This is a cul noir cup. It is an old folk art piece made in Northern France, where "cul" means "butt" and "noir" means "black. Made of reddish-brown or biscuit-colored clay, the front side is white, and the back is almost black. It is shaped like a mug, but given the background of a coffee culture at the time, it may have been used to hold utensils such as scales or soup at the table. There is a chip in the glaze on the bottom, so please avoid prolonged use in water.