At the present time blue, green, red and violet pigments come into the market under the name of ultramarine. The green and blue have been commercial articles for about seventy years; the violet and red were introduced about the year 1860.

At first the name ultramarine was restricted to a natural blue pigment obtained from lapis lazuli, which was extremely costly. Accounts of the payments of Italian artists, still extant, show the expensive nature of the ultramarine blue used in their paintings. At that time, when artists were compelled themselves to make the majority of their pigments, ultramarine was made in a most laborious manner from lapis lazuli, for which incredibly high prices were paid. In order to make the mineral easier to powder the lumps were heated, and, whilst hot, thrown into water. They were then powdered as finely as possible. The powder was mixed with melted resin, and the mixture kneaded under water for a long time. The ultramarine suspended in the water by this crude method of levigation was obtained by allowing the wash waters to settle. Few places are known at which lapis lazuli occurs in quantity. It is chiefly obtained in China and Thibet, and, considering the little intercourse between Europe and these distant countries at that time, it is no wonder that the price of ultramarine was fabulously high. One ounce cost about £8—a price which is explained by the small yield of ultramarine from the best lapis lazuli. By the most careful work not more than from 2 to 3 per cent. of the mineral was obtained, the residue consisted of foreign minerals.

The enormously high price of this pigment was the stimulus for the endeavours to make it artificially. The attempts are to be regarded not only as completely successful, but it must be allowed that science has gone a considerable step further than nature, since the researches have made it known that there is not only a blue ultramarine, but also a green, and, according to the latest researches, there exist in addition violet, red and white compounds, also to be described as ultramarines. With the discovery of the methods by which ultramarine can be made artificially, the preparation of this pigment from lapis lazuli came to an end, and has now but historic interest. The discovery of artificial ultramarine is due to the French chemist Guimet and to the great German chemist Gmelin. Ultramarine was first manufactured in Germany in the year 1828 by A. Köttig, as a branch of the porcelain works at Meissen in Saxony, where the manufacture was continued for about fifty years.

The discovery of the aniline dyes is rightly called a triumph of human intellect. The artificial manufacture of ultramarine deserves the same description in no less degree, although it has not effected so great a revolution as the former.

Attempts to make ultramarine artificially would naturally be based on the analysis of the natural product. The following comparison of the compositions of natural and artificial ultramarines shows how nearly the artificial product approaches the natural:—

NATURAL ULTRAMARINE.

ARTIFICIAL ULTRAMARINE.

I. Szilasi found three samples of green ultramarine to have the following composition:—

There are many other analyses in addition to those we have given and agreeing with them, so that there is no doubt as to the composition of ultramarine, but as to the manner in which the elements are grouped nothing is definitely known. Some chemists are of the opinion that the colouring principle of ultramarine is a sulphur compound of iron, whilst others oppose this view and consider that the colour is due to the combination of a double silicate of alumina and soda with an unknown sulphide of sodium. Although no blue or green compound is known of corresponding composition, the majority of chemists incline to the latter view. Experience has shown that the presence of iron in any of the materials used in the manufacture of artificial ultramarine is very dangerous to the success of the operation, and at the least considerably injures the beauty of the product.

Although the manufacture of ultramarine is now very well known, it cannot be denied that some works produce a pigment of a shade which cannot be obtained by others. These works keep their method very secret, so that it is not possible to say with certainty whether they have introduced a process varying from that commonly known, or whether, by carefully watching the process, they have achieved great technical dexterity in the manufacture of this product. The latter appears the most probable, for in order to obtain a good result, many experiments, and an accurate knowledge of the raw materials, are necessary.

The raw materials used are as follows: pure aluminium silicate, sodium sulphate, soda, sulphur, coal. The aluminium silicate is used in the form of fine china clay or kaolin, sodium sulphate and soda must be used in the anhydrous form, the sulphur is the ordinary commercial substance purified by distillation. Charcoal or coal containing little ash can be used.

