corundum, sapphire, Malawi.12, treatments.6, heat treatment, gems

Controlled heating experiments examined whether the colour of pale greenish-blue sapphires from Chimwadzulu Hill, Malawi, could be improved by treatment in air. The material consisted of circular, step-cut stones weighing 0.44–0.69 carats, supplied to the Institute of Geological Sciences for examination. Twenty-four sapphires were divided into eight closely matched colour groups of three stones. Within each group, one stone was retained as an untreated comparison, while the others were heated for one and five hours, respectively, at the assigned temperature. Experiments covered 500–1200 °C in 100 °C increments, using an electric muffle furnace and an unglazed porcelain boat. Heating to the selected temperature took approximately 40 minutes at 500 °C and two hours at 1200 °C; stones subsequently cooled overnight inside the furnace. Visual examination revealed no obvious colour change after treatment at 500–700 °C or after one hour at 800 °C. Five hours at 800 °C weakened the blue colour, while treatment at 900–1200 °C eliminated the residual blue and left the stones very pale green. Examination of pleochroism showed that the untreated sapphires displayed a pale aquamarine-blue ordinary ray and a very pale green extraordinary ray. The initial fading at 800 °C affected the ordinary ray, with no discernible alteration of the extraordinary ray. At 900 °C and above, the ordinary ray became very pale, slightly greyish-green, differing only subtly from the slightly yellowish-green extraordinary ray. The latter showed little, if any, change compared with untreated material. The original absorption spectrum comprised a distinct narrow band at 450 nm, a faint line at 460 nm, general absorption between these features and a very faint line at 471 nm. No detectable spectral changes followed heating. Refractive indices remained constant at 1.761 for the extraordinary ray and 1.769 for the ordinary ray, and no weight changes were recorded. Under the tested conditions, heating failed to enhance blue colour and instead progressively removed it, contrasting with the frequent enhancement of blue in heat-treated aquamarine.