The mineral occurs naturally as encrustations where weathering has oxidized minerals containing sulfides and potassium. Crystals of potash alum can also be manufactured by the processing of alunite (alum rock or alum stone) and alum shale (alum ore or alum schist).
During the late Middle Ages, Genoa and Venice dominated the trade of potash alum from the Middle East and volcanism-related deposits around the Mediterranean Basin.
From medieval to pre-modern times, potash alum was used for a variety of purposes: as a mordant in the dyeing of textiles, and in the manufacture of pigments as a siccative (drying) agent. Amongst its other uses are the tawing of hides, sizing of paper, and manufacture of glue.
Extensive description of potash alum and its use in papermaking here.