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. The earliest documents are written in iron gall ink on papyrus date back into the first centuries after Christ.
Because of its indelibility, it was the ink of choice for documentation from the Middle Ages to the twentieth century. Iron gall ink is primarily made from tannin (most often extracted from galls), vitriol (iron sulfate), gum, and water.
Tannins are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. Galls are a kind of swelling growth on the external tissues of plants, fungi, or animals.
The earliest recipes for oak gall ink come from Pliny the Elder who was a Roman author, naturalist and natural philosopher, naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. Many famous and important manuscripts have been written using ferrous oak gall ink, including the Codex Sinaiticus, the oldest, most complete Bible currently known to exist, handwritten over 1600 years ago.
Iron gall inks ease to make, along with its permanence and water resistance made it the favorite back in the day. Its use and production started to decline only in the 20th century when other waterproof formulas (better suited for writing on paper) became available.
Today, libraries’ biggest issue is the puzzle of how to care for one of the most widely used and inherently damaging historic inks – iron gall ink. Works include such iconic ‘Treasures’ of the Library as Magna Carta and the Lindisfarne Gospels, and range from illuminated manuscripts to personal correspondence and formal maps to impromptu sketches including those of Leonardo Da Vinci.
The iron gall ink corrodes. When applied to paper or vellum the ink ‘burns’ into it leaving a mark that is insoluble in water or alcohol, and which cannot be erased. Over time the ink attacks the paper or parchment, weakening the material and causing areas of text to be damaged or lost.
Ideally, a complete and effective treatment of iron gall ink corrosion must work on three fronts.
- It should arrest current and future acid hydrolysis by removing water-soluble acid groups from the paper and introducing an alkaline buffer.
- It should also block or retard oxidative degradation accelerated by the presence of excess iron.
- It should strengthen the physical condition of the ink and its underlying support.
Aqueous treatment combined with an alkaline buffer and iron complexing agent will minimize oxidation and hydrolysis reactions. However, the use of water to treat objects with unstable inks has some serious disadvantages. Water itself is a necessary component for acid hydrolysis.
In addition, a large proportion of ink and paper degradation products are water-soluble, and treatment with water may significantly alter the color of the ink and the overall tonal balance of the object. Furthermore, degradation products of ink corrosion may be deposited far beyond the ink line and throughout the paper when using water during treatment.
The study of preservation techniques is continuing with the focus on:
- Art technology – better understanding of artifacts made with iron gall ink
- Degradation – better insight in the decay processes
- Conservation – development of innovative preservation and conservation strategies