Alum isn't a spice. It is a family of chemical compounds that contain Aluminum. Alum is sold in the baking and spice sections of grocery stores and is a white powder.
The exact history of the alum we use today is difficult to determine because the term Alum has been used for various compounds over the years. However, we know that Alum was being used long before common era. It was mostly used for dying and tanning, but was also used in pottery and as an eastern medicinal compound.
Alum occurs naturally in the earth, but can also be manufactured. It is mined in many different areas.
Traditionally Alum has been used in pickling cucumbers and watermelon rinds to help maintain crispness. It is recommended to use no more than 1/4 t per pint. Greater amounts can actually decrease firmness and can introduce bitterness and cause stomach discomfort. Alum is only needed in fermented pickles and has little to no effect on quick pickles. The USDA does not recommend using Alum in pickling. Pickle crispness can be improved by using fresh produce, trimming blossom ends and soaking cucumbers in ice water prior to pickling.
Alum is used in cleaning snails in Nigeria.
Alum was used to increase the whiteness of bread in Victorian England. It caused other nutrients in the bread not to be absorbed by the body.