Flame fusion

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The flame fusion method (also known as the "Verneuil process") to grow synthetic gemstones was created in 1893 by Victor Auguste Louis Verneuil (1856-1913) as a means to grow synthetic corundum. In 1904 Verneuil shared his knowledge with the world in a publication in the French journal "Extrait des Annales de Chimie et de Physique" and his technique is still used today, either with or without slight adjustments, to grow a variety of synthetic gems.
Due to the high polish this synthetic corundum can take (and its relative high hardness) the produced crystals were, and still are, in high demand by the watch making industry. Either as bearings or as watch glasses.

Simplified flame fusion (Verneuil process) diagram to grow synthetic corundum.

In order to grow synthetic corundum (or another mineral that is suitable), a feed of source powder is held in a container with a sieve like floor. On tapping with a hammer, the container releases a certain amount of the powder that is fed into a chamber and mixed with compressed oxygen. One can compare this with tapping against a kitchen sieve to dose flour or powdered sugar.
The powder with the extra oxygen falls down into an ignited oxyhydrogen flame (at around 2200° C) and the feed powder starts to sinter onto a ceramic rod (named a "candle"). At first the powder is allowed to create a "starter" mass on which the rest of the sintered powder will grow.
When the "starter" boule has formed, the rate at which the hammer hits the powder container is decreased and the oxygen/hydrogen mixture is adjusted to the most favorable circumstances. The ceramic candle is lowered from the flame at a constant rate to keep the top almost in the flame to let the boule grow.


References

  • Gemology - An Annotated Bibliography - Vol.2 (1993) - John Sinkankas ISBN 0810826526