As a rule, barytes is first roughly crushed in edge-runner mills or stamps, and then ground to the extreme degree of fineness obtainable in ordinary mills. Even with the greatest care, however, it is impossible by this means to obtain sufficient fineness of division for mixing with fine colours, the only way in which this can be accomplished being by levigation.
Given a fairly pure white barytes to begin with, levigation furnishes a handsome white pigment that can be mixed with colours of any kind; but when used by itself in association with oil or varnish, its covering power is very low and the colour never perfectly white. Native barytes is therefore unsuitable, as such, for paint.
Varieties that are not pure white are sometimes corrected with ultramarine, added in the grinding-mill. If the yellow tinge is due to iron compounds, this can often be remedied by treating the finely ground material with hydrochloric acid, which dissolves them out, this treatment being followed by a thorough washing with pure water.
As already mentioned, white lead is most frequently mixed with barytes, this being usually added when the white lead is being ground, by feeding the two materials to the mill and grinding them together.
The crudeness of mechanical methods of reduction is clearly exemplified by comparing the most carefully ground and levigated barytes with that obtained by artificial means. The permanent white largely used in the production of wall-paper, and quite unalterable in air, is, chemically speaking, identical with native barytes, viz. barium sulphate. The two also seem to be identical in crystalline habit, as is usual in the case of one and the same mineral, whether native or prepared by artificial means. Artificial barytes is obtained by treating a soluble salt of barium with sulphuric acid, or a solution of sodium sulphate (Glauber salt), so long as a precipitate continues to form.
This precipitate is barium sulphate, which subsides completely on account of its extreme insolubility, this being greater than that of any other salt known. The rapid rate of deposition results in the formation of extremely small crystals, which, being colourless and reflecting the light completely, appear to be perfectly white. Even when permanent white is applied in very thin layers to any surface, its covering power is very considerable, by reason of the extremely fine subdivision of the material.
This behaviour of artificial barytes in comparison with that of the natural product, affords an important hint in connection with the preparation of earth colours, namely, that in order to obtain products of specially good quality, the endeavour should be to reduce the raw materials to the finest condition possible. This result is accomplished most securely by bestowing the greatest care on grinding and levigation; and it is therefore highly important that the manufacturer should select, from the various apparatus used in reducing the materials, those that are best adapted for the purpose.
Carbonate of Magnesia
Although carbonate of magnesia is seldom used alone as a pigment, it can be advantageously employed as such when circumstances permit. It is met with not infrequently, in Nature, in a crystalline form, as magnesite or bitter spar, the latter name arising from the fact that the soluble salts of magnesia have a bitter taste. Still more frequently, magnesia occurs in association with calcium carbonate, in the mineral dolomite, which contains up to 20% of magnesia.
A less abundant native mineral is hydromagnesite, which consists of basic magnesium hydrocarbonate. Hydromagnesite is a very light, chalk-white mass, with a non-greasy feel, which, when reduced to a soft powder, forms an excellent material for paint. It is highly inert, in a chemical sense, and can therefore be mixed with the most delicate colours, having no other effect thereon than to render them lighter in shade.