Abstract:
Geuda is a low-quality variety of corundum which can be converted into blue sapphire by heat treatment. Colour of the blue sapphire is mainly due to the presence of Iron (Fe) and Titanium (Ti) and their adjacent order in the lattice structure. The process of colouring is called Intervalence charge transfer, where an electron moves from one transition element to an another and the process absorb part of white light resulting in a blue colour. This study identified UV visible spectroscope absorptions behaviours of Geuda before and after gas-fired heat treatment using “Lakmini” furnace and electrically operated Lenton EHF 17/3 furnace. 6 Samples were randomly collected from Ratnapura, Niwithigala, Lunugala and Ambalangoda gemstone markets. Maximum temperature in Lakmini furnace was 1750°C with soaking time of 30 minutes under reduced environmental condition inside the furnace. Lenton EHF 17/3 was used with 1700 °C temperature and three different soaking times (three days, one week, one month). The optical absorption spectra were obtained using multiskan go microplate spectrophotometer through with SkanIt 4.1 software. Data were analysed using OriginPro 2018 software. A peak height difference was obtained at around 560-600nm between gas heated and combined heated (electric furnace heat treatment after gas furnace heat treatment) samples at maximum temperature with increased soaking times. This was maily due to the formation of [FeTi]6+ complex where the ate of formation of [FeTi]6+ was high with prolonged soaking. Therefore, results confirmed that the presence of both Iron (Fe) and Titanium (Ti) is required to produce the desired blue colour in heat treated Geuda stones.
