The bird feces is farmed by going home to home where large pigeon populations gather and then scooping up the bird feces with their bare hands. The person collecting the bird poop has to access small cramp spaces where dried poop is heavy in the air. They breathe it in, soak it into their clothes and cover their hands with it.
Animal skins are placed into large vats, which contain a mixture of water, limestone, and pigeon droppings. The limestone helps to remove hair from the skins while the acid in the pigeon droppings softens the hides. Three days later the skins are removed and washed.
Dozens of men stand waist deep in bird feces churning the mixture with their own bodies. Pigeon poop contains ammonia that acts as softening agents that allows the hides to become malleable so they can absorb dye. The tanner uses his bare feet to knead the hides for up to three hours to achieve the desired softness.
The stench of rotting flesh and ammonia are overwhelming for most people
The tanning pits of Fez, Morocco, were built in the 11th century and leather goods have been produced there using the exact same method for more than a thousand years.