Purity: Ruling on Objects Made from Impure Materials Exposed to Fire
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 190
A clay pitcher or something similar made from impure clay is impure. As for charcoal made from impure wood, in the event that none of the former physical properties is in it, it is pure. If impure clay is changed by fire into crockery or bricks, then based on obligatory precaution, it remains impure.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)