- Also called:
- flame-fusion process
- Related Topics:
- boule
- star ruby
- crystal growth
Verneuil process
-
What is the Verneuil process?
The Verneuil process, or Verneuil method, is a way to make large crystals like synthetic rubies and sapphires, spinel, rutile and strontium titanate. French chemist Auguste Verneuil originally developed it in 1902.
In this process, highly purified alumina powder is added to the top of a rod. A flame is directed vertically downward, maintaining the rod's end at or near its melting point. The alumina sifts down into a chamber and is carried by oxygen into an oxyhydrogen flame, where it fuses and falls as droplets onto the boule's (alumina mass) molten upper surface. The temperature is held just above the melting point. The furnace is shut down, and the boule cools when it reaches the desired size.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Why was the Verneuil process invented?
The Verneuil process was invented in 1902 by French chemist Auguste Verneuil for producing synthetic rubies and sapphires. It creates a boule, a mass of alumina possessing the same physical and chemical characteristics as corundum, using finely ground alumina (Al₂O₃) and an inverted oxyhydrogen torch that leads into a ceramic muffle. Slight modifications allow the production of materials like spinel, rutile, and strontium titanate. Before 1940, synthetic corundum was manufactured in Switzerland, Germany, and France. Synthetic ruby, created using a mixture of aluminum and chromium oxides, was the primary product, with the amount of chromium oxide affecting the color.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How does the Verneuil process create synthetic gems?
The Verneuil process, developed in 1902 by French chemist Auguste Verneuil, is a method for creating synthetic gems like rubies and sapphires. The process involves using an inverted oxyhydrogen torch that opens into a ceramic muffle to produce a boule (a mass of alumina with the same physical and chemical characteristics as corundum) from finely ground alumina (Al₂O₃).
In the Verneuil process, highly purified alumina is placed in a container with a fine sieve at its base. When the container is tapped, the alumina sifts down into an 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 as droplets on the molten upper surface of the boule. The temperature of the boule's upper surface is held just above the melting point, which is about 2,030° C for colorless sapphire. When a boule reaches the desired size, the furnace is shut down, and the boule is cooled.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What materials are used in the Verneuil process?
The Verneuil process employs highly purified alumina (Al₂O₃) placed in a container with a fine sieve. Oxygen carries the fine alumina particles into the intense heat of an oxyhydrogen flame, where they fuse and fall on the molten upper surface of the boule as droplets. The temperature of the upper surface of the boule is held just above the melting point, which for colorless sapphire is 2,030° C.
With slight modifications, this method is also used to produce spinel, rutile, and strontium titanate.
1 Britannica SourceCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How does a Verneuil furnace work?
The Verneuil process, developed in 1902 by French chemist Auguste Verneuil, is a method for creating synthetic gems like rubies and sapphires. It involves using an inverted oxyhydrogen torch that opens into a ceramic muffle to produce a boule of alumina.
In this process, highly purified alumina is placed in a container with a fine sieve at its base. When the container is tapped, the alumina sifts down into an enclosed chamber. Oxygen then carries the fine alumina particles into the intense heat of the oxyhydrogen flame, where they fuse and fall as droplets onto the molten upper surface of the boule. The flame and the rate at which the powder is fed, as well as the boule lowering, are adjusted to ensure a uniform diameter. The boule's upper surface is kept just above the melting point, around 2,030° C for colorless sapphire. Once the boule reaches the desired size, typically 150 to 200 carats, the furnace is shut down, and the boule is cooled.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How can you tell the difference between Verneuil synthetic gems and natural gems?
It can be difficult to distinguish between natural and Verneuil synthetic gems. However, there are a few key differences that can help identify them.
Natural gems have microscopic, irregularly shaped gas and liquid inclusions, while synthetic gems usually show microscopic cracks along and normal to the intersection of facets. Under the microscope, colored synthetic stones may show curved lines parallel to the upper growth surface of the boule, representing uneven distribution of pigmentation. Occasionally, especially in blue sapphire, these curved lines are visible to the unaided eye.
Star rubies and sapphires are created by adding titanium oxide to the starting powder and then heat treating them. The synthetic gems have sharper and more distinctly developed stars than the natural crystals.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Verneuil 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.
" height="20" alt="HEW Logo">
Synthetic corundum.
Before 1940 all synthetic corundum was made in Switzerland, Germany, and France. For several years after the discovery of the process of manufacture, all of the production was used for gemstones. Synthetic ruby was the chief product and was produced by using an intimate mixture of aluminum and chromium oxides; 5 percent chromium oxide (Cr2O3) yields a pale-pink boule and 6 percent a deep-red one. The higher the percentage of chromium oxide, the more difficult it is to control boule growth and the greater the loss from cracking on cooling. Blue sapphire is produced by adding iron and titanium, green by cobalt, and yellow by nickel and magnesium oxides.
Star rubies and sapphires, first developed in 1947 in the United States, are made by adding one percent rutile (titanium oxide, TiO2) to the starting powder, forming the boules in the usual manner, and then heat treating them at temperatures between 1,100° C and 1,500° C. The rutile forms small needlelike crystals that are oriented along the hexagonal crystal planes within the boule; these are similar to the same crystals in natural star sapphire. The synthetic gems have sharper and more distinctly developed stars than the natural crystals.
It is very difficult to distinguish between natural and synthetic colourless sapphires. The natural crystals have microscopic irregularly-shaped gas and liquid inclusions, whereas the synthetic gems usually show microscopic cracks along and normal to the intersection of facets. Under the microscope, coloured synthetic stones show curved lines parallel to the upper growth surface of the boule. They represent uneven distribution of pigmentation. Occasionally, especially in blue sapphire, they are visible to the unaided eye.
Synthetic spinel.
Spinel boules have a square cross section with round corners but otherwise are like corundum boules in manufacture, size, and appearance, although they do not develop internal stresses during cooling. They are made in all colours by adding appropriate pigments.
The trusted destination for professionals, college students, and lifelong learners.
SUBSCRIBE
Synthetic rutile.
Synthetic rutile, first produced in 1948 by the Verneuil process, is far superior to the natural material as a gemstone, because natural rutile is dark in colour and the pure synthetic boules may be produced in nearly any shade by the addition of appropriate pigments. Rutile boules have a square cross section similar to spinel boules but rarely exceed 100 carats in weight. When removed from the furnace they are black in colour, because some titanium is still unoxidized; on heating in an oxygen atmosphere, however, the oxygen is slowly absorbed and the colour fades until only a faint yellow tinge remains in unpigmented boules.