2. Why do leather become hard?
The oil introduced into the leather will evaporate over time in its natural state and be lost. Or due to other reasons so that the oil film is destroyed, such as water hydrolysis or in a high temperature environment, the oil film is destroyed, the oil evaporated away, so the leather internal fibres will be bonded to each other, so that the leather becomes hard, brittle. In short, the root cause of leather hardening and brittleness is due to the lack of lubrication between the internal fibres of leather, bonded to each other.