Alum is a class of chemical compounds containing sulphuric acid or selenate alumina, an alkali and water. They are used industrially as dying agents, as well as food preservatives and flame retardants, and to purify water, staunch bleeding and enhance immune response in some vaccines. They are also used as an ingredient in toothpaste and in artist's clays as a firming agent.
The main kinds of alum are potassium (or potash) alum, soda alum, ammonia alum and chrome alum, all of which contain sulphur, and the selenate alums, which contain selenium instead of sulphur. The selenate alums are used as antiseptics because they are strong oxidizing agents.
The sulphur-based alums are the most widely used, especially potassium alum. Potassium alum is used throughout the food industry in baking powder and as a preservative, especially in pickle brine. It is also used in the textile and leather industry for tanning and dying fabric.
Soda alum is also used in the food industry as a constituent of baking powder and as an acidulent.
Ammonia alum is used in the food industry for many of the same purposes as the two above, as well as being used as a flame retardant and in the manufacture of glue. Ammonia and potash alum are both used to filter water. They are also both used as deodorant crystals.
Chrome alum is not as common as the three other sulphur-based alums, but it is used in the leather industry for tanning skins.