Purity: Purification of Melted and Solidified Impure Materials like Metal or Plastic
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 151
If some impure metal, plastic, or suchlike is melted, and in the melting process the inside of it also becomes impure, then in the event that it is washed with water after it has solidified, its exterior becomes pure.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)