Optimization treatment
heat treatment
Heat treatment can change the color and type of zircon.
1, change color
Zircon from different places will appear different colors after heat treatment.
1) can generate blue or colorless zircon by heat treatment under reducing conditions. The most important one is Vietnamese reddish-brown zircon, which produces colorless, blue and golden yellow after heat treatment, and is the most common variety in gem jewelry.
The heat treatment steps are as follows:
Firstly, put the sample into a closed crucible (usually 1kg), put it into a furnace, and heat it at 900 ~ 1000℃ under reduced pressure, so that about 30% of the sample can reach the gem level. The remaining 70% blue zircon with poor color but good clarity can produce colorless zircon after further heating. The purpose of this thermal step is to remove earthy tones from zircon, thus simultaneously producing colorless zircon and white fog effect.
2) After heat treatment under oxidation conditions, golden yellow and colorless zircon can be produced when the temperature reaches 900°C, and some samples can be red. Samples that have not reached the gem-grade color after the above two steps of heat treatment can also be heat-treated into colorless or golden zircon under this oxidation condition.
Colorless, blue, yellow and orange zircon can be obtained by heat treatment. Zircon is still regarded as a natural gem in jewelry appraisal because no other substances are added during the optimization of heat treatment. Another treatment method (this method is generally rare) is to put zircon, nitric acid and cobalt, potassium and iron salts together and burn them in a crucible for 6-8 hours. Zircon produced by this treatment method can change color again under extreme high temperature and pressure conditions. For example, blue zircon can be transformed into golden yellow or bright red under extreme high temperature and pressure. Sometimes, this partially converted zircon can be completely converted into blue by putting it in a crucible filled with charcoal and heating it to 800 ~ 900℃ again. This phenomenon is closely related to the difference of impurity composition in zircon caused by different producing areas.
Step 2 change the type
After heating to 1450℃ and continuous heat treatment, silicon and zircon can be recrystallized, and low-type zircon can be changed into high-type zircon. After this treatment, the density of low, medium and high zircon can be increased (up to 4.7g/cm3), the refractive index is high, the absorption line is clear, and the transparency and brightness are also improved. The recrystallization caused by heat treatment can also produce fibrous microcrystals and form cat's eyes.
After heat treatment, the surface or edge of zircon is fuller and the gloss is improved to some extent.
Irradiation therapy
The results of irradiation treatment and heat treatment of zircon are opposite, and they are opposite changes. After irradiation (X-ray, γ-ray, high-energy electron, etc.), the high-type zircon modified products obtained by heat treatment can almost restore the color before heat treatment, even darken the color. ). Natural zircon will also change color under irradiation. For example, colorless zircon can become crimson, maroon, purple or orange under X-ray irradiation. Blue zircon will turn brown-red under X-ray irradiation. However, the discoloration process of irradiated zircon is reversible and can be restored to its original state under extreme high temperature and pressure.