Cleaning leather boots and gloves when they are dirty removes the natural oil in the leather. You can use many different types of oil for leather care, replacing the natural oils. Two examples are neatsfoot oil and mink oil, both of which are made from animal byproducts.
Mink oil was originally made from mink, a small animal which is usually bred for its fur. Now, oil made from other types of animal fat is often sold as mink oil. The fur is used in fur coats and stoles. The oil is extracted from the bodies of the animal after the fur has been removed.
Neatsfoot oil is made from the tallow, hoofs and other discarded parts of cattle. The oil has a similar composition to that of the original oils which permeate leather. Since it is processed, it can darken the leather which it is applied to.
Both mink oil and neatsfoot oil are used to manufacture leather treatments. Both types of oil penetrate into the leather and replace the fats which have been removed by cleaning and mechanical wear. The oil is often mixed with wax or silicone to coat the leather and make it waterproof. Different oils are also mixed together to create leather treatments.
Mink oil is used for cosmetics, but not neatsfoot oil. The extracted and refined mink oil is mixed with aromatic compounds and packaged for use in skin treatments. The oil softens the human skin as well as leather.