Surface analysis of heat-treated Mong Hsu rubies

Mong Hsu rubies have been heat treated in air at 1100, 1200, 1300, 1400, 1500 and 1600 °C. Their visual appearance and surface analysis (XPS) after each stage of heating have been monitored. The characteristic blue core regions of untreated ruby become slightly faded at 1100 °C and completely disappear at temperatures above 1500 °C. Trace amounts of Na, Ca, Si and Fe were found on the surface of untreated stones. Ti was first detected after heating to 1100 °C. Plots of detected surface concentrations of elements versus temperature show that the highest concentration of Fe occurred at 1300 °C while surface concentrations of Ti appeared to show two maxima near 1300 and 1500 °C. The results suggest that both the changing oxidation state of Fe2+ to Fe3+ and the diffusion of the Fe and Ti ions with temperature are responsible for the color changes through decreasing Fe2+ to Ti4+ charge transfer.