Kaolin, China Clay, Bolus Alba, or Porcelain Clay, is a fine, white clay, which consists chiefly of aluminium silicate. It is also official in the U.S.P. It is derived from the decomposition of the felspar of granitic rocks, and contains approximately 47 per cent. of silica, 40 per cent. of alumina, and 13 per cent. of water. It is found in large quantities in Cornwall, and is freed from gritty particles by elutriation. It occurs as a soft, white, or slightly yellowish-white, powder, which is odourless when dry, but develops a clay-like odour when moistened. It has an earthy taste, is insoluble in all the ordinary solvents, and is unaffected by most chemical reagents. It darkens in colour on being moistened with water. When kaolin is fused with alkalies, silicates and aluminates of the alkali metals are formed. The fused mass dissolves partially in water, the solution depositing silica in a hydrated condition when filtered and acidified with hydrochloric acid, while the acid filtrate when made alkaline with ammonia yields aluminium hydroxide. Kaolin should not contain more than traces of iron, and on ignition it should not leave less than 85 per cent. of non-volatile residue. Fuller's earth is an impure variety of kaolin, containing iron and magnesia. For toilet purposes it should be sterilised by roasting; cases of tetanus have arisen from the use of common varieties. White fuller's earth is a purer form, and is suitable for use as a dusting powder; preparations of this are known under the trade-names "Cimolite" and "Emol Keleet."
Action and Uses.—Specially prepared varieties of kaolin are used in place of fuller's earth as dusting and toilet powders; they are absorbent, and prevent irritation due to friction. Kaolin is also employed for its absorbent properties in the preparation of pill masses, especially in the presence of substances which are readily oxidised or reduced (e.g., Pilula Phosphori Composita). In the form of Massa Kaolini, it is used to prepare pills of potassium permanganate, silver nitrate, silver oxide, and other similar substances, which would be reduced by contact with the organic matter of ordinary pill excipients. Kaolin forms a silicate basis for many carbolic and similar disinfectant powders. It is used as the basis of a cleanly form of poultice (see Cataplasma Kaolini), intended to replace the domestic linseed or bread poultice. Kaolin is also used as a filtering medium, to clear syrups and cloudy solutions of volatile oils.
- Cataplasma Kaolini, B.P.C.—KAOLIN POULTICE.
- The formula differs slightly from that for Cataplasma Kaolini, U.S.P., the quantity of glycerin being increased by 5 per cent. Preparations similar to the above, though differing somewhat in composition, are sold under different trade-names. Kaolin poultice is used as a cleanly and antiseptic substitute for Cataplasma Lini. The poultice is made hot and applied with a spatula in a thin layer, and covered with cotton wool or lint.
- Massa Kaolini, B.P.C.—KAOLIN MASS. Syn.—Unguentum Kaolini; Kaolin Ointment. 1 in 4.
- Employed in the preparation of pill masses, containing substances that are readily reduced by contact with the organic matter of the usual pill excipients, e.g., potassium permanganate, silver nitrate and oxide, and gold chloride.
The British Pharmaceutical Codex, 1911, was published by direction of the Council of the Pharmaceutical Society of Great Britain.