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The sulphur is the ordinary roll sulphur or brimstone. The coal and charcoal are the ordinary commercial varieties; but the coal used must be free from pyrites. Both articles are ground before using. The quartz should be as free from impurities as possible; the better the quality of the quartz, the better the quality of the ultramarine made from it. The infusorial earth or kieselguhr is the well-known commercial article. The rosin used is the best commercial variety obtainable. The manner in which these are mixed together depends upon the variety of ultramarine to be made, and it also varies in different works, each of which has its own formula, although there is not much variation in the essential points. Gentele lays down the following rules:–1st, That the soda used be sufficient to neutralise half the silica present in the kaolin or clay and silica used; 2nd, that the proportions of soda and sulphur le such as to produce a polysulphide of soda. ULTRAMARINE MANUFACTURE.-There are two processes in use for the manufacture of ultramarine; the oldest, called the indirect process, is used for making both sulphide and soda ultramarines, and is the only process by which the former can be made. Indirect Process of Making Ultramarine.—This consists of the two stages or operations, viz.:
(a) The calcining operation.
(a) Calcining Operation—Manufacture of Ultramarine Green.—A mixture of the ingredients named above is made ; if sulphate ultramarine is required, sulphate of soda is used ; if soda ultramarine is to be made, then soda carbonate is used. Some works use a mixture of the two soda salts.
The various ingredients are ground together with water into a very fine paste; the finer the grinding, the better will be the quality of the ultramarine ; after the grinding, the paste is dried. In some works the water is omitted, it being considered unnecessary, while the subsequent drying adds to the expense of making.
The following are examples of the mixings used in different works:—
For sulphate ultramarine.
Kaolin, - -
Coal, . - -
2. 3. Parts. Parts. 100 100 41 220 17 --41
When sodium sulphate is used less sulphur is required; in proportion as the latter is decreased so the proportion of the
former must be increased.
Kaolin, - -
For soda ultramarine rich in silica—
Quartz, . - -
Coal, - - - - -
The second recipe gives a dark ultramarine; the more sulphur there is used in making soda ultramarines, the deeper is the
shade of the blue produced;
mixture is packed rather tightly into these; sometimes what are called seggars are used, but the crucible form is better, as giving a firmer pile when placed in the furnace. In some works open, flat, round capsules are used of such a size that they hold when # full, about 9 ozs. of the material, which forms a layer 14 to 1} inches thick; these are piled one above the other in a furnace capable of holding about 216 arranged in 9 layers of 24 capsules, each formed of lots of 6 by 4. The furnace in which these pots of material are placed varies in form in different works. Fig. 20 shows one form of ultramarine furnace. The furnace chamber, B, is an almost exact
cube in form; the back is completely closed in, and the front, C, is open, but is made up with firebricks when the furnace has been filled with the crucibles. The fireplace, A, is under the furnace chamber, the flames and heat in it pass through openings, e e e, in the floor of the chamber; similar openings, ff, in the roof, D, of the chamber serve as outlets for the waste heat and gases of the furnace into the flue, E E. A number of these furnaces are built side by side and back to back, forming a range or bench of furnaces, but are not all worked together, for while some are being filled, others are being emptied, and others, again, are being heated. In some works a kind of muffle furnace, similar to Fig. 21, is used. After the furnace has been charged with the crucibles, the front is made up with bricks, and the interstices between these filled with a mixture of sand and clay, a small sight-hole being left so that the temperature of the furnace can be observed; if necessary, this sight-hole is stopped with an easily removable plug of clay. The temperature of the furnace is then slowly raised to a bright red heat, at which it is maintained for from 7 to 10 hours, the time varying with the nature of the composition and determinable only by actual practice. Sulphate ultramarine requires a higher temperature than soda ultramarine; if a muffle furnace is used, the temperature is often raised to a bright yellow for from 2% to 3 hours only. When the calcination is considered to be complete the fire is drawn and the furnace allowed to cool; this must be done as slowly as possible, and care must be taken that no air enters into the furnace during the cooling, because while hot the crude ultramarine is very susceptible to the action of the oxygen of the air, and the yield as well as shade of the colour would be injured. When cold the crucibles are removed and the furnace is ready for another charge. This first burning of the ultramarine is a most important operation, and great care must be exercised in carrying it out; access of air to the contents of the crucible must be carefully avoided; the temperature should not be too high nor too prolonged, as then the material would be overburnt and will not give a satisfactory blue; on the other hand, underburning is just as bad, for then the colour will not be homogeneous. A furnace such as that shown can be charged three times per week. The colour of the burnt mass varies somewhat; usually it is of a green colour, mostly of a bluish tone (which is generally indicative of good burning), but sometimes it is of a yellowishgreen shade, and at others it passes more into a blue, while, if not properly burnt, it will have a brown shade. The crude green ultramarine, which is somewhat cindery in appearance, is now thrown into water for the purpose of washing out all the soluble soda salts; the last washings of one batch are often used as the first wash-waters of another batch for the purpose of economising the water. While still wet the ultramarine is ground up in mills into as fine a form as possible, in order to effect the completest practicable extraction of the soluble matter. The ground-up green ultramarine is then dried, when it is ready for the next operation. In this form it is sold under the name of green ultramarine for use as a pigment. The wash-waters contain a large proportion of sodium salts, chiefly in the form of sodium sulphide. In many works it is customary to evaporate the liquors to dryness by means of the waste heat of the furnaces, and to use the dry residue for another mixing. (b) Colouring Operation—Manufacture of Ultramarine Blue.—The green ultramarine obtained in the first stage has now to be converted into the blue, which is done by heating it with sulphur in a furnace at a low temperature. There are three ways of carrying out this colouring operation. (1) On trays, (2) in a cylinder, and (3) in a muffle. 1. Tray Method.—A form of muffle furnace is built in which the muffle is filled with a number of trays or shelves. On these trays the green ultramarine is spread in layers of about an inch thick, and over them is sprinkled some sulphur; the muffle door is closed, the furnace is lighted, and the heat continued until the sulphur takes fire; then the fires are drawn and the sulphur allowed to burn itself out, after which the crude pigment is taken from the muffle and finished in the manner described further on. 2. Cylinder Method.—It is also known as the German method. Small cast-iron cylindrical vessels are imbedded in brickwork over an ordinary fireplace; these cylinders are closed at the back end, but open in front, which is fitted with a door made of wrought iron; in this door are two apertures for the purpose of charging the cylinder with sulphur, while a pipe from the top of the cylinder carries off the gases produced by the burning of the sulphur. An agitator is fitted to the cylinder, by means of which its contents can be kept well mixed during the progress of the operation. From 27 to 34 lbs. of the ground green ultramarine is charged into the furnace, the door closed and the fire lighted. When the temperature is sufficient to ignite sulphur, 1 lb. of sulphur is thrown into the cylinder; when this has burned away and fumes have ceased to issue from the cylinder, another pound of sulphur is thrown in and allowed to burn; a small sample is now drawn from the furnace and its colour noted ; if not blue enough, more sulphur is thrown in at intervals until a sample taken out of the cylinder shows that the blue has properly formed ; after the cylinder has cooled down the pigment is scraped into a box, and is ready for the finishing operation.