Praise be to Allah.
The skins of animals that become halaal through proper slaughter are pure (taahir), because they become good through the process of proper slaughter, such as the skins of camels, cattle, sheep, gazelles, rabbits and so on, whether they are tanned or not tanned. As for the skins of animals whose meat we cannot eat, such as dogs, wolves, lions, elephants and the like, it is impure (naajis), whether it is slaughtered or it dies or is killed, because even if it is slaughtered it does not become halaal and cannot be good, so it is naajis, whether it is tanned or not tanned. This is according to the most correct opinion, because the most correct opinion is that impure skins cannot be made pure through tanning if they come from animals which we are not permitted to slaughter for food.
As for the skins of animals which have died before they could be slaughtered properly, if these are tanned then they become pure, but before they are tanned they are impure. So now animal skins may be divided into three types:
The first type: those which are pure whether they are tanned or not, which are the skins of animals that may be eaten if they are slaughtered correctly.
The second type: skins which cannot be pure either before or after tanning, because they are impure. These are the skins of animals whose meat we cannot eat, like pigs.
The third type: skins which become pure after tanning, but are not pure before before tanning. These are the skins of animals whose meat may be eaten if they are slaughtered properly but not if they die otherwise.