As CentrX describes above, the term "alum" is actually used to describe several aluminum compounds. However, in this writeup I'm going to focus on the most common alum, potassium aluminum sulfate (K2SO4·Al2(SO4)3·24H2O), also known as potash alum, cake alum and pickle alum. This white powder occurs naturally in a mineral called alunogenite. It was originally extracted directly from the mineral but today it is synthesized in laboratories. Alum has been used in many different fields, including photography, papermaking, medicine, and food preparation. It was mainly used during the last century and is rarely used today. However, alum can still be found in pharmacies and in spice sections of most supermarkets.
The main use of alum in the previous century was in food preparation, especially for pickling vegetables like cucumbers and onions. A very small amount of alum (about one quarter of a teaspoon per pint) was added to the brine solution to keep the pickles crisp and crunchy. It was also added to bottled fruits such as maraschino cherries. Alum was also a common component of baking powder. It served as an acid that reacted with a base in the baking powder to form gas that help baked goods rise.
Alum is not commonly used in food today because of health concerns. Studies have shown that aluminum sulfates are mildly toxic and higher doses can irritate the skin, eyes, and lungs. It can also cause gastrointestinal problems when consumed and may even result in death. Fortunately, most people would have had to consume extremely large amounts of baking powder or pickles that contained alum to be at risk. Most companies decided they didn't want the risk and have stopped using alum in their products. Today, commercial baking powder, pickles, and maraschino cherries do not contain alum.
If you make your own pickles keep in mind there are alternatives to alum. Calcium salts or food-grade lime can be used to keep pickles crisp. However, if you want to not use any chemicals at all, you can simply soak the cucumbers in ice water for several hours before pickling. The cold water also works well to keep the pickles crisp.
Alum also has various medical uses because of its strong astringency. It can be applied after shaving to soothe the skin. Also, many people swear that putting a small dab of alum on a canker sore a couple times a day helps relieve the pain and helps it to heal faster. Alum will most likely cause the canker sore to burn and may make the mouth pucker from its astringency. If you try this remedy, it's important to rinse your mouth out with water after a few minutes and not swallow the alum. This is because alum is also an emetic when ingested at high doses.
www.straightdope.com/mailbag/malum.html
http://education.jlab.org/itselemental/ele013.html
http://www.bulkfoods.com/alum.htm | 
