Purity: Ruling on Purifying Soap When Exterior or Interior Becomes Impure
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 159
If the exterior of soap becomes impure, it is possible to purify it; however, if its interior becomes impure, it is not possible to purify it. If a person doubts whether impure water has reached the soap’s interior or not, its interior is pure.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)