Does tanning purify the skins of all animals that were not pure in life?
Chapter on Utensils
Al-Mughni
Book of Purification
Primary text
If we affirm the purity of skins through tanning, then the skin of an animal that was not pure in life is not purified by tanning. Ahmad explicitly stated that such skins are purified. The textual basis supporting this general purification is the statement, 'Any hide that is tanned has become pure,' which encompasses both permissible and impermissible food animals. The exception made is for what was impure in life, as tanning only affects the impurity arising from death; thus, it remains under the general principle in other cases.
Supporting text
Some of our colleagues state that only the skins of animals whose flesh is permissible to eat become pure through tanning. This is the established view of Al-Awza'i, Abu Thawr, and Ishaq. Their evidence rests on the narration stating, 'The tanning of leather is its slaughter (dhakāh).' They equate tanning to ritual slaughter (dhakāh), which is only effective for creatures whose meat is permissible to consume. Furthermore, since tanning is one of the purifying agents for the hide, it should not affect what is not permissible to eat, similar to the effect of slaughter.