Been reading labels again, have you?
Alum is a salt that in chemistry is a combination of an alkali metal, such as sodium, potassium, or ammonium and a trivalent metal, such as aluminum, iron, or chromium. The most common form, potassium aluminum sulfate, or potash alum, is one form that has been used in food processing. Another, sodium aluminum sulfate, is an ingredient in commercially produced baking powder. (Have you never noticed the faint metallic taste in baking powder? It comes from the alum.)
The potassium-based alum has been used to produce crisp cucumber and watermelon-rind pickles as well as maraschino cherries, where the aluminum ions strengthen the fruits' cell-wall pectins.
Alum is approved by the U.S. Food and Drug Administration as a food additive, but in large quantities - well, an ounce or more - it is toxic to humans. As a result, efforts have been made and are being made to wean us of our alum dependency. The U.S. Department of Agriculture says that if good quality produce and modern canning methods are employed, there is no need to use alum to bolster the crispness of our pickles and cherries. In any event, the department says, even if alum is used to soak the pickles, it should not be used in the final pickling liquid.