An exceptionally versatile chemical, alum has been especially vital as a mordant in the dyeing industry and in the area of water treatment.
Anyone who has ever tie-dyed a shirt or dyed a piece of cloth is familiar with the challenge that not all of the color remains permanently attached to the fabric. Indeed, the tendency for dyes to leach out of clothing is why it is important to wash whites separately from colors lest the whites turn gray or pink from the escaping colorants.
Many brilliantly colored dyes do not easily bond to the fibers in cloth, so the solution to the problem is the application of a mordant. Mordants serve as a chemical bridge between the fiber and the dye molecule so that the color is more permanently retained.
The mordant alum, or potassium aluminum sulfate, has historically been an essential component of the dyeing process, which is curious since alum itself is colorless. In medieval times, alum was imported to Europe from the Near East and the Greek Islands, but with the Turkish occupation of Constantinople that trade route was eliminated. Fortunately, alum deposits were discovered on land owned by the Pope in Italy, and the fortunes of the Medici family were made as the administrators of this resource. Indeed, alum was such a profitable commodity that it has been suggested that King Henry VIII of England specifically married Anne of Cleaves in order to gain control of her family’s alum mines.
Completely separate from its history as a mordant, alum is currently used for numerous applications, including water treatment. As early as 1500 BC, Egyptians used alum to reduce the cloudiness or turbidity of water. Alum continues to find related uses given its ability to clear water of suspended particles that are too fine to be filtered out or precipitated.
More information about alum as a mordant can be found at http://www.earthues.com/aboutmordants.html