The New Student's Reference Work/Alum

Alum, common alum, a sulphate of potassium and aluminum, is a salt used in the arts and in medicine. It forms colorless, octahedral crystals containing much water. It is sometimes found in a natural state, but is usually manufactured. There are several kinds of alum, and the one containing ammonium in place of potassium is often used instead of the more common compound. Alum is used in the manufacture of calico, in tanning and dyeing. Mixed in the milk it helps in the separation of the butter, and bakers sometimes use it to whiten their bread. If added in small quantities to turbid water, in a few minutes it will make it perfectly clear without any bad taste or quality, but it should be used with caution in articles of food and drink on account of its astringent properties.