Chullin 123 - Flaying
The hide of an animal, although not in the same category as its flesh, can nevertheless transmit ritual impurity to and from the animal. In the case of a kosher animal, a person might be impure, and he may transmit his impurity to the carcass. In the case of a non-kosher animal, its meat is ritually impure, and it may transmit impurity to the person working with it.
If one is flaying a dead animal, whether domestic or wild, kosher or non-kosher, small or large, and his goal is to make a spread out of it, he first cuts the hide from head to hindquarters. The beginning of flaying is the hardest, and until he peels enough hide to grasp the carcass, the hide is considered a handle, and it transmits impurity both ways. If he wants to make a leather flask and strips the hide whole from the neck, the hardest is flaying the breast, so until the breast the hide is considered a handle and it conveys impurity. However, if in flaying for a flask, he begins from the hind legs, then all of the flayed hide is considered a handle, until he flays the breast.
Art: Vincent Van Gogh - Still Life With Four Stone Bottles Flask And White Cup