Verneuil Process

Verneuil process, also called flame-fusion process, method for producing synthetic rubies and sapphires. Originally developed (1902) by a French chemist, Auguste Verneuil, the process produces a boule (a mass of alumina with the same physical and chemical characteristics as corundum) from finely ground alumina (Al₂O₃) by means of an inverted oxyhydrogen torch that opens into a ceramic muffle. With slight modifications, this method is used to produce spinel, rutile, and strontium titanate.

Highly purified alumina is placed in a container with a fine sieve at its base. When the container is tapped by a mechanically-activated hammer, the alumina sifts down into the enclosed chamber. Oxygen passes into this chamber and carries the fine alumina particles into the intense heat of the central part of an oxyhydrogen flame, where they fuse and fall on the molten upper surface of the boule as droplets. Flame characteristics and the rate of powder feed and boule lowering are adjusted to produce a boule of uniform diameter. The temperature of the upper surface of the boule is held just above the melting point, which for colourless sapphire is 2,030° C (about 3,690° F). When a boule reaches the desired size, normally 150 to 200 carats, the furnace is shut down, and the boule is cooled.

Strain develops during cooling, because the outer surface cools faster than the interior; this phenomenon causes considerable loss from cracking during the manufacturing process. The strain is relieved by splitting the boule longitudinally, which is induced by snapping off its elongated stem. Some residual strain not disadvantageous for gem and most industrial uses is left in the half-boule developed by splitting. Strain-free, whole boules may be produced by annealing at 1,950° C.