Aluminium ore is called bauxite.
Bauxite contains aluminium oxide, water, iron oxide and other impurities.
The purified dry ore, called alumina, is aluminium oxide - Al2O3.
The alumina must be molten for electrolysis to occur,
since the ions are not free to move in the solid state.
Unfortunately, alumina has a high melting point (2040 'C)
and it is not practical to do electrolysis at this temperature.
In the middle of the nineteenth century,
it was found that alumina dissolved in cryolite.
Cryolite is sodium aluminium fluoride - Na3AlF6.
A solution of alumina in cryolite has a lower melting point,
allowing electrolysis at about 950 'C.
