Vegetable tanning

Using tannins
''One autumn, a busy and inexperienced tanner had left and forgotten a stack of cleaned hides outside near his house, under a grove of trees. The hides remained there for weeks and hadn't been properly packed: they soaked with water from the humid ground and constant rains of autumn, covered and surrounded by fallen twigs and leaves from the nearby trees. When the tanner realized his mistake, he was quite surprised to find the hides in such good condition: not only had they resisted putrefaction from the humidity, but they looked more flexible than the ones usually processed through drying and smoking. What was the cause of this miracle? Looking at the brown water from the decaying leaves and wood in which the hides had soaked all that time, the tanner felt he already knew the answer to that.''

Potential Requirements
???