A beautiful permanent blue pigment, until recently obtained from the lapis-lazuli, or azure-stone. (See the article Azure-Stone, where that process is described.) A method of forming ultramarine artificially has, however, been recently discovered by M. Gmelin. This gentleman was led to consider sulphur as the colouring matter of ultramarine, from an observation made by M. Tasseart, that he had noticed a substance resembling ultramarine, which was found in a furnace used in the manufacture of soda. The following is the process by which we are told (in the Annates de Chimie, xxxvii. p. 109,) ultramarine may be infallibly prepared.
Pulverized quartz is to be fused with four times its weight of carbonate of soda, the mass dissolved in water, and then precipitated by muriatic acid: thus a hydrate of silica will be formed. A hydrate of alumina is now to be prepared, by precipitating alum by ammonia. These two earths are to be carefully washed with boiling water; the proportion of dry earth in each is then to be ascertained, by heating a small quantity and weighing it. The hydrate of silica used by M. Gmelin contained 56 per cent., and the hydrate of alumina 3.24 per cent.
As much hydrate of silica is then to be dissolved in a hot solution of caustic soda as it will take up, and the quantity determined; then such proportion is to be taken as contains 72 parts of an hydrous silica, and a quantity of the hydrate of alumina, equivalent to 70 parts of dry alumina added to it, and the whole evaporated together, being continually stirred until it becomes a damp powder.
This combination of silica, alumina, and soda, is the basis of ultramarine, and is now to be coloured by asulphuret of sodium in the following manner. A mixture of two parts of sulphur with one part of an hydrous carbonate of soda, is to be put into a Hessian crucible, covered up, and then gradually raised to a red heat, until it is well fused; then the mixture is to be thrown, in very small quantities at a time, into the midst of the fused mass. As soon as the effervescence occasioned by the water in one portion has ceased, another portion is to be added. Having retained the crucible at a moderate heat for an hour, it is to be removed from the fire, and allowed to cool. It now contains ultramarine, mixed with excess of sulphuret: the latter may be separated by water. If sulphur is in excess, a moderate heat will dissipate it. If all the parts are not equally coloured, a selection should be made, and then the substance reduced to a fine powder.
This cheap artificial product is equal in brilliancy, clearness, and durability, to the mineral ultramarine, for which we paid, a few years ago, as much as five guineas an ounce; and it is now so extensively manufactured as to be capable of being substituted for cobalt, from motives of economy.