Alum

From Oxford English dictionary

1. An astringent mineral salt, typically occurring as colourless or whitish crystals, that is used as a mordant for dyeing, in tanning, for sizing paper and fireproofing materials, in water purification, and in medicine (esp. as a styptic); common alum

2. Any of various substances resembling this compound in chemical properties, crystal structure, etc.; any of a series of isomorphous crystalline double sulphates of a monovalent metal or group and a trivalent metal, analogous to common or potash alum. Frequently with distinguishing word.