Alum means many things to many people. To most people,
alum means hydrated aluminum sulfate, which is the salt of sulfuric acid (a
strong acid) and aluminum hydroxide (a weak base). Since this single salt is the
one used predominantly by industry, it is the commercial grade. It is also known
as papermakers' or filter alum.
Chemically speaking, however, alum is a generic term. It
refers to a group of crystalline double salts, of which the most common are
ammonium-aluminum sulfate and potassium-aluminum sulfate. These double salts
occur naturally and were the only ones available until the development of
economical production methods for the simple single salt. Since both the single
and double salts had much the same properties, and were used for approximately
the same purpose, they were both called alum.
Almost all the alum produced in the United
States today is manufactured from bauxite, bauxitic clays and other clays, which
are found primarily in Alabama, Arkansas, Georgia, and Missouri. Deposits are
also located in Jamaica, in Guyana, Surinam, and other South American countries
and in Asia.