With Adobe Photoshop or Lightroom we can change the color of iron gall ink drawings from brown to black and see, at least vaguely, how the drawings would have originally appeared. When an artist uses iron gall ink it starts out gray, quickly oxidizes to blue-black, and over years changes to brown. It is hard to grasp that so many of the warm brown drawings we know were conceived in black.
The color in iron gall ink comes from gall and vitriol–yes, a negative ring. Recipes call for ground wasp’s galls (tannic acid), iron or copper filings (vitriol or sulfuric acid), gum arabic, and water or wine. The photographs of the galls here are on oak trees in Italy. Galls, also called gall apples, are nests built by wasps for their larvae. This kind of ink was made in antiquity and again starting in the 13th century. It was the most commonly used ink until the 19th century. Part of its appeal was that it was indelible, unlike carbon ink. Since it eats into the paper, it could not be altered, a positive attribute for scribes with their official documents.
Most old master drawings in brown ink were created with iron gall ink. The other important brown inks are bister and sepia, both of them more stable. One easy, and unfortunate, way to tell the difference between these inks, is that the iron gall ink bites into the paper, making it look seared or burnt. Some greatly damaged drawings appear lacy with all their holes. In this example by the artist Mola, we can see how the areas with concentrations of ink, especially in the eye and cuff sections, are weakening the paper. This is the action of the acid in the ink.
The Ink Corrosion Website details the scope of the problem of deteriorating documents, drawings, and music scores created with iron gall ink and has information on what conservators are doing about it. The excellent site also gives recipes for the ink.