Antimony Oxychloride, or Powder of Algaroth, is obtained by dissolving stibnite in strong hydrochloric acid, the operation being performed under a chimney to carry off the sulphuretted hydrogen. The solution of antimony trichloride, when the impurities have settled, is poured into a large vessel of water. At once a pure white precipitate is formed, which quickly sinks; it is washed with water until the washings have no acid reaction. Washing should not be continued too long or the oxychloride will be further decomposed; by washing with hot water it is almost entirely changed to antimony trioxide. The precipitate, after washing with cold water, has generally the composition expressed by the formula SbOCl.
Antimony oxide and oxychloride are both very crystalline powders, and have in consequence small covering power.
Bismuth White is not used as a painters’ pigment; it has no advantage over the white pigments previously described, and is much more expensive. Its only use as a pigment is for the preparation of white cosmetics, and even for this purpose zinc white is now frequently used; it is cheaper and quite as satisfactory.
Bismuth white is prepared by treating metallic bismuth with fuming nitric acid. The white precipitate at first formed completely dissolves in an excess of acid, and when the solution is poured into a large quantity of water, basic bismuth nitrate—bismuth white—separates.
Pure bismuth white is a soft, heavy powder, brilliantly white; it must be preserved in air-tight vessels as soon as it is dry, otherwise it acquires a yellowish tinge. Bismuth compounds are, if possible, more susceptible to the action of sulphuretted hydrogen than lead compounds. The yellow colouration, turning to black in the course of time, is due to black bismuth sulphide.
Tin White is used for earthenware enamels. It is obtained by treating granulated metallic tin with very strong fuming nitric acid. The heavy, white powder which is formed is separated from the undissolved tin by floating. Tin white has no application as a pigment; when mixed with glazes it gives them a handsome, milky appearance.
Manganese White.—When large quantities of a solution of impure manganese chloride are at hand, such as are produced in the preparation of chlorine, manganese carbonate may be obtained. A small quantity of soda solution is first added and the liquid left for several days so that the oxide of iron may separate. When this is the case an addition of soda gives a pure white precipitate.
Magnesia White or Mineral White is obtained, according to T. H. Cobley, by mixing a solution of magnesium sulphate with calcium chloride, adding 10 per cent. of aluminium chloride to the mixture and stirring in slaked lime so long as a precipitate is formed. A cheaper process is to precipitate mixed solutions of magnesium and aluminium sulphates by slaked lime.
Annaline.—A white pigment is recommended for use under this name; it consists of dead-burnt gypsum, which has been converted into a fine powder by grinding and levigating. (Dead-burnt gypsum has been so strongly heated that it is not able to again unite with water.)
To obtain paler shades of certain colours, additions are made of natural pigments, such as chalk, which has been converted into a very fine powder by levigation. An addition of chalk to a heavy mineral pigment, such as chrome yellow, is not advisable; it would, besides, seriously diminish its covering power.