gem heat treatment technology principles

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In the process of heat treatment, there are many changes in the interior of gem. The basic principle of heat treatment is to discover and use those changes that help to show the potential aesthetic feeling of jewelry and jade through repeated tests and corrections. Gem heat treatment equipment: box furnace, vacuum furnace; according to the internal change mechanism of gem in the heat treatment process, the heat treatment principle is divided into the following categories for detailed description:

(1) Change the color of a gem to produce a color change.

These chemical components can be the main components of gemstones, and can also be the trace chromogenic elements of gemstones.

For example, for organic gemstones such as pearls, ivory, corals and amber, the organic matter in them will be oxidized by heat treatment. If the temperature is too high, the color will gradually darken and turn black. If heating continues, the phenomenon of “carbonization” will appear. People often use this function to control the temperature and implement the “distress” treatment of organic gems. For gemstones, heat treatment tends to oxidize low valence cations into high valence states, thus changing the color. The most typical example is heating Aquamarine with a green hue in the air to remove the green and turn it 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. Color center is a structural defect that can produce color. It can be understood as the center of “the electron falling into the hole trap absorbs a certain amount of energy in the visible light to produce the electron transition to display the color”. The color of the hole trap is different with the concentration. If there are hole traps of different depths at the same time, the color will show mixed color. Heating this type of gem is equivalent to adding a certain amount of energy to the trapped electrons, which can excite the electrons in the color center to a higher energy level. If the energy provided by the outside exceeds the trap energy, the electrons in the trap will jump out of the trap and escape, the color center of the trap energy will be destroyed and the color will be eliminated. People use this principle to master the heating temperature and time, eliminate the color center color of trapping energy, and make the color center color with high trapping energy achieve the purpose of improving color.

For example, the irradiation method changes the color of blue topaz. When the colorless topaz is irradiated, the sample is brown. This is due to the different color mixing caused by the color center of different trapping energy. The problem of low trap energy is eliminated by heat treatment. In the color center, you can get a beautiful sea blue. Pink Topaz turns yellow, Amethyst turns yellow or green, smoked crystal turns yellow green or colorless, which is also caused by the change of color center caused by heat treatment.

(3) To change the color of a gem by diffusing impurities or changing its existing state.

Some gemstones have chromogenic ions, but because of their poor state, the color of gemstones is not good or can not cause color. Heating can make the chromogenic ions diffuse evenly in the gem and enter into the lattice particle position or lattice defects, thus changing the color of the gem. For example, heating a brown Ruby into a red ruby, heating a white sapphire into a blue sapphire. In some gemstones, the color causing ions are polymerized, which makes the color unsightly. After heating and diffusion, they can form beautiful colors. For example, coarse stone produced in Sri Lanka is known as “geuda” corundum. Before treatment, it is translucent milky white filiform corundum luster. It’s not a gem. It’s usually used for paving and decoration. In the flower bed, the titanium element in corundum exists in the form of rutile mineral. If heated at high temperature, rutile minerals may melt (corundum minerals do not melt at this time).

(4) Dehydrates some hydrous gemstones and causes color changes.

Some gemstones contain not only adsorbed water but also 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 discharged, and the gem will be dehydrated. Thus, the structural stability of the gem is destroyed. Of course, some gemstones will change color at this time, but this color change is usually not desirable. If the beautiful opal is heated to about 300 ℃, it will lose moisture, thus destroying its discoloration effect. Therefore, when using heat treatment to optimize the color of jewelry and jade, it is necessary to master the heating temperature.

(5) Change the crystal structure of some gemstones.

With the increase of temperature, some gemstones will change the type of lattice structure, thus changing the color. For example, heating can transform low type zircon into high type zircon, and the color changes from brown red to colorless transparent. People often use this method to obtain colorless zircon with high refractive index as imitation diamond. If heated in a reductive environment, attractive light blue zircon can also be obtained.

(6) In order to achieve the goal of optimization, some gemstones were reorganized, regenerated and purified.

For amber and other organic gemstones, heat treatment at a lower temperature will make them soften or melt. After cooling, it will become amber with high transparency and pure texture. If it is pressed during the softening process, a beautiful cracking pattern will appear, commonly known as “sunshine”.

(7) The inclusion in gemstone is eliminated, and the transparency and clarity of gemstone are improved.

Gemstones usually contain inclusions, which not only affect the transparency of gemstones, but also sometimes affect the transparency of gemstones. High temperature heat treatment (usually close to the melting point of gemstones) can melt or eliminate impure impurities in gemstones, so as to improve the transparency of gemstones. For example, heat treatment of ruby can eliminate mercerization. According to the data, many high-end Ruby and sapphire sold on the market are processed at high temperature.

(8) Sudden temperature changes can cause jewelry and jade to break.

Since most jewelry 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 will be produced and cracks are easy to occur. In addition, some jades contain more gas-liquid inclusions. At high temperatures, these inclusions may rupture, leading to cracks or inclusions similar to fingerprints.