Research on Heat Treatment of Ruby from Burma and Vietnam

This research aimed to investigate the heat treatment changing the colour and clarity of natural rubies from Burma and Vietnam.The samples are rosy with purple and bluish violet hue and have much fracture.To support experiment with the preparation,the natural rubies were analyzed by LIBS and EDXRF to get the main elements and the colour causing elements.It shows that there are Al,Fe,Cr,Ca,Mg,Ti and Si and the purple and bluish violet is caused by Fe and Ti ions.It is known that the Cr³⁺is responsible for the red colour of ruby and the d-dtransition of Cr³⁺leads to absorption band at 410and 540nm and absorption line at 692nm.The bluish violet colour in ruby is the result of intervalence charge transfer between Fe²⁺and Ti ⁴⁺ions and it causes absorption at 564nm.In nature,Fe presents as Fe³⁺ions and substitutes Al ³⁺,giving a slight yellow colour in the ruby.However,it affects little the red colour of the ruby.The heat treatment was divided into two parts according to the temperature setting and components of fluxing agent. One is in an oxidizing atmosphere with the fluxing agent of sodium borate at temperature of 1 500 C and 1 580 C. After each heat treatment, USB 4000 was employed to measure the absorption spectrum. The results suggested that after heat treatment, the colour of samples changed from deep purple red to slight purple red, but the change is not much obvious. In additional, when the temperature was setting at 1 500℃, there was some white substance on the surface of samples. And the white substance entered into the fractures when the temperature was setting at 1 580℃ The element of white substance was tested to be Na. It illustrated that the temperature of 1 580℃ contributed to the fluxing agent diffusion into the ruby. The absorption caused by Cr³⁺ according to the result of USB 4000 was enhanced, which can illustrate that after heat treatment the bluish violet hue diminished, The other part of the heat treatment was also setting in an oxidizing atmosphere at the temperature of 1 580℃, but the difference is in fluxing agent added, including none fluxing agent,the fluxing agent of sodium borate and the fluxing agent of sodium borate and alumina.In natural, the sodium borate fluid melts Al₂O₃ very well in the high temperature. The γ-Al₂O₃, has high activity and strong adsorption features,which makes Fe and Ti precipitation easier and the gemstones brighter and more transparent, Compared with the results of heat treatment in three different fluxing agent, it showed that the purple and bluish violet hue diminished most and the samples had a brilliant rosy appearance under the condition of the sodium borate and alumina fluxing agent, However the rubies were broken into three or two pieces, and there was white fluxing agent residue at the appearance of the rubies. The transparency was improved but not very well. After heat treatment, USB 4000was employed to measure the absorption spectrum and it showed that the absorption spectrum caused by Cr³⁺ was enhanced. Comparing with the absorption spectrum of the heat treatment rubies, it suggested that the red absorption weakened the violet, yellow and green absorption region were stronger under the sodium borate and alumina fluxing agent condition, and in this condition the colour of the treated rubies were the most beautiful.According to the study, it can be concluded that it is important to diminish the purple and bluish violet hue and make the rosy colour more brilliant in the oxidizing atmosphere with the fluxing agent of sodium borate and alumina at the temperature of 1 580℃. It is believed in the literature that the heat treatment with fluxing agent can improve transparency.However, in this observation the transparency of samples does not enhance much, because there are a lot of fractures in the samples.