Optimization of heat treatments of V̇ietnamese rubies

Abstract

A suitable heating condition designed for a particular type of ruby is used to enhance the quality to increase its value. Heating Vietnamese rubies in an oxygen atmosphere may improve color and clarity, and thus increase their prices. The color of rubies is due to the presence of trace amounts of Cr 3+. Fe3+ yields a pale yellow color and the charge transfer mechanism between Fe2+ and Ti4+ gives ruby an undesirable bluish color. Reducing this mechanism is possible by heating the ruby in oxygen so that most of the Fe2+ is converted into Fe 3+ ions and thus they appear a more intense red. By using XRD we found that the c/a ratio of the hexagonal structure was smallest after heat treatment at 1300°C. The number of Fe2+ ions converted to Fe 3+ was detected by an electron spin resonance spectrometer and found to increase with temperature. The ruby appeared a most intense red after heating at 1500°C for 12 hours. The color change is due to both the decrease in c/a ratio and the increase in the number of Fe3+ ions. ESR experiments on ruby crystals by rotation about their c-axis show that Fe3+ ions are a little off axis before heat treatment. After heat treatment at 1300°C they move to new asymmetric equilibrium positions towards larger O2-triangles. It is not clear, however, if this Fe3+ movement is related to the change in color. The asymmetry may arise due to some O2-vacancies.