Colcothar

Colcothar is a term with historical significance, primarily referring to a red pigment and a residue from alchemical processes. Historically, it was known as a bright red iron oxide, similar to Venetian red or red ochre, and was used as a pigment in paints and dyes. The exact composition could vary depending on its preparation.

In alchemy, colcothar was specifically the residue obtained after the calcination or dry distillation of sulfuric acid with iron. This process was part of attempts to produce various substances, including potential medicines or philosophical mercury. The alchemists believed this red residue held important transformative properties.

The term "colcothar" itself is thought to derive from Greek words, possibly relating to its color. While its alchemical significance has faded with the decline of alchemy as a scientific pursuit, its historical use as a pigment is documented. Modern industrial processes for producing iron oxide pigments have largely superseded the alchemical methods associated with colcothar. Today, the term is more likely to be encountered in historical texts on alchemy or the history of chemistry and pigments.