A novel heat treatment method to convert Sri Lankan Deguna stones to Padparadscha sapphire

Abstract

Red and blue combined deguna stones are traditionally being converted to pink sapphire or ruby around 1,200 0C under oxidizing ambient air condition inside the Lakmini furnace. Under these conditions, already available red tone because of chromium (Cr3+) ions is highlighted as fading of blue colour due to the dissociation of ferro-titanium bi-particle ([Fe.Ti]6+). The aim of this study to deduce a novel heat treatment method to convert pale reddish blue colour deguna stones to padparadscha sapphire by further oxidizing these dissociated Fe2+ ions in to Fe3+ ions. In this method, pale reddish blue colour deguna stones were heat treated at 1,400 0C providing 30 minutes maximum soaking period, under high oxidizing ambient air conditions inside the Lakmini furnace. The heat treatment practice was carried out using an alumina crucible with both bottom and side ventilation holes. Also, 5 0C/min heating rate and 4 0C/min cooling rate were maintained to achieve resilience of the treated stone. The mineralogical and chemical studies of yielded product were revealed that the initial pale reddish blue colour deguna stones have been converted to padparadscha sapphire or golden sapphire depending on the availability of initial lemental impurities within the corundum host of deguna stones. However, physical properties and internal bulk chemistry of initial stones have been remained unchanged during this heat treatment method. The change in valence stages of elemental impurities may cause the colour change from pale reddish blue colour to typical padparadscha colour.