Aluminum resembles silver in its whiteness but is much lighter. The ore of this metal is obtained from pure clay, a substance with which man was familiar for ages without suspecting the treasure it contained. The metal is not found in a pure state although its compounds are common. It is now made on a large scale by an electrolytic process which has greatly reduced its cost.
When an electric current is passed through cryolite (a fluoride of aluminum and sodium found abundantly in Greenland), the aluminum oxide which is added is decomposed and the aluminum goes to the negative pole and is collected in the molten state.
Pure aluminum is acted upon neither by the oxygen of the air, by water, nor by dilute acids, but the commercial grade is impure and is rapidly oxidized by steam at a high temperature. Dilute or concentrated sulphuric or nitric acid does not, when cold, act on aluminum, but when heated dissolves the metal rapidly. Dilute or concentrated hydrochloric acid also will dissolve the metal readily. Since it is not affected to any great extent by organic acid, and the salts of aluminum are quite harmless, this metal is widely used for cooking purposes in place of tin and copper. Caustic alkalies readily dissolve aluminum with a formation of aluminates and the liberation of hydrogen.
Aluminum in a pulverized condition is gray in color, is very malleable and ductile, and has a specific gravity about that of glass (2.7). It melts at a red heat and vaporizes at a very high temperature.