Verneuil method

History

Since the study of alchemy began, there have been attempts to synthetically produce precious stones, and ruby, being one of the prized cardinal gems, has long been a prime candidate. In the 19th century, significant advances were achieved, with the first ruby formed by melting two smaller rubies together in 1817, and the first microscopic crystals created from alumina (aluminium oxide) in a laboratory in 1837. By 1877, chemist Edmond Frémy had devised an effective method for commercial ruby manufacture by using molten baths of alumina, yielding the first gemstone-quality synthetic stones. The Parisian chemist Auguste Verneuil, who applied to work with Frémy at age 17, became Frémy's assistant in 1876 and helped develop the method, but soon went on to independently develop the flame fusion process, which would eventually come to bear his name.

One of Verneuil's sources of inspiration for developing his own method was the appearance of synthetic rubies sold by an unknown Genevan merchant in 1885. These "Geneva rubies" were determined to be artificial at the time but are now believed to be the first rubies produced by flame fusion, predating Verneuil's work by several years. After being introduced to the "Geneva rubies" by a mineralogist at the Natural Museum of History in Paris, Verneuil came to the conclusion that it was possible to recrystallise finely ground aluminium oxide into a large gemstone. This realisation, along with the availability of the recently developed oxyhydrogen torch and growing demand for synthetic rubies, led him to design the Verneuil furnace, where finely ground purified alumina and chromium oxide were melted by a flame of at least 2,000 °C (3,630 °F), and recrystallised on a support below the flame, creating a large crystal. Verneuil documented his work in sealed documents during the 1890s and publicly announced his work in 1902, publishing details outlining the process in 1904.

By 1910, Verneuil's laboratory had expanded into a 30-furnace production facility, with annual gemstone production by the Verneuil process having reached 1,000 kg (2,200 lb) in 1907. By 1912, production reached 3,200 kg (7,100 lb), and would go on to reach 200,000 kg (440,000 lb) in 1980 and 250,000 kg (550,000 lb) in 2000, led by Hrand Djevahirdjian's factory in Monthey, Switzerland, founded in 1914. The most notable improvements in the process were made in 1932, by S. K. Popov, who helped establish the capability for producing high-quality sapphires in the Soviet Union through the next 20 years. A large production capability was also established in the United States during World War II, when European sources were not available, and jewels were in high demand for their military applications such as for timepieces.

The process was designed primarily for the synthesis of rubies, which became the first gemstone to be produced on an industrial scale. However, the Verneuil process could also be used for the production of other stones, including blue sapphire which required oxides of iron and titanium to be used in place of chromium oxide. The basic process can be used to form even more elaborate gemstones such as star sapphires, where titania (titanium dioxide) was added and the boule was kept in the heat longer, allowing needles of rutile to crystallise within it. In 1947, the Linde Air Products division of Union Carbide pioneered the use of the Verneuil process for creating such star sapphires, until production was discontinued in 1974 owing to overseas competition.

Despite some improvements in the method, the Verneuil process remains virtually unchanged to this day, while maintaining a leading position in the manufacture of synthetic corundum and spinel gemstones. Its most significant setback came in 1917, when Jan Czochralski introduced the Czochralski process, which has found numerous applications in the semiconductor industry, where a much higher quality of crystals is required than the Verneuil process can produce. Other alternatives to the process emerged in 1957, when Bell Labs introduced the hydrothermal process, and in 1958, when Carroll Chatham introduced the flux method. In 1989 Larry P Kelley of ICT, Inc. also developed a variant of the Czochralski process where natural ruby is used as the 'feed' material.