What is corundum?

Corundum is a very pure mineral that belongs to the mineralogical category of oxides. In fact, it represents the crystalline form of aluminium oxide (Al₂O₃). The term corundum derives from the Sanskrit kuruvinda (ruby). In 1725 John Woodward named this mineral corinvindum, but the current name was coined by the Irish Richard Kirwan in 1794.

Corundum does not contain respirable crystalline silica, is non-toxic and absolutely free of substances harmful to health. It has physical characteristics that should allow a rapid recognition of this mineral:

• Regular angular shape
• Resistant to high temperatures; corundum melts at a temperature of at least 2000°C
• Insoluble for all acids; even corrosive chemical agents find it difficult to dissolve corundum
• High hardness, corundum ranks second in hardness after diamond; 9 Mohs, only one point lower than diamond
• Allochromatic; it is present in all colours. Its colouring depends on impurities that do not alter its chemical composition

Types and applications

Corundum crystals are used in different applications based on its purity and particle size. The common varieties of corundum are mainly used in the preparation of abrasives while the valuable ones are used as gems.

NATURAL CORUNDUM

Corundum is present in nature as a mineral and some of its natural crystals are used in the jewellery sector as precious stones among the most sought after diamonds. Depending on their colour, they take on different names: sapphire (blue), ruby (red), emerald (green), amethyst (purple). However, for a long time now, jewellers all over the world have been full of artificial gems.

SYNTHETIC CORUNDUM

Corundum can be easily produced artificially thanks to a sintering process. The result is a synthetic corundum, a product with exceptionally hard and impact-resistant grains which finds application in various industrial sectors thanks to its low price compared to the price of natural corundum.

Also, in this case the colour gives the corundum different roles:

• Toughness and the ability to quickly abrade any type of material make brown (or red-brown) corundum grains to stand out. The brown red corundum is used for sandblasting with recycling system thanks to their high impact resistance during the removal of rust, oxides and burrs from iron and steel constructions. They are also suitable for deburring, anti-reflective finishing, surface roughening and pre-painting. No less important is the use of corundum in the ceramic sector where it is used as an anti-slip and anti-wear additive in glazes for floor tiles and in the production of ceramic products thanks to its surface hardness. The different grain sizes available allow users to choose the degree of engraving suitable for every need.

• The white coloured grains are less hard but purer than the red-brown ones. Being less aggressive, they are often used as an abrasive to obtain perfect surfaces in the surface processing of glass, noble metals, plastics and stainless steel. White corundum grains can be used at high abrasion rates which thanks to its characteristics avoids the harmful heating of sensitive materials such as refractories, steels and metals of various kinds during processing such as sharpening, polishing, grinding and precision milling.