Joan Bastard was a blacksmith established in Perpignan in the early 15th century. Nevertheless, he seems to have developed his main professional activity as a millstone trader. Between 1410 and 1422, he sold more than 100 millstones in Roussillon, Vallespir and Bas-Conflent. Notarized documents related with this trade are of an exceptional quality. They allow not only to study a particular commercial system but also to know the millstone in use in the north side of Oriental Pyrenees. Joan Bastard sold his millstones at credit. He offered almost imported millstones of “Montjuïc stone”, a sandstone that made possible to obtain a flour finer than ones produced using millstones of local sandstone and granites.