Reactivation of calcinated oxides
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Most oxides sold in hardware stores as pigments tend to be calcinated at high temperatures, which changes their crystalline structure and makes them resistant to the attack of most corrosive reagents, like acids, aqueous alkali, metal halides or halogens. Indurated oxides can be reactivated by fusing them with molten alkali or sodium bisulfate, though this too may not always work.
Note that some oxides, like titanium dioxide and zirconium dioxide are normally (almost) inert without calcination.
Successful methods
Fusion with molten alkali
Mixing the inert oxide with an alkali hydroxide, like sodium hydroxide, potassium hydroxide and melting the resulting mixture for a few hours will break down the oxide's inert structure and make it more reactive. May not work with all oxides, such as calcium oxide or magnesia. Reaction with chromium(III) oxide can give chromate or dichromate, while with iron(III) oxide will give ferrate.
Fusion with molten sodium bisulfate
Molten sodium bisulfate (NaHSO4) has been claimed to react with calcinated oxides.
Dissolution in hydrofluoric acid
Hydrofluoric acid is the only acid that can attack any metal oxide, however since many fluorides are almost insoluble in water, dissolving large amounts of inert oxide will take forever. And since working with hydrofluoric acid is very dangerous, this route is both expensive and extremely hazardous.
Dissolution in hot concentrated sulfuric acid
Some metal oxides like titanium dioxide will react with hot conc. sulfuric acid, forming TiOSO4, which hydrolyzes in the presence of water, giving raw titanium dioxide. If the TiOSO4 is kept in sulfuric acid, and hydrogen peroxide is added, an orange colored peroxo titanium oxide compound is obtained.
Reduction to metal
Using a reducing metal, like aluminium or magnesium or a non-metal, like carbon into a thermite reaction to obtain the metal from the oxide is another accessible route. To obtain the oxide again, dissolve the metal in an acid, precipitate it with aq. ammonia then heat the product until it decomposes. While attractive, this method will consume plenty of useful aluminium or magnesium powders that would be of better use elsewhere. The thermite reaction also produces plenty of slag, which may be inert.
Failed methods
Calcination with ammonium chloride
Heating the oxide with ammonium chloride gave no results.
Fusion with alkali nitrates
Melting together a calcinated metal oxide, like chromium(III) oxide with molten potassium nitrate will give a brownish product, insoluble in water, without any visible reactivity.
Safety
Working with molten alkali and salts is very dangerous and proper protection equipment should be worn all the times.
