linguistic:voice_change, structural:scholar_edition
Modern understanding of the alums

During the early 1700s, G. E. Stahl claimed that reacting sulfuric acid with limestone produced a sort of alum. The error was soon corrected by Johann Heinrich Pott and Andreas Sigismund Marggraf, who showed that the precipitate obtained when an alkali is poured into a solution of alum, namely alumina, is quite different from lime and chalk, and is one of the ingredients in common clay.

Marggraf also showed that perfect crystals with properties of alum can be obtained by dissolving alumina in sulfuric acid and adding potash or ammonia to the concentrated solution. In 1767, Torbern Bergman observed the need for potassium or ammonium sulfates to convert aluminium sulfate into alum, while sodium or calcium would not work.

The composition of common alum was determined finally by Louis Vauquelin in 1797. As soon as Martin Klaproth discovered the presence of potassium in leucite and lepidolite, Vauquelin demonstrated that common alum is a double salt, composed of sulfuric acid, alumina, and potash. In the same journal volume, Chaptal published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum, and an alum manufactured by himself, confirming Vauquelin 's result.