Skull Melting Process (Synthesizing Synthetic Cubic Zirconia)

This process was perfected in USSR specifically for crystallizing synthetic cubic zirconia, for uses in optical, electronic and laser equipment. Cubic zirconium oxide has a very high melting point and is a very reactive material. No container can hold this melt since cubic zirconia has a melting point of 2750°C, and hence a cold crucible or skull is used.

How gemstones are synthesized by skull melting process?

• A container is filled with powdered zirconia, stabilizer and some pieces of zirconium metal.
• These are rapidly heated by a radio frequency field (similar to a microwave oven).
• The zirconium oxide powder is used as its own insulation and container as the temperature at the core is raised to extreme levels.
• The metal reacts with the oxygen and melts, with the exception of a skin of about 1/15 inches thick at the outside, which is kept solid by contact with the liquid – cooled copper tubes around – the skull.
• Zirconia and the stabilizer contents are kept molten for several hours to provide uniformity.
• The power is reduced and the skull is slowly lowered out of the coil.
• Crystal growth begins at the bottom of the skull and columnar crystals grow upwards until the entire melt solidifies.
• The individual single crystal columns that are readily available are generally 2 inches in diameter.
• Light tapping will separate the crystal columns.
• Dopants are used to produce various colours.

Identification:

• Colourless when pure (without any dopants) – diamond simulant.
• The S.G. ranges from 5.60 to 6.20 (as per dopants used)
• Isotropic character.
• The dispersion is 0.060
• Some varieties exhibit a strong yellow fluorescence under ultra violet light.
• The characteristic inclusions seen are negative crystals, streams of zirconium oxide powder which appear as tiny white spherical bubbles.