Synthetic sapphire (Verneuil flame fusion process)

In 1902, the French chemist Auguste Verneuil developed a process for producing synthetic sapphire crystals.[20] In the Verneuil process, named for him, fine alumina powder is added to an oxyhydrogen flame, and this is directed downward against a mantle.[21] The alumina in the flame is slowly deposited, creating a teardrop shaped "boule" of sapphire material. Chemical dopants can be added to create artificial versions of the ruby, and all the other natural colors of sapphire, and in addition, other colors never seen in geology. Artificial sapphire material is identical to natural sapphire, except it can be made without the flaws that are found in natural stones. The disadvantage of Verneuil process is that the grown crystals have high internal strains. Many methods of manufacturing sapphire today are variations of the Czochralski process, which was invented in 1916.[22] In this process, a tiny sapphire seed crystal is dipped into a crucible made of the precious metal rhodium,[23] containing molten alumina, and then slowly withdrawn upward at a rate of one to 100 mm per hour. The alumina crystallizes on the end, creating long carrot-shaped boules of large size, up to 400 mm in diameter and weighing almost 500 kg.[24]

Synthetic sapphire is industrially produced from agglomerated aluminum oxide, sintered and fused in an inert atmosphere (hot isostatic pressing for example), yielding a transparent polycrystalline product, slightly porous, or with more traditional methods such as Verneuil, Czochralski, flux method, etc., yielding a single crystal sapphire material which is non-porous and should be relieved of its internal stress.