Geuda sapphire charcoal furnace reduction: sealing crucible lids with refractory clay mud?

Thermal alteration of milky Geuda corundum in Ratnapura requires maintaining strict reducing conditions at temperatures between 1600°C and 1750°C. Rough stones are packed into high-purity graphite or sillimanite crucibles filled with carbonaceous charcoal powder. The crucible lid is sealed with a thick collar of refractory fireclay mixed with zircon sand and sodium silicate. When fired inside oil-and-air blown Lakka furnaces, the charcoal consumes internal oxygen (forming carbon monoxide CO gas). This reducing atmosphere forces iron ions from Fe3+ into Fe2+, which bonds with Ti4+ to trigger intense blue charge-transfer coloration.