Try google on 'ISTIHALAH' concept used in Msia and throughout the world. Example :
"Istihala is when something becomes pure. It was najis (impure) but it is now taahir (pure). A good example would be maitah (animal carcass): it is najis, but should it be burned and become ashes, or decompose and become earth, then it is taahir, it is no longer najis. This can happen with dung or feces or whatever. Whenever something changes from one property to another, then the ruling likewise changes.
Example: Let us say that someone uses the fat of a dead animal to make soap. That fat is najis, but the chemical change that it was put through makes it taahir. "
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