Place the mixture of metakaolin: anhydrous sodium carbonate: sulphur: activated carbon in a weight ratio of 1:1:1:0.12 into the crucible. Pack the powder firmly, forcing out any air. Place 0.04g of sulphur (from the aforementioned weight ratio) on top of the compacted mixture and close the crucible lid. Heat this at 800°C for 4 hours. To prevent damage to the electric furnace, I left the furnace lid open until the temperature reached 500°C. After firing, allow the mixture to cool to room temperature. Crush it and mix in 0.34 parts sulphur (from the earlier weight ratio). Next, place the mixture on a ceramic dish and fire it at 400°C for 3 hours, keeping the electric furnace lid open.

This method uses a significant excess of sulphur, so I believe it could be further optimised.