Aluminum Trioxide, Alumina (Al2O3)

Formula: Al2O3

Melting point: 2072ºC

Density: 3.987 g/cc (all modifications)

Solubility: Only in HF and hot bases.

Alumina has been recognized since antiquity through its gemological forms: ruby and sapphire, both varieties of the mineral corundum, which is simply Al₂O₃ in its most stable crystalline form. These precious stones were valued for millennia without their exact composition being known until the development of modern chemistry. During the 19th century, industrial methods for obtaining pure alumina were perfected through the controlled oxidation of metallic aluminum or the calcination of minerals rich in this element, laying the foundation for its mass production. 

Aluminum oxide is a material of extreme hardness (9 on the Mohs scale), surpassed only by compounds such as diamond or silicon carbide (SiC, known as moissanite in its gem form). It possesses great toughness, high abrasion resistance, and outstanding thermal behavior, positioning it as one of the most important technical ceramics.

The term “alumina” applies to the polycrystalline, milky-white form that is industrially produced in large volumes. Meanwhile, “corundum” is the natural or synthetic monocrystalline phase of Al₂O₃, notable for its transparency and structural purity. Although their structures differ, both forms share practically identical mechanical and chemical properties: high thermal stability, chemical inertness, and excellent wear resistance.

A special variant, activated alumina, is porous and hydrophilic, capable of adsorbing marginal amounts of H₂O. It is used as a desiccant and should not be confused with dense alumina, which is completely inert to water and exhibits no appreciable solubility.

Corundum, with a density close to 4 g·cm⁻³, is denser than metallic aluminum because the Al–O bonds are shorter and stronger than the Al–Al bonds, allowing more atoms to be packed per unit volume. This constitutes a notable exception compared to many other metallic oxides, which are usually less dense than their parent metals.

Al₂O₃ is extensively used as an abrasive, in crucibles, in laboratory parts resistant to corrosion and heat, and in high-performance ceramic components. Due to its unique combination of hardness, toughness, and thermal stability, it is one of the most economical and, at the same time, most versatile technical ceramics.

The monocrystalline form (corundum) can be synthesized with high purity in laboratories using processes such as the Verneuil method or plasma beam melting. Thanks to this, blocks of synthetic sapphire of up to almost one cubic meter are manufactured, used in high-strength optical windows, lenses for laser systems, electronic substrates, and transparent components subjected to extreme environments. The “synthetic sapphire” found in high-end watch faces is actually monocrystalline Al₂O₃, identical or superior to natural sapphire in quality and purity.

çSimilarly, mechanical watches that claim to have “20 or 23 rubies” use synthetic rubies (corundum doped with chromium to give the characteristic red color) as high-precision bearings. These crystals minimize wear on internal axles, offering minimal friction and outstanding durability.

Ruby and sapphire do not differ in their chemical structure: both are pure corundum, and the color is due to traces of transition metals. Chromium imparts the red hue of ruby, while the combination of iron and titanium produces the blue of sapphire; other impurities generate yellow, green, violet, or pink colors.

Today, aluminum trioxide is indispensable in sectors ranging from jewelry to metallurgy, microelectronics, and advanced engineering. Its natural abundance, ease of synthesis, and balance between cost and performance make it one of the world's leading technical ceramics.