ULTRAMARINE  , a

blue pigment, consisting essentially of a double silicate of aluminium and sodium with some sulphides or sulphates, and occurring in nature as a proximate component of lapis lazuli (q.v.) . As early at least as the Irth century the art of extracting a blue pigment from lapis lazuli was practised, and from the beginning of the 16th century this pigment began to be imported into Europe from " over the sea," as azurrum ultramarinum . As the mineral only yields from 2 to 3% of the pigment, 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) . In 1814 Tassaert 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'Industrie to offer, in 1824, a prize for the artificial production of the precious colour . Processes were devised by Guimet (1826) and by Christian Gmelin (1828), then professor of chemistry in Tubingen; but while Guimet kept his process a secret Gmelin published his, and thus became the originator of the " artificial ultramarine " industry . The details of the commercial processes are trade secrets . The raw materials used in the manufacture are: (I) 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 : Al20, demanded by the formula assigned to ideal kaolin (a deficit of silica, however, 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 . " Ultramarine poor in silica " is obtained by fusing a mixture of soft clay, sodium sulphate, charcoal, soda and sulphur . The product is at first white, but soon turns green (" green ultra-marine ") when it is mixed with sulphur and heated . The sulphur fires, and a fine blue pigment is obtained . " Ultramarine rich in silica " is generally obtained by heating a mixture of pure clay, very fine white sand, sulphur and charcoal in a muffle-furnace .

A blue product is obtained at once, but a red tinge often results . The different ultramarines—green, blue, red and violet—are finely ground and washed with

water . Artificial, like natural, ultramarine has a magnificent 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 and milk of sulphur . 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 is largely used for wall painting, the printing of paperhangings 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 . The composition of the pigment is quite similar to that of lapis lazuli; but the constitution of both is uncertain . By treating blue ultramarine with silver nitrate solution, " silver-ultramarine " is obtained as a yellow powder . This compound gives a blue potassium- and lithium-ultramarine when treated with the corresponding chloride, and an ethyl-ultramarine when treated with ethyl icdide . Selenium- and tellurium-ultramarine, in which these elements replace the sulphur, have also been prepared . It has been suggested that ultramarine is a compound of a sodium aluminium silicate and sodium sulphide .

Another view is that the colour is due to some comparatively

simple substance suspended in a colourless medium .