Purity: Purifying Large Containers Such as Cauldrons and Barrels

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 150

If a large container like a cauldron or barrel becomes impure, in the event that it is filled and emptied three times, it becomes pure. The same applies if water is poured into it from above three times in a manner that it reaches all its sides and each time the water that collects at the bottom is emptied out; and the recommended precaution is that on the second and third time, the container that is used to empty out the water should be washed with water.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)