Ratnapura gem heat treatment ovens: electric muffle furnaces and borax flux cleaning?
4 Answers from travellers
Thermal enhancement of rough corundum in Ratnapura utilizes high-temperature electric muffle furnaces reaching up to 1750°C. Cloudy, milk-colored sapphires containing dense rutile silk (milky corundum / Geuda) are coated in fine borax flux powder and packed into high-purity alumina crucibles. The furnace ramps up slowly over 12 hours. At extreme temperatures, rutile needles dissolve back into the corundum crystal lattice, transforming milky geuda into transparent blue or yellow gems.
Borax flux acts as a glass-forming solvent that seals surface-reaching fractures and prevents stones from fusing together inside hot crucibles.
Disclosure of thermal treatment on export lab memos is mandatory under NGJA trade regulations to protect consumer transparency.
Unheated natural stones with high clarity command substantial price premiums over heat-treated counterparts.
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