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Principles of gem heat treatment technology

Time：2020-10-09 13:11 Click：

　　There are many internal changes in gems during heat treatment. The basic principle of heat treatment is to find and use those changes that are conducive to showing the potential beauty of jewelry and jade through repeated trials and repeated corrections. Gem heat treatment equipment: box furnace, vacuum furnace
According to the mechanism of internal changes of gems during heat treatment, the principles of heat treatment are divided into the following categories for detailed explanation:

(1) Change the color of the gemstone to produce a color change.
These chemical components can be the main components of gems, or they can be trace color-causing elements of gems.
For example: for organic gemstones such as pearls, ivory, coral, amber, etc., heat treatment will oxidize the organic matter in them, and if the temperature is too high, the color will gradually darken and become black. If you continue to heat it, the phenomenon of "carbonization" will appear. People often use this feature to master the temperature and implement the "distressed" treatment of organic gems. For gemstones, heat treatment tends to oxidize the low-valence cations into high-valence states, thereby changing the color. The most typical example is that aquamarine with a green hue is heated in the air to remove the green color and turn the color into blue.

(2) The original color center of the gem is destroyed and the color changes.
The color of some gemstones is mainly caused by the color center. The color center is a structural defect that can produce color. It can be understood as "the electron falling into the hole trap absorbs a certain energy in the visible light to produce an electron transition to display the color" center. The concentration of hole traps is different, and the displayed color is different. If hole traps of different depths exist at the same time, the color will show a mixed color. Heating this type of gemstone is equivalent to adding a certain amount of energy to the electrons falling into the trap, so that the electrons in the color center can be excited to a higher energy level. If the energy given by the outside world exceeds the trap energy, the electrons in the trap will jump out of the trap and escape, the color center of the trap energy is destroyed and the color is eliminated. People use this principle to master the heating temperature and time, eliminate the color center color of the trap energy, and leave the color center color with high trap energy to achieve the purpose of improving the color.
For example, the irradiation method changes the color of blue topaz. When the colorless topaz is processed by irradiation, the sample is brown-tan. This is caused by the mixing of different colors produced by the color centers of different trap energy. The low trap energy is eliminated by heat treatment. The color center, you can get a beautiful sea blue. Pink topaz turns yellow, amethyst turns yellow or green, and smoky crystal turns yellow-green or colorless, which are also caused by heat treatment changing the color center.

(3) Diffusing impurities in the gemstone or changing the existing state to change the color.
There are chromogenic ions in some gemstones, but due to their bad state, the color of the gemstone is not good or can not cause color. Heating can make the chromogenic ions diffuse uniformly in the gemstone and enter the lattice particle position or lattice defect, thereby changing The color of the gem. For example, brown rubies are heated to become red rubies, and white sapphires are heated to become blue sapphires. The color-causing ions in some gems are in a polymerized state, which makes the color not beautiful. After heating and diffusion, they can form beautiful colors.
For example, a rough stone produced in Sri Lanka is called "Geuda" corundum. Before treatment, it is a translucent, milky white, silky luster corundum. It is not a gem and is often used to pave garden paths and decorate. In flower beds, the titanium element in this corundum exists in the form of rutile mineral. If high temperature heating is applied, the rutile mineral can be melted (the corundum mineral does not melt at this time). , A beautiful blue sapphire.

(4) Dehydrate some water-bearing gemstones and cause color changes.
Some gems not only contain adsorbed water, but also contain structured water. In the process of heat treatment optimization, if the temperature does not destroy the structured water, the color change task can be completed; if the heating temperature is too high, the structured water will be driven out and the gems will be dehydrated. , Thereby destroying the structural stability of the gemstone. Of course, some gems will change color at this time, but this kind of color change is often undesirable. If the beautiful opal is heated to about 300 ℃, it will lose water, thereby destroying its color changing effect. Therefore, the heating temperature must be mastered when heat treatment is used to optimize the color of jewelry and jade. The pink chalcedony turns orange, red or brown; the heating of tiger's eye stone produces dark brown to reddish brown and is related to dehydration.
(5) Change the crystal configuration of some gems.
As the temperature rises, some gemstones will change the type of lattice structure and thus change the color. For example, heating can transform low-type zircon into high-type zircon, the color changes from brown-brown red to colorless and transparent. People often use this method to obtain colorless zircon with high refractive index as a diamond imitation; if in a reducing environment Heating can also get charming light blue-blue zircon.

(6) Reorganize, regenerate and purify certain gems to achieve the goal of optimization.
For organic gemstones such as amber, heat treatment at a lower temperature can soften or melt it. After cooling, it will become amber with high transparency and pure texture. If it is pressurized during softening, a beautiful bursting pattern will appear, usually Call it "sun ray".

(7) Eliminate inclusions in gemstones and improve the transparency and clarity of gemstones.
There are often inclusions in gems, which not only affect the clarity of the gem, but sometimes also affect the transparency of the gem. High temperature heat treatment (usually close to the melting point of gems) can melt or eliminate impure inclusions in gems, so as to improve the transparency and clarity of gems. For example, heat treatment of rubies can remove mercerization. According to data, many of the high-end rubies and blue sapphires sold on the market are processed at high temperatures.

(8) Sudden changes in temperature may cause cracks in the jewelry and jade.
Since most jewels are poor thermal conductors, the coefficient of thermal expansion is relatively small. When the heating speed is too fast or the cooling process is too fast, internal stress is generated and cracks are easily generated. In addition, some jade contains more gas-liquid inclusions, At high temperatures, these inclusions may burst, resulting in cracks or fingerprint-like inclusions. For example, the "halo" around inclusions in sapphire. We often use this feature of gemstones to create many effects, such as halo quartz.

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