A mordant is an ion that scientists add to a stain to give color to various organisms so they can better identify them. The mordant binds to the chemical dye, helping to hold it so it remains stuck to the organism. The word mordant actually applies to any chemical that keeps a dye in its place.
There are many types of mordants. Some examples of mordants are cochineal, madder, fustic, osage and logwood. Mordants can come in many chemical forms, whether as salt, hydroxide, aluminum or chromium. Furthermore, mordants can dye a large variety of substrates from textiles and leather to flowers and wood, so they are not strictly relegated for the microscopic realm.
Alum is the cheapest and safest mordant available on the market. Thus common mordant comes in handy for dyeing textiles, especially. Potassium aluminum sulfate is considered a "true alum" because of its chemical composition as a double salt of aluminum. Chemists also use aluminum sulfate, but it has subtle difference in its chemical properties from the former, being that is the result of when bauxite is refined. Iron-free grade aluminum sulfate is useful for natural dyeing, but it may be more expensive than other mordants.
There are many other uses for alum in all its forms. Water treatment plants, glue manufacturers, paper mills, skin products and cloths are some other examples of how alums are widely used in industrial settings.