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the natural or synthetic oxide of aluminum, Al 2 O 3 , occurring in nature in a pure crystal form as corundum.
Historical Examples

The garnet is but as hard as quartz, and is a silicate of alumina with lime and a little iron.
The A B C of Mining Charles A. Bramble

It is composed of silica, alumina, carbonate of lime, magnesia and oxide of iron.
Museum of Antiquity L. W. Yaggy

Alizarin only stains the untreated woollen, whilst the piece treated with alumina is dyed red, and that with iron, purple.
The Chemistry of Hat Manufacturing Watson Smith

At the end of this time it must be washed, to extract the sulphate of alumina.
A Dictionary of Arts, Manufactures and Mines Andrew Ure

The electricity keeps on working, and now it separates the alumina into its two parts.
Diggers in the Earth Eva March Tappan

It usually contains a little silica, alumina, and oxide of iron.
A Dictionary of Arts, Manufactures and Mines Andrew Ure

To make bottle-glass, oxide of iron and alumina is added to the silica, lime, and soda.
Round-about Rambles in Lands of Fact and Fancy Frank Richard Stockton

The affinity of alumina for the colouring matter is very remarkable.
A Dictionary of Arts, Manufactures and Mines Andrew Ure

The liquors are previously mixed with a little milk of lime, which frees them from ferric oxide, alumina, and silica.
Cooley’s Cyclopdia of Practical Receipts and Collateral Information in the Arts, Manufactures, Professions, and Trades…, Sixth Edition, Volume I Arnold Cooley

The alumina may be separated from the ferric oxide by potash-lye.
A Dictionary of Arts, Manufactures and Mines Andrew Ure

noun
another name for aluminium oxide
alumina
(ə-l’mə-nə)
Any of several forms of aluminum oxide used in aluminum production and in abrasives, refractories, ceramics, and electrical insulation. Alumina occurs naturally as the mineral corundum and, with minor traces of chromium and cobalt, as the minerals ruby and sapphire, respectively. In its hydrated form it also occurs as the rock bauxite. Also called aluminum oxide. Chemical formula: Al2O3.