Iron gall ink
Iron gall ink (also known as iron gall nut ink or oak gall ink) is a purple-black or brown-black ink made from iron salts and tannic acids from vegetable sources. It was the standard writing and drawing ink in Europe, from about the 5th century to the 19th century, and remained in use well into the 20th century.
Preparation and use 
The ink was generally prepared by adding some iron(II) sulfate (FeSO4) to a solution of tannic acid (C6H2(OH)3COOH), but any iron ion donor (e.g. nails, iron metal scraps, etc.) can be used. The gallotannic acid was usually extracted from oak galls (also known as "oak apples"), or galls of other trees; hence the name. Fermentation or hydrolysis of the extract releases tannic acid, which yields a darker black ink.
The fermented extract was combined with the ferrous iron(II) sulfate. After filtering, the resulting pale-gray solution had a binder added to it, (most commonly gum arabic) and was used to write on paper or vellum. A well-prepared ink would gradually darken to an intense purplish black. The resulting marks would adhere firmly to the vellum or parchment, and (unlike india ink or other formulas) could not be erased by rubbing or washing. The marks could only by erased by actually 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 oxygen a ferric tannate pigment is formed. This complex is not water-soluble, contributing to its permanence as a writing ink.
The gradual darkening 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 had to be stored in a well-stoppered bottle, and often became 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 organometallic compound. The specifics of the chemistry of iron gall ink can be found at realscience.com.
While a very effective ink, the formula was less than ideal. Iron gall ink is acidic ranging from roughly equivalent to a lemon (pH ≈ 2) to that of a cup of black coffee (pH ≈ 5). In chemistry, pH is a measure of the activity of the (solvated) hydrogen ion, where a lower pH level indicates a more acidic solution. For this reason some makers of iron gall ink used crushed egg shells (which contain calcium carbonate (CaCO3)) to temper the ink solution acidity, bringing it closer to a neutral pH (pH = 7) value. 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 vellum or paper.) Also any excess of ferrous ions remaining in the ink over years, decades, and centuries, could create a rusty halo around the marks and ultimately it might eat holes through the surface it was on.
Paper has its own special problems with iron gall ink. The iron-tannic pigment did not make chemical bonds with the cellulose fibers. The ink stuck firmly to the paper, but largely by mechanical bonding; namely, the dried ink penetrated the spaces between the fibers and, after drying, became entangled in them. The process of decaying the writing surface is accelerated on paper when compared to vellum, doing the damage in decades or years that could take more than a millennium on vellum.
The acidity of iron gall ink is well known but it must also be observed that the case for the acidity of iron gall ink is somewhat overstated. There are several thousands of manuscripts, some of them well over 1,000 years old, with iron