Sapphire
Produced by the Verneuil-Process (flame melting): An ammonium-aluminum alum is melted by a hydrogen gas compressor. The flame is aligned straight downward and heats the top of the growing crystal. The flame and the molten material that steadily hits the top surface creates a molten film of Al2O3. On the bottom of this film, crystallization slowly takes place. Using temperatures above 2000 °C (3632 °F), the crystal grows over several days. During this time, growth conditions must be kept constant to receive a high quality crystal.
- Transmission range: 0.5-4.5 µm
- High hardness
- Small refractive index
- Parallelism: 3 arcminutes
- Planarity: 2 wavelengths at 633 µm
- Surface quality: 60/40
- Bakeable up to 350 °C (662 °F)
