270 JOSEPH STRUTHERS 



good ore varies from 1 to 3 and the specific gravity from 2.4 

 to 2.55. 



When exposed to extreme heat bauxite becomes so hard 

 that it is almost impossible to make any impression upon it 

 with steel tools. This property in connection with its 

 infusibility makes it an excellent material for crucibles and 

 for furnace linings, since it resists chemical and calorific ac- 

 tions. The chief reason why bauxite is not used more ex- 

 tensively as a basic furnace lining is due to the impurities 

 that are usually present in the mineral. 



Four principal substances are prepared from bauxite, 

 viz., aluminum sulphate, alum, artificial emery, and alumi- 

 num hydroxide. From the last named substance the metal 

 aluminum is manufactured. 



Aluminum sulphate (Al3(S04)3,18H20) is a chemical 

 salt technically known as concentrated alum. It is prepared 

 by decomposing bauxite with sulphuric acid. If the mineral 

 is dissolved directly in the acid, the product will contain a 

 large quantity of iron, forming the so-called alumino-ferric 

 cake, which is used for many purposes where iron and free 

 acid are not objectionable, as in the precipitation of sewage 

 and of waste liquors from dyeworks. Pure aluminum sul- 

 phate is now generally prepared by the Bayer process, which 

 consists in adding powdered alumina to the solution of sodium 

 aluminate, containing one part of ALO3 to 1.8 parts of NaoO. 

 The reaction causes a crystalline precipitate of aluminum 

 hydroxide. The silica and the iron present remain dis- 

 solved in the solution. The precipitated aluminum hydroxide 

 is separated from the solution and is thoroughly washed 

 and later dissolved in pure, hot, concentrated sulphuric acid 

 until the frothing ceases. The solution is then transferred 

 to shallow leaden pans and allowed to cool; thus the pure 

 aluminum sulphate is separated out in the form of a solid 

 crystalline mass. Owing to its greater purity and greater 

 strength, aluminum sulphate has largely replaced alum in 

 the arts. Aluminum sulphate is extensively used as a mor- 

 dant in dyeing; in preparing size for paper; for making alum 

 and aluminum salts (red liquor, etc.); in tawing skins; for 



