This product can also be prepared artificially, by treating a dissolved magnesium salt with a solution of carbonate of soda, the result being the formation of a pure white precipitate, which is very brilliant when dry, and is characterised by unusually low specific gravity. In some places, conditions are such that this preparation can be made on a large scale at very low cost. For instance, there is a spring at Bilin, in Bohemia, the water of which contains large quantities of alkali carbonates in solution; whilst in the vicinity of Saidschütz is a spring fairly rich in magnesia salts. The waters from these two springs are concentrated by evaporation, and mixed in large tanks; and when a sufficient deposit of the resulting basic carbonate of magnesia has accumulated, it is taken out of the tanks, placed on linen filters and washed with water. The residue is dried slowly, without the employment of a high temperature, and then forms a white powder, which is very light and can be used for a number of purposes, chiefly medicinal, though it is also well adapted as a material for paint.
For this latter purpose it is, however, far too expensive; but since the conditions obtaining at Bilin are certain to occur elsewhere, we have included carbonate of magnesia among the earth colours.
On account of its specific lightness, carbonate of magnesia is specially adapted for making pale shades of certain delicate lake colours, which, if toned with even perfectly pure chalk, would undergo alteration in course of time. Carmine, for instance, can be graded, by the addition of carbonate of magnesia, into every possible variety of shades between the pure red of carmine itself and the palest pink; and the resulting colours are quite permanent whether mixed with gum solution or any other vehicle.
Talc
Although this mineral is not used as a pigment by itself, it must be mentioned here because it is not infrequently employed for mixing with other colours, and is also used in the wall-paper industry. It also serves to distribute certain pigments in a state of fine division, the “rouge végétal” of the perfumer, for example, usually consisting of talc and a small quantity of very fine carmine.
In commerce the name talc is sometimes applied to two separate minerals, true talc and steatite or soapstone. The former is rarely met with native as well-defined crystals, mostly occurring as scaly masses in primitive rocks. Thin pieces exhibit a certain degree of flexibility. The hardness of this mineral is so small that it can be scratched with the finger-nail; and its sp. gr. is 2·9–2·8. Talc is easily scraped, and the powder remains sticking to the knife, a property which renders the substance difficult to reduce to powder, because it balls together and takes a very long time to convert into a fine flour. The process is facilitated by calcining the talc and quenching it in cold water, this treatment increasing the hardness and at the same time making it more brittle, and thus more easy to pulverise.
A characteristic feature of all the talc minerals is their peculiar greasy appearance and feel. The colour varies, white pieces alone being of any use to the colour manufacturer. The yellow- or green-tinged varieties owe their shade to the presence of ferric and ferrous oxides. In chemical composition, talc consists of a combination of magnesium silicate with hydrated silica, the supposed formula being: 4MgO . SiO2 + H2O . SiO2, and the percentage composition: silica, 62·6%; magnesia, 32·9%; water, 4·9%.
Steatite or Soapstone
Steatite so closely resembles talc in most of its properties, that the two minerals were long regarded as identical. Whereas, however, talc is scarcely acted upon at all by the strongest acids, steatite is completely decomposed by prolonged boiling therewith, although both minerals have exactly the same composition.
As a pigment, steatite is far more important than talc, and, as French chalk, is largely used for drawing or writing. To prepare it for this purpose pure white steatite requires no preliminary treatment, beyond cutting the large lumps up into quadrangular prisms, which are mounted in wood, like lead pencil, and used for writing on the blackboard. The powder produced in the cutting process is made up into pastel crayons. With this object, the powder is mixed with a sufficient quantity of some mineral pigment to produce a mass of the desired shade, and is kneaded to a stiff paste with water containing an adhesive such as gum, glue or tragacanth mucilage. The mass is shaped into prisms, which, when dry, are cut into pencils and mounted in wood. Steatite being like talc, without action on even the most delicate colours, can be used as a diluent in the preparation of light shades.