green colour, and was proposed as a substitute for the arsenical greens; owing to its cost it has never come into use.

ZINC GREEN.-Under this name there is frequently sold greens made in a similar way to the Brunswick greens, by mixing together zinc chrome, Prussian blue, and barytes; such greens possess the advantage that they are not affected by sulphur as much as the Brunswick greens. They are best made in the dry way (see p. 151).

The green lakes and the pigments made from coal-tar colouringmatters will be found described in the chapter on Lakes.

Blue, as a colour, enters very largely into the decoration of objects, both alone and in combination with other colours, to form a large and very useful series of tints and shades. Although so important as a colour, yet there are few blue pigments; but these possess the merit of being more permanent, and, therefore, more useful than any other group of pigments. The list of blue pigments includes ultramarine (a curious compound of silica, alumina, and soda, which was at one time obtained exclusively from natural sources, but is now mostly prepared artificially), and Prussian blue with its varieties (a most valuable blue, whose base is iron), which are the most predominant blue pigments used. Cobalt is the base of cobalt blue and smalts, while copper forms the basis of several unimportant pigments.

ULTRAMARINE.

Ultramarine is one of the most important pigments possessed by the painter; being used in painting, in printing of all kinds (letterpress, wall-papers, calico), and in bleaching; it is undoubtedly the best blue for the laundry, and in soapmaking it is used to produce the blue mottled soap.

Ultramarine has been known for centuries, but its extended use has only been possible during the last half century. Prior to about 1820 the natural supplies were small, and the processes so expensive that it could only be used by artists who did not find the cost prohibitive; but about the year named, discoveries were made by several chemists, which resulted in ultramarine being made artificially at such a cheap rate that it is the cheapest blue pigment known; consequently, its consumption is now measured by tons.

Natural Ultramarine.-The source of natural ultramarine is a blue mineral, lapis lazuli, found in small quantities in Persia, China, Siberia, and a few other places. This mineral is found in streaks and small patches distributed through an earthy matrix

or gangue, from which it has to be separated by mechanical means. The production of natural ultramarine has declined very much during the last fifty years, it having been displaced by artificial ultramarine; but the mineral is still sought for in fair quantities, for use in the production of inlaid ornamental work, as the peculiar blue colour of the mineral cannot be obtained by other means.

The process of extraction of the pigment from the mineral consists in grinding the mineral to a fine powder, after separating as much of the gangue as possible; it is then mixed with a compound of resin, wax, and linseed oil, and the mixture put into cloths and kneaded under hot water; the colour comes through the cloth into the water, several waters being used; after the working, the waters are placed on one side for the colour to settle. The blue thus obtained varies in shade in the different waters; that which settles out of the first water is the deepest in colour, and the brightest, and is sold as ultramarine; that which comes from the last waters has a blue-grey colour, and is sold as ultramarine ash. After the colour has settled, it is usual to grind it still finer, so that the beauty of the pigment shall be developed as much as possible. No better process for extracting ultramarine has been devised, although it is so tedious and gives such poor results. Very little is now so produced, as the natural variety has been almost replaced in European and other countries by the artificial variety.

The chemical composition and constitution of ultramarine early became the subject of research by chemists, which researches were partly undertaken with a view to its artificial production; for it was recognised that, from the beauty of its colour and its permanent qualities, ultramarine would, if it could be produced cheap enough, have a wide field of use. Several analyses were made by different chemists, but these vary very much, owing, as is probable, to the difficulty of obtaining the pigment quite free from its matrix. Those by Clement and Desormes and by Gmelin, which are, perhaps, the most typical, are here given.

It is evident that with such discrepancies in the analyses nothing could be satisfactorily inferred as to the chemical composition and constitution of ultramarine, and it is no wonder that none of them led to its artificial production.

Artificial Ultramarine.-Early in the present century, soon after soda began to be produced on the large scale from salt by the Leblanc process, many persons noticed the formation of a substance resembling ultramarine in colour; Tessart and Kuhlmann recorded, in 1814, that they had seen this blue colour in a soda furnace. Vauquelin, on examining it, found it to be a compound of silica, alumina, lime, soda, and sulphur, and showed that it had a similar composition to ultramarine. It was recorded that it was formed only when sandstone was used in the construction of the furnace; when bricks were used it was not formed.

Guimet, an eminent French manufacturing chemist, studied the production of ultramarine. In 1828 he succeeded in making it on a large scale, and obtained a prize of 6,000 francs, offered by the Société d'Encouragement of France to any one who made ultramarine in a wholesale way. Guimet's process is still used by his successors, but has not been published.

Gmelin also interested himself in the production of ultramarine, and in 1828 he published an elaborate description of his method of making it.

Kottig, the director of the Miessen Porcelain Works also, about the same time, observed the production of ultramarine in his furnaces, and, as the result of his researches, succeeded in making the pigment on a large scale; the Miessen ultramarine was for many years one of the leading brands; the works are now closed.

About 1834 Dr. Leverkus, working by Gmelin's process, started its manufacture in Germany at works which are still in existence.

The great bulk of the ultramarine used is made in Germany; there are two or three works in England, a few in France, and one in America. Several writers have given descriptions, more or less complete, of the process of making ultramarine; but the best and most complete description is that by J. G. Gentele,* and more recently one by Rawlins.t

Varieties of Ultramarine.-There are two principal varieties of artificial ultramarine-1st, sulphate ultramarine, which is of

Technologiste, vol. xviii., pp. 389-411. + Journ. Soc. Chem. Ind., 1887, p. 791.

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, Og; if the proportions much exceed these the shade will be poor, while if they reach those indicated by the formula, 3 Si O2, Al, Og, 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.