Alum miners at work in miniature. Bibliotheque Nationale de France, Paris.
Alum is a vitriolic clay composed of sulfuric acid, aluminum,and potassium. It comes in many variants, including rock alum, Yemenite alum, Roman alum, and shale alum. It was used as a mordant for dyeing throughout the ancient world, including India. Until the 15th century, it was processed mainly in Constantinople, Aleppo, and a site near Smyrna (modern Izmir) called Rocca. In the mid-15th century, a Genoese merchant who had travled to the East and spent some time in Rocca founded the first alum factory in Europe on the island of Ischia. (Francois Delamare and Bernard Guineau)