An indicator of its time: two millennia of the iron–gall-nut test†

The reaction between iron salts and the extract or tincture of gall-nuts is a unique example of an analytical reagent, for its use has been known and recorded for around 2000 years. In its time it became an important but controversial method for detecting iron in solution, before being supplanted by more effective reagents. The purpose of this paper is to review the history of the reagent and to show that not only is it an indicator of iron, but also an indicator of the chemical understanding of the times: ‘a litmus test’ of chemical knowledge.