Inks can be very complicated mixtures, but two key ingredients are the pigment used to color the ink and the solvent used to dissolve (or at least suspend) the pigment particles. While modern inks come in every color imaginable for a host of different pen types, historically inks fell into one of two major categories.

The first is carbon-based inks. Residues from burning wood or oil, like soot, were used as coloring in these inks, which were suspended in the sap of the acacia tree (known today as gum arabic).

The other type of ink used historically is called iron gall. The iron was usually added in the form of iron sulfate (Fe²⁺SO₄^2-); “gall” refers to gallotannic acid that was extracted from growths, or galls, on oak trees. Iron gall ink slowly darkens as the iron ions undergo oxidation from Fe²⁺ to Fe³⁺. The acidity of the ink solution can cause damage to the paper it is used on, so preservation of historical documents that were written with iron gall ink is challenging.