corundum, China.12, treatments.6, gems

Wang Chuanfu, Yang Yaoshan, and Li Guoxun compare conventional heating with an electrolytic oxidation treatment intended to improve the appearance of dark sapphire from Shandong Province, China. Much of the available corundum is described as unsuitable for gemstones because of its dark colour and pronounced pleochroism, appearing bluish violet perpendicular to the crystallographic c-axis and green or greenish yellow parallel to it. High-temperature heating can improve apparent clarity but commonly produces commercially undesirable bluish-grey or grey stones. The authors investigate whether stronger oxidation at a substantially lower temperature can lighten the material while retaining a desirable blue hue. Their alternative treatment uses a molten fluoride electrolyte containing cryolite, aluminium fluoride, calcium fluoride, titanium dioxide, and alumina near its saturation concentration. Sapphire specimens are attached to a platinum anode and immersed in the bath at 940 °C, with a carbon crucible serving as the cathode. Electrolysis is conducted at 3.0 V for 30–45 minutes. For comparison, other specimens are heated in carbon crucibles in air at 1,700 °C for ten hours. Treated samples are prepared with parallel polished faces and their absorption spectra measured perpendicular to the c-axis. Most electrolytically treated specimens change from dark to light blue and show improved clarity without the grey component observed after conventional heating. However, slight dissolution in the molten salts causes an approximately 8% weight loss. High-temperature treatment produces only modest clarity improvement and generally bluish-grey or grey colour. The proposed explanation centres on oxidation of ferrous to ferric iron, together with differences in titanium retention. The authors associate spectral changes near 450 nm and beyond 570 nm with increased ferric iron and interpret them as evidence of oxidation. In conventionally heated specimens, diminished absorption near 550 nm and titanium detected in the crucible are taken to indicate some titanium loss, which they link to greying. At 940 °C, titanium diffusion out of the sapphire is considered more restricted; the titanium-rich electrolyte may also permit inward diffusion. Persistence or strengthening of the approximately 550 nm absorption is presented as consistent with this possibility. The study concludes that molten-salt electrolysis can improve the colour of these dark sapphires through oxidation while retaining, or potentially increasing, titanium. Its mechanistic interpretation is offered as a model based on the observed appearance, absorption spectra, and limited supporting chemical evidence.