The Chaouwara tanneries are one of the city’s most iconic sights (and smells). Head east or northeast from Place as-Seffarine and take the left fork after about 50m; you’ll soon pick up the unmistakeable waft of skin and dye that will guide you into the heart of the leather district (the touts offering to show you the way make it even harder to miss).

http://www.lonelyplanet.com/morocco/the-mediterranean-coast-and-the-rif/fes/sights/farms-workshops-factories/chaouwara-tanneries