corundum, sapphire, Sri Lanka.12, heat treatments.6, gems

Ediriweera and Perera investigate the spectral changes accompanying the conversion of Sri Lankan geuda corundum into blue sapphire by heat treatment. Approximately fifty initially non-transparent stones, ranging from white through yellowish brown to light blue, were heated for varying periods between 1200 and 1900 °C. Specimens were cut and polished with parallel faces, placed in alumina crucibles, and heated in an alumina furnace chamber using an oxyacetylene flame adjusted to neutral or slightly reducing conditions. After successive heating stages, transmitted-light spectra were measured over 420–720 nm using a monochromator and radiometer–photometer. Above 1500 °C, approximately 90% of the samples became blue sapphires of differing colour intensities, with higher temperatures producing darker blue colours and improved clarity. Three recurring spectral changes accompanied the transformation: development of a broad absorption band beyond 560 nm, weakening or disappearance of an initially weak band at 540 nm, and persistence of a weak absorption at 450 nm. A representative silky bluish-grey specimen, initially brownish in transmitted light, became blue after heating at 1700 °C and dark blue after further treatment at 1800–1850 °C. Its successive spectra illustrate the relationship between the emerging blue colour and these absorption changes. Drawing on published assignments, the authors associate the broad absorption beyond 560 nm with an iron–titanium pair denoted (Fe–Ti)⁶⁺, the 540 nm band with Ti³⁺, and the 450 nm band with Fe³⁺. They interpret the observations as evidence that pair formation produces the blue colour, while the concentration of isolated Ti³⁺ decreases appreciably and that of Fe³⁺ changes relatively little. Their proposed model assumes that both trivalent ions are initially present, with iron in excess. Heating promotes iron–titanium pair formation, consuming enough titanium to markedly diminish its individual absorption, whereas the larger iron population retains a detectable and comparatively unchanged absorption. The study thus links visible colour development to systematic spectral changes and offers a model for their interpretation, with the inferred changes in ionic populations based on absorption-band behaviour.