Whilst the remaining raw materials are always of similar composition, the china clay from different localities possesses a very varying composition. This substance must be carefully chosen. There is hardly any kaolin which is naturally of sufficient purity to be used without purification; it is well known that the china clay used for porcelain is subjected to a thorough preparation before it is used. Kaolin, like all clays, has been produced by the decomposition of feldspar; when the aluminium silicate so formed was able to deposit without foreign admixtures, that mineral was formed which is the purest of all clays and is called kaolin. The more foreign substances are mixed with the aluminium silicate the further is the clay removed from kaolin. The impurities which generally accompany the aluminium silicate are quartz sand, chalk and ferric oxide. We distinguish accordingly between kaolin, white clay or pipe clay, clay, and lastly marl, a clay containing much chalk.

Even the purest kaolin contains certain impurities, of which quartz sand is the principal and the least harmful. Before kaolin can be used in the ultramarine manufacture it must always be purified by levigation; it is then ignited at a low temperature and powdered under stamps or in mills. The other materials required are generally produced by the chemical works in a condition of such purity that they can be at once used.

Occasionally Glauber’s salt contains iron, which would spoil the shade of the ultramarine. The iron can be easily removed by dissolving the crystallised salt in water, adding a little milk of lime and leaving the liquid for several days, stirring frequently. The lime neutralises every trace of free acid, and at the same time produces an ochre-yellow precipitate of ferric hydrate. The solution of Glauber’s salt thus freed from iron is evaporated in reverberatory furnaces, in which the salt is then calcined. This operation is not only simpler than evaporating the solution in iron pans to crystallisation and subjecting the dried salt alone to calcination, but it also guards against fresh contamination by iron, which might be caused by the use of iron evaporating pans.

The sulphur and coal are employed in a soft powder, which is most readily obtained by placing the coarsely powdered materials in rotating drums containing a number of iron balls. By continued rotation the materials are converted to any desired degree of fineness without the production of dust. The powder is then put through fine sieves by which the larger particles are retained.

The proportions in which the raw materials are mixed vary within certain limits; fixed quantities can only be given for a kaolin of definite composition. Definite formulæ are known by which ultramarine is made, but these can only be regarded as approximate. The proportions employed by French manufacturers differ considerably from those usual in German works. This variation is chiefly due to the difference in the composition of the kaolin employed in the two countries.

From the composition of the different mixtures one conclusion may be drawn with certainty—sufficient sodium is always used to neutralise half the silicic acid of the kaolin and to form some quantity of sodium sulphur compounds. In the successful process of the German makers a portion of the soda unites with the silica during heating. By the action of the coal on the Glauber’s salt it is reduced to sodium sulphide, which, since sulphur is present, unites with a further quantity of that element. The sodium sulphur compounds then unite with the silicates of aluminium and sodium to form a green compound, which is converted into blue ultramarine by a further treatment with sulphur in the presence of air.

Instead of using Glauber’s salt, which must always be decomposed in the first process, the sodium sulphide may be formed by the action of sulphur on soda in the presence of coal. This procedure is adopted in the French process. The proportions of the mixture used in different works vary. If the kaolin employed is assumed to be bisilicate of alumina—a somewhat arbitrary assumption—the following mixture can be successfully used:—

In working by the French method the following formula is suitable:—

These formulæ are not to be regarded as unalterable. In the different works such varied mixtures are used that it may be said with truth that each works has its particular formula for the mixture, the composition of which depends on the nature of the clay used in the works. The composition of the mixtures used in works is kept secret as far as possible. The formulæ given above refer to a clay approximating in composition to the bisilicate of alumina.

It will appear from the description of the manufacture of ultramarine that certain quantities of sodium sulphide are produced. The process is, however, conducted so that sodium sulphide shall be formed; thus the soda and Glauber’s salt used in the mixture may be replaced by the sodium sulphide produced in previous operations. The liquors in which the sodium sulphide is contained are evaporated to dryness, and as much of the residue added to the clay as corresponds to the quantity obtained from the usual amounts of soda and Glauber’s salt. Assuming that these latter materials are pure, 80 parts of the sodium sulphide residue correspond to 100 parts of soda, and 60 parts to 100 parts of Glauber’s salt.