~Mordants and dyeing.~ Aluminium hydroxide has the peculiar property of combining with many soluble coloring materials and forming insoluble products with them. On this account it is often used as a filter to remove objectionable colors from water. This property also leads to its wide use in the dye industry. Many dyes will not adhere to natural fibers such as cotton and wool, that is, will not "dye fast." If, however, the cloth to be dyed is soaked in a solution of aluminium compounds and then treated with ammonia, the aluminium salts which have soaked into the fiber will be converted into the hydroxide, which, being insoluble, remains in the body of it. If the fiber is now dipped into a solution of the dye, the aluminium hydroxide combines with the color material and fastens, or "fixes," it upon the fiber. A substance which serves this purpose is called a _mordant_, and aluminium salts, particularly the acetate, are used in this way. ~Aluminium chloride~ (AlCl_{3}.6 H_{2}O). This substance is prepared by dissolving the hydroxide in hydrochloric acid and evaporating to crystallization. When heated it is converted into the oxide, resembling magnesium in this respect: 2(AlCl_{3}.6 H_{2}O) = Al_{2}O_{3} + 6HCl + 9H_{2}O. The anhydrous chloride, which has some important uses, is made by heating aluminium turnings in a current of chlorine. ~Alums.~ Aluminium sulphate can be prepared by the action of sulphuric acid upon aluminium hydroxide. It has the property of combining with the sulphates of the alkali metals to form compounds called _alums_. Thus, with potassium sulphate the reaction is expressed by the equation K_{2}SO_{4} + Al_{2}(SO_{4})_{3} + 24H_{2}O = 2(KAl(SO_{4})_{2}.12H_{2}O). Under similar conditions ammonium sulphate yields ammonium alum: (NH_{4})_{2}SO_{4} + Al_{2}(SO_{4})_{3} + 24H_{2}O = 2(NH_{4}Al(SO_{4})_{2}.12H_{2}O). Other trivalent sulphates besides aluminium sulphate can form similar compounds with the alkali sulphates, and these compounds are also called alums, though they contain no aluminium. They all crystallize in octahedra and contain twelve molecules of water of crystallization. The alums most frequently prepared are the following: Potassium alum KAl(SO_{4})_{2}.12H_{2}O. Ammonium alum NH_{4}Al(SO_{4})_{2}.12H_{2}O. Ammonium iron alum NH_{4}Fe(SO_{4})_{2}.12H_{2}O. Potassium chrome alum KCr(SO_{4})_{2}.12H_{2}O. An alum may therefore be regarded as a compound derived from two molecules of sulphuric acid, in which one hydrogen atom has been displaced by the univalent alkali atom, and the other three hydrogen atoms by an atom of one of the trivalent metals, such as aluminium, iron, or chromium.