Comparison of Heat Treatment at High Temperature and Low Temperature on Ruby from Yen Bai, Vietnam

The comparative experiments of heat treatment at high temperature (1 600℃)and low temperature (1 000℃) on rubies with blue and violet hue from Yen Bai, Vietnam, were carried out according to the colour saturation.In addition, the main technology parameters were obtained, including the heat treatment temperature, holding time, and fluxing agent, to explore the best heat treatment scheme and compare the applicable range of the two sets of temperature parameters.The results showed that the colour and local transparency of the ruby samples improved obviously after heat treatment at high temperature.The optimum holding time of ruby samples with light purplish red, purplish red and deep purplish red got longer from 6 hours to 12 hours with the increase of the colour saturation.And the mixture of borax and strong adsorptive γ-Al₂O₃ can be used to help adsorb the foreign ions to remove the internal tawny bands effectively.While only keeping the holding time for over 12 hours can help improve the colour of the ruby samples obviously at low temperature.Contrasting the effect of heat treatment at different stages, ruby samples with fissures were not suitable for low temperature heat treatment in terms of transparency.For ruby samples with few fissures and off-colour, the effect of low temperature heating for a long time was similar to that at high temperature for a short heating time, however, from the view of market acceptance, low temperature heat treatment is more easily accepted due to its fuzzy identification characteristics.UV-Vis absorption spectra of ruby samples were tested for verifying the results of colour alteration.