Explanatory notes
… an electric furnace. The aluminium oxide may contain small proportions of other oxides (e.g., titanium oxide, chromium … artificial corundum and other substances, such as zirconium dioxide, are excluded ( heading 38.24 ). Artificial corundum … it is more resistant than ordinary aluminium oxide to the action of air and acids, and is very hard. It is used, e.g., …