a pale greenish-blue colour; 2nd, soda ultramarine, which has a violet-blue colour. Of the latter there are two varieties—one contains more silica than the other, and is mostly used by paper-makers, owing to its resisting the action of acids and alum better, while the variety poor in silica is used for all other purposes.

The materials used are nearly the same for both kinds, and comprise kaolin or china clay, sodium sulphate (Na, S O4), sodium carbonate (Na, CO), sulphur, coal or charcoal, rosin, quartz, and infusorial earth. All these are not used in the same operation; some makers using one kind of mixture, others another. The quality of the materials is a matter of very great importance.

The kaolin or china clay should be as free as possible from any earthy matrix; a trace of lime has no injurious influence, but the clay must be free from iron, which has a tendency to dull the colour of the ultramarine. It has been found from experience that every sample of china clay does not give equally good results, although all may be pure and of good quality. It has been found that the relative proportions in which the silica, Si O2, are combined with the alumina, Al, O„, is a matter of some importance, and in china clays from different localities there is wide differences in this respect; then, again, a china clay which will work well for sulphate ultramarine will not do for soda ultramarine. For making sulphate ultramarine the china clay should contain the silica and alumina in the proportion of 2 silica to 1 alumina, 2 Si O2, Al, O; if the proportions much exceed these the shade will be poor, while if they reach those indicated by the formula, 3 Si O2, Al2O3, the clay will not make sulphate ultramarine. On the other hand, while almost all clays will make soda ultramarine, yet the best results are obtained with clays containing from 2 to 3 parts of silica to 1 part of alumina; the larger the proportion of silica, the redder the shade of the ultramarine made from it, and the more resisting power it has to the action of acids and alum. The china clay is prepared for use by a process of grinding and levigating, so as to obtain it in as fine a form and as free from impurity as possible.

The sodium salts are used in the anhydrous state; both should be as pure as possible, especially should they be free from iron, which has a most deleterious influence upon the shade of the ultramarine made from the salts. Although other impurities are of small moment, still, where first-class ultramarine is required, it is best to purify the commercial products.

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 be 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.

(b) The colouring 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.

*13

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.

For soda ultramarine poor in silica

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;

on the other hand, by reducing the quantity of sulphur and silica the blue obtained is not so deep, but is rather more brilliant in hué.

The mixture is then placed in crucibles, about 6 inches by 4 inches in size, and fitted with lids, which are somewhat saucershaped, so that the crucibles can be piled one above another in a furnace. Fig. 19 shows the shape of the crucible and its lid. The

Fig. 19.

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 1 to 12 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, ƒƒ, 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 ultra