal; therefore, it is not liable to be deleteriously affected. Further, the mill can be kept cleaner than is possible with a metal hopper

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and bed, especially where the mill is only occasionally used. The mill is under-driven.

The same firm make a similar mill with iron edge runners working in an iron hopper or bed. These mills are made in several sizes, having various capacities; a useful size is the one described above. Such a mill requires 2 borse-power to drive it, and is capable of grinding from 1 to 1} tons of colour per day, but the quality will necessarily vary with the character of the material which is being ground. Some natural oxides are rather hard, and 1 ton may be taken as a fair day's work; on the other hand, soft ochres may be ground at the rate of 2 tons a day. Much depends upon the degree of fineness of the

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