Alum

Alum was a metallic mineral ore. It was a double sulfate composed of a trivalent metal, aluminium, and a univalent metal, potassium or sodium, used as either a clarifying agent or a hardening agent. Overexposure to fumes could lead to alum mineral sickness, a common affliction of alum miners such as Haldo. Alum was a common material in the production of Mandalorian armor, though the incredibly resilient properties of beskar iron made it the preferred choice for armor crafting among Mandalorians.

During the Galactic Civil War, members of the Death Watch—a radical and violent Mandalorian splinter group led by a Death Watch Overlord—excavated a vast network of alum mines located beneath their bunker on the forest moon of Endor. Some time around 1.5 ABY, a group of spacers defeated the contingent of Death Watch soldiers at the base, and went on to use the group's alum supply to manufacture Mandalorian armor for themselves.

Text and facts from the “Alum” article on Wookieepedia, licensed under CC BY-SA 3.0. Modified for this site.