Fes is one of Morocco's Four Imperial Cities (Rabat, Meknes and Marrakech are the others). In the medina of Fes you will come across the Leather Souq, easily found by the stench, that has been in operations for something like nine centuries. The tannery in the Souq is reported to be the oldest in the world.
The process of curing and dying the sheep and goat skins has not changed in all that time. The hides are first soaked in a solution of pigeon excrement, which is slightly acid. This solution softens the hides and causes the hair of the animal to be more easily removed from the hide. Once the hide has been prepared this way it is then put into a stone vessel with the dye. From a vantage point of a balcony in the showroom of the tannery, the lucky tourist is able to look down on the honeycomb appearing tannery. The time we were there red dye was the prominent color being used, but this changes on about a three week schedule.
The men working in the tannery are dyed too---their arms and legs bearing witness to the colors being used during a particular dye cycle.
Once the hides are dyed the appropriate color, they are left to dry in the sun on the rooftops in the Souq.
The tannery has a lovely shop, with poufs, leather jackets, handbags, cases, bags, traditional Moroccan slippers (babouche) and the like for sale. I bought my beautiful red, upturned-toe babouches at the tannery shop, along with a lovely, red (!!! do you sense a theme???) shoulder bag. I won't say they were dirt cheap, but they were a very good value. The leather is soft and beautiful and the workmanship is very good.