The word of the day is alum:
- Also called potash alum, potassium alum. a crystalline solid, aluminum potassium sulfate, K 2 SO 4 ⋅Al 2 (SO 4 )3 ⋅24H 2 O, used in medicine as an astringent and styptic, in dyeing and tanning, and in many technical processes.
- one of a class of double sulfates analogous to the potassium alum, as aluminum ammonium sulfate, having the general formula R 2 SO 4 ⋅X 2 (SO 4) 3 ⋅24H 2 O, where R is a univalent alkali metal or ammonium, and X one of a number of trivalent metals.
- (not in technical use) aluminum sulfate.
early 14c., "whitish mineral salt used as an astringent, dye, etc.," from O.Fr. alum, from L. alumen "alum," lit. "bitter salt," cognate with Gk. aludoimos "bitter" and Eng. ale.
"The production of one mordant, pure alum, from pyrite has been described as the point of origin of the modern chemical industry, because the process required not only the manufacture of a chemical substance but also its purification."
- David Rickard, "The Many Faces of Fool's Gold", May-June 2016 American Scientist (http://www.americanscientist.org/issues/feature/2016/3/the-many-faces-of-fools-gold/1)