Iron gall ink (also known as common ink, standard ink, oak gall ink or iron gall nut ink) is a purple-black or brown-black ink made from iron salts and tannic acids from vegetable sources. It was the standard ink formulation used in Europe for the 1400-year period between the 5th and 19th centuries, remained in widespread use well into the 20th century, and is still sold.

The ink was traditionally prepared by adding some iron(II) sulfate (FeSO₄) to a solution of tannic acid, but any iron ion donor can be used. The gallotannic acid was usually extracted from oak galls or galls of other trees, hence the name. Fermentation or hydrolysis of the extract releases glucose and gallic acid, which yields a darker purple-black ink, due to the formation of iron gallate.

The fermented extract was combined with the iron(II) sulfate. After filtering, the resulting pale-grey solution had a binder added to it (most commonly gum arabic) and was used to write on paper or parchment. A well-prepared ink would gradually darken to an intense purplish black. The resulting marks would adhere firmly to the parchment or vellum, and (unlike india ink or other formulas) could not be erased by rubbing or washing. The marks could only be erased by scraping a thin layer off the writing surface.

By mixing tannin with iron sulfate, a water-soluble ferrous tannate complex is formed. Because of its solubility, the ink is able to penetrate the paper surface, making it difficult to erase. When exposed to air, it converts to a ferric tannate, which is a darker pigment. This product is not water-soluble, contributing to its permanence as a writing ink.

The darkening process of the ink is due to the oxidation of the iron ions from ferrous (Fe²⁺) to ferric (Fe³⁺) state by atmospheric oxygen. For that reason, the liquid ink needs to be stored in a well-stoppered bottle, and often becomes unusable after a time. The ferric ions react with the tannic acid or some derived compound (possibly gallic acid or pyrogallol) to form a polymeric metal-organic compound.

While a very effective ink, the formula was less than ideal. Iron gall ink is acidic. Depending on the writing surface being used, iron gall ink can have unsightly "ghost writing" on the obverse face of the writing surface (most commonly parchment or paper). Ultimately it may eat holes through the surface it was on. This is accelerated by high temperature and humidity. However, some manuscripts written with it, such as the Book of Magical Charms, have survived hundreds of years without it damaging the paper on which it was used.

The earliest recipes for oak gall ink, though vague, come from Pliny the Elder. Many famous and important manuscripts have been written using ferrous oak gall ink, including the Codex Sinaiticus which is the oldest, most complete Bible known to exist, thought to be written in the middle of the fourth century. Due to the ease of making iron gall ink, its permanence, and water resistance, it became favored by scribes in the European corridor and around the Mediterranean Sea. Surviving manuscripts from the Middle Ages as well as the Renaissance bear this out as the vast majority are written using iron gall ink, the balance being written using lamp black or carbon black inks. Many drawings by Leonardo da Vinci were made with iron gall ink. Laws were enacted in Great Britain and France specifying the content of iron gall ink for all royal and legal records to ensure permanence.

The popularity of iron gall ink spread across the world during the colonization period and beyond. The United States Postal Service had its own official recipe that was to be used in all post office branches for their customers. It was not until the invention of chemically-produced inks and writing fluids in the latter half of the 20th century that iron gall ink fell out of common use.