When it is desired to mix other pigments with a white substance, to lighten the shade, permanent white can be specially recommended, since it is quite insensitive to atmospheric influences and has no chemical action on the colour, so that it can be used with even the most delicate colours without risk. In this way, not only can the colours be considerably cheapened, but over-dark colours can be shaded to the desired extent. Another advantage of such mixtures is that a smaller quantity of oil or varnish is required, barytes only needing about 8% of its own weight of vehicle to form a workable mixture, whilst other pigments take five times as much, or even more. In many cases the low covering power of barytes enables large quantities to be added, and this reacts favourably on the consumption of varnish. Another barium mineral is witherite, or barium carbonate. This is not used direct as a pigment, but--in contrast to heavy spar--is readily soluble in hydrochloric acid, and therefore serves as raw material for the preparation of artificial barytes and other barium compounds, the first-named being obtained by treating a solution of barium chloride with sulphuric acid, insoluble barium sulphate being precipitated. _Talc_, _Soapstone_, _Steatite_ Talc occurs in Nature either as a pure white mass, of greasy lustre, or occasionally as yellow, green or grey masses, all distinguished by a peculiar greasy appearance and a soapy feel. This appearance is common to all the minerals of the steatite group, and is the cause of their generic name, soapstone. Although the steatites have a very low degree of hardness--most of them can be scratched by the finger-nail--some difficulty is encountered in reducing them to fine powder. Calcination usually increases the hardness considerably, so that, in some cases, the calcined mineral gives off sparks when struck with a steel instrument. Soapstone is composed of magnesium silicates, containing varying proportions of magnesia and silica, together with a small quantity of water, apparently in a state of chemical combination, a very high temperature, approaching white heat, being required to effect its complete expulsion, the residue then attaining the aforesaid high degree of hardness. The composition of talc can be expressed by the symbol H{2}Mg{2}(SiO{3}){4}, corresponding to 63·52% of silica, 31·72% of magnesia, and 4·76% of water. In some varieties of talc, a portion (1–5%) of the magnesia is replaced by ferrous oxide. Talc is quite unaffected by the action of dilute acids, boiling concentrated sulphuric acid being required to decompose it, with separation of silica. Owing to its low specific gravity and chemical indifference, talc is suitable for lightening the shade of certain lake pigments. It can also be used as a pigment by itself, and also as a gloss on wall-paper, for mixing with paper pulp, and for various other purposes. _Clay_