Encyclopædia Britannica, Ninth Edition/Ultramarine

ULTRAMARINE, a magnificent blue pigment, which occurs in nature as a proximate component of Lapis Lazuli (q.v.). Lapis lazuli has long been known as a precious stone, and highly valued as such, and as early at least as the 11th century the art of extracting a blue pigment from it was practised. From the beginning of the 16th century this pigment began to be imported into Europe from "over the sea," as azurrum ultramarinum. To extract it, the stone, after having been powdered coarsely, is heated to redness and thrown into cold water to facilitate its con version into a very fine powder, which is next treated with dilute acetic acid to remove the carbonate of lime which is present in almost all specimens. The insoluble blue residue is mixed up into a "dough" with a composition of resin, pitch, and linseed oil, and this dough is then kneaded under water, which is renewed as long as it runs off with a blue colour. The blue liquor, when allowed to stand, deposits a fine precipitate, which is collected, washed, dried, and sold as ultramarine. As the yield amounts to only 2 to 3 per cent, of the mineral used, it is not surprising to learn that the pigment used to be weighed up with gold. It was valued chiefly on account of its brilliancy of tone and its inertness in opposition to sunlight, oil, and slaked lime (in fresco-painting).

Lapis lazuli has the composition of a double silicate of lime and soda combined with sulphates and sulphides of the metals named. Of the many analyses made (compare Lapis Lazuli) we quote the following, carried out by Schultz in Rammelsberg's laboratory:—combined sulphur (not SO₃), 3·16; combined sulphuric acid, SO₃, 5·67; silica, 43·26; alumina, 20·22; oxide of iron, calculated as Fe₂O₃, 4·20; lime, 14·73; soda, 8·76.

In 1814 Tassaer observed the spontaneous formation of a blue compound, very similar to ultramarine, if not identical with it, in a soda-furnace at St Gobain, which caused the "Societe pour l'Encouragement d'lndustrie" to offer a prize for the artificial production of the precious colour. The problem was solved almost simultaneously by Guimet and by Christian Gmelin, then professor of chemistry in Tübingen; but while Guimet kept his process a secret (it has indeed never become known) Gmelin published his, and thus became the originator of an industry which flourishes to this day chiefly in Germany. There are very few ultramarine works in other countries, and none, as far as we know, in Great Britain. The raw materials used in the manufacture are—(1) iron-free kaolin, or some other kind of pure clay, which should contain its silica and alumina as nearly as possible in the proportion of 2SiO₂: Al₂O₃ demanded by the formula assigned to ideal kaolin (a deficit of silica, how ever, it appears can be made up for by addition of the calculated weight of finely divided silica); (2) anhydrous sulphate of soda; (3) anhydrous carbonate of soda; (4) sulphur (in the state of powder); and (5) powdered charcoal or relatively ash-free coal, or colophony in lumps. The numerous modes of manufacture may be viewed as modifications or combinations of three processes.

(1) In the Nuremberg process the soda is used as sulphate, or partly as such and partly as carbonate. The following recipe gives an idea of the proportions in which the materials are used: kaolin (calculated as anhydrous matter) 100 parts; calcined sulphate of soda 83 to 100 (or 41 of sulphate and 41 of carbonate); charcoal 17; powdered sulphur 13. These ingredients are mixed most intimately; they are then rammed tight into fire-clay crucibles and kept at a nearly white heat for 7 to 10 hours, access of air being prevented as far as possible. The product obtained is a greyish or yellowish green mass, which is soaked in and washed with water; the porous residue is ground very fine in mills, again washed, dried, and again ground in the dry state and passed through sieves. The product at this stage has a green colour, and is sometimes sold as "green ultramarine," although it has not a high standing amongst green pigments. For its conversion into blue ultramarine it is heated with sulphur in the presence of air to a relatively low temperature. Of the various apparatus used for this important stage of the manufacture, the easiest to describe is a large muffle, heated from the outside. On its floor the green ultramarine is spread out to a depth of 21/2 to 3 inches, and heated (with closed doors) to a temperature at which sulphur powder when thrown in catches fire spontaneously. This temperature being maintained, a shovelful of sulphur is thrown in and allowed to burn off while the mass is being constantly agitated with iron rakers. Another dose of sulphur is then added, and so on until a sample taken out is found to have come up to the highest attainable brilliancy and depth of blue. The product is then lixiviated with water, which removes a deal of sulphate of soda formed in the process; it is then ground up very fine, and finally subjected to elutriation to produce a graduated series of ultramarines of different degrees of fineness. In some works the process of sulphuration is divided into two or more periods, after each of which the product is washed, dried, and ground before being returned to the muffle to produce a higher degree of homogeneity.

(2) In the carbonate of soda process the soda is used solely, or at least principally, in the carbonate form. The following is one of many recipes: kaolin (calculated as anhydrous matter) 100; carbonate of soda 100; charcoal 12; sulphur 60. The mixture is heated in a reverberatory furnace to form in the first instance a white mass, which is so porous that it readily passes, by oxidation, into green and partly even into blue ultramarine. Green ultramarine, saleable as such, cannot be produced in this manner. The half-blue product is finished by sulphuration pretty much as explained above for the Nuremberg process. Well-made soda-ash ultramarine has a richer colour than the Nuremberg variety.

(3) Silica ultramarine is soda-ash ultramarine in whose preparation a quantity of finely divided silica, equal to 5 to 10 per cent, of the weight of the kaolin, has been added. It is distinguished by a reddish tinge, which is the more fully developed the greater the proportion of added silica. It is more highly proof against the action of alum solution than non -siliceous ultramarine is.

Since 1873 the Nuremberg works have been producing four varieties of magnificently violet ultramarine. The mode of manufacture has not transpired. At the Paris Exhibition in 1867 a magnificent block of ultramarine exhibited by the Kaiserslautern works attracted attention. In its manufacture the roasting (blueing) pro cess is said to have been continued for three weeks.

Artificial, like natural, ultramarine has a magnificently blue colour, which is not affected by light nor by contact with oil or lime as used in painting. Hydrochloric acid at once bleaches it with liberation of sulphuretted hydrogen gas and milk of sulphur. The natural pigment is proof against dilute acetic acid and solution of alum; the artificial pigment is even alum-proof only in a higher or lower relative sense. Hence there is room for progress in one direction at least. The composition of the pigment is quite similar to that of lapis lazuli; but the constitution of both is still a chemical enigma. It is remarkable that even a small addition of zinc-white (oxide of zinc) to the reddish varieties especially causes a considerable diminution in the intensity of the colour, while dilution with artificial precipitated sulphate of lime ("annalin") or sulphate of baryta ( blanc fix ") acts pretty much as one would expect. Ultramarine being very cheap (it sells at 7d. to lOd. per K>), it is largely used for wall painting, the printing of paper hangings and calico, &c., and also as a corrective for the yellowish tinge often present in things meant to be white, such as linen, paper, &c. Large quantities are used in the manufacture of paper, and especially for producing that kind of pale blue writing paper which is so popular in Great Britain. Only the very finest ultramarine can be used for paper tinging, because the least admixture of coarse particles becomes visible in the paper as dark spots or stains.