Alum is a complex chemical compound. It represents a single compound – hydrated potassium aluminium sulphate (or Potassium alum). It is also the name used to denote a class of compounds with a related empirical formula. It appears as a whitish coloured rock-like solid crystalline form with irregular surfaces and is used in a series of industrial processes. It is water-soluble, with an astringent almost sweet taste. When heated the crystalline alum melts into a liquid form and further heating leaves an amorphous powder.
In the middle of the 15th century, the earliest European Alum mines were exploited north of Rome in Central Italy. Roman Alum that was recovered from here was perhaps one of the best varieties of alum available for paper-making. Alum was imported into England mainly from Middle East from the 15th century onwards. It was initially used as a mordant or a dye-fixer for wool, which happened to be England’s primary Industry at that time.
Alum has been used in water purification by Indians since many centuries. Ayurveda describes Alum as ‘sphatika’ and is believed to have antiseptic, hemostatic and astringent properties. It has antibiotic, anti-pyretic and anti-inflammatory properties.
Alum can be obtained from alunite by calcinating it followed by air exposure for considerable lengths of time. While airing it is continuously moistened with water, at the end of which a fine powder is formed. Then the Alum is allowed to crystallize after hot water and sulphuric acid wash. Alum may alternatively be prepared from clays or bauxite, by calcination, mixing with sulphuric acid and heating to a boil. Crystallisation is then initiated.
Types of Alum
- Potash alum is employed as an astringent and antiseptic agent in certain food preparation stages such as pickling and fermentation. As such its most common use remains that of a flocculant for water purification.
- Soda alum is used as an acidulant in foods and also in the manufacture of baking powder.
- Ammonium alum, chrome alum and Aluminium sulphate are the other forms of alum which are used for industrial purposes.
One of the principal culinary uses of Alum is in pickling recipes to retain crispness of fruits and vegetables. Alum forms the acidic component in some commercial baking powders. Some of the medicinal uses include application to wounds to prevent or treat infections. It is used in some vaccines as an adjuvant to enhance the body’s response to the vaccines.
Industrial uses include water purification, paper production, and in foamite of fire extinguishers among others.
As a water treatment agent because of its acidic and antibacterial function it makes insoluble compounds in water to form a dense residue, sinks to the bottom and can then be filtered out.
The FDA – Food and Drug Administration has approved alum use in oral healthcare products as an astringent and also in certain stages of flour and cheese manufacture.
In folk magic, alum powder is believed to help stop people from gossiping about someone.