Nicholson 1795/II

William Nicholson, A Dictionary of Chemistry, Exhibiting the present State of the Theory and Practice of that Science, its Application to Natural Philosophy, the Processes of Manufatures, Metallurgy, and numerous other Arts dependant on the Properties and Habitudes of Bodies, in the Mineral, Vegetalbe, and Animal Kindoms… II, London [George G. Robinson – John Robinson] 1795.

pp. 1009–1010

Ultramarine

ULTRAMARINE. A blue colour, made from lapis lazuli. The manipulations for making it are variously described by authors, and consist in pulverizing the stone very finely, by ignition, quenching it in water, and subsequent levigation. This powder is then carefully stirred by small portions at a time into a mass of resinous composition, from which it is again extracted by washing and kneading in hot water. Some artists use the mere powder of lapis lazuli without any subsequent management.

I find it difficult to form an opinion as to what happens in this process, which seems to be merely of a mechanical nature. Whether it differs at all from simple elutriation and subsidence, by which the finer parts of powders are separated from their coarser parts; and if so, in what respect, are questions which demand the test of experiment. What may be the use of the resinous compound? Do the parts of the finest colour adhere less forcibly to the resin, and are therefore washed out first? It seems hardly probable, more especially if, as Neri * informs us, the same process may be applied to blue enamel, by which I suppose he means glass tinged with cobalt.

Kunckel † describes a process which he says successful in his hands; and though upon perusal it seems capable of being much simplified, I shall give it unaltered in this place, because at least as simple as any of the other receipts.

Lapis lazuli was reduced to pieces about the size of peas, then ignited and extinguished in stronger vinegar. The author says, distilled vinegar is preferable. The stone was in the next place triturated with vinegar to an impalpable powder. This is said to be the most important part of the operation. Of a composition of equal parts very pure virgin wax and colophony, a quantity was taken of the same weight as that of the powdered stone. This was melted in a glazed earthen plate, and the powder was added by a little at a time, and well stirred in. The mass was then poured into cold water and left for eight days, at the end of which, two vessels of glass were filled with water so hot as barely to suffer the hand to be plunged in it. A piece of the composition was then kneaded under water till it was supposed that the finest part of the colour was extracted. The mass was then transferred to the other water, and afforded a paler and less valuable blue than the first. The waters were left at rest for four days and then decanted off, and the powder carefully collected. The quantity of the best blue is very small; but the same mass, by kneading in different waters, especially when the quantity operated upon is large, affords three or four different kinds. Cleanliness of the hands, and of all the vessels and materials made use of, is of the utmost consequence in the manufacture.

p. 1010

Umber

UMBER. A brown ochreous earth used as a pigment. It acquires a reddish colour when slightly heated, but in a stronger heat becomes again brown, and magnetic, and in a still stronger is fused into a black glass. It does not effervesce with acids before roasting, but after that process the iron it contains is soluble.

Kirwan also mentions under this name, in his Argillaceous Genus, a brown or blackish substance which stains the fingers, and is very light. This was long supposed to be a clay; but M. Hupsch, in the Berlin Memoirs for 1777, shews that it consists of particles of decayed wood mixed with bitumen.

p. 1060

Vitriol

VITRIOL. The saline combination of copper with the vitriolic acid is called blue vitriol, in commerce. Vitriolic acid and iron form green vitriol, or the green copperas of the shops. Vitriol of zinc is called white copperas. The looseness and inaccuracy of these denominations are evident. Some chemists have endeavoured to generalize the term vitriol, by applying it to all combinations containing vitriolic acid. Thus they would say, vitriol of iron, or of copper, or of lime; but this method has never been generally adopted. It is usual however to call all the metallic salts vitriols, which contain vitriolic acid. See ACID, VITRIOLIC; SULPHUR; PYRITES, and the several Metals.

pp. 1064–1065

Zaffre

ZAFFRE, or SAFFRE, is the residuum of cobalt, after the sulphur, arsenic, and other volatile matters of this mineral have been expelled by calcination. It is therefore a calx of cobalt, of a gray or reddish colour. The use of cobalt is to produce a very fine blue colour, when it is melted with fusible and vitrifiable matters.

The calx of cobalt is the sole cause of the blue colour produced by zaffre. But as the quantity of regulus contained in cobalt is variable, some zaffres furnish more blue than others. The heterogeneous fixed matters contained in cobalts contribute likewise according to their quantity to the greater or less intensity of the blue colour. For this reason the manufacturers of zaffre from cobalt make frequent essays of the roasted ore, by mixing it with vitreous matters, to discover the intensity and beauty of the blue colour.

Good cobalt calcined would form too deep a blue, and almost a black glass, if it were not previously mixed with a certain quantity of vitreous fritt. In the manufacture of zaffre, therefore, the calx of cobalt, the strength of which has been previously determined by essays, is mixed with such a quantity of sand, or of powdered flints and quartz, that with the addition of some saline flux a deep blue glass may be formed.

The zaffre that is commonly sold, and which comes from Saxony, is a mixture of calx of cobalt with some vitrifiable earth. It is of a gray colour, as all the calces of cobalt are before vitrification. Some zaffres are dearer than others, according to the intensity of the colour which they are capable of producing.

Zaffre is employed in the manufacture of pottery and of porcelain, for painting the surface of the pieces of ware, upon which it is applied together with some saline flux, previously to the baking or glazing, that the same fire may also vitrify this colouring material.

The blue of zaffre is the most solid and fixed of all the colours that can be employed in vitrification. It suffers no change from the most violent fire. It is successfully employed to give shades of blue to enamels, and to the crystal glasses made in imitation of some opake and transparent precious stones, as the lapis lazuli, the turquois, the sapphire, and others of this kind. See AZURE; COBALT; INK, SYMPATHETIC; and SMALT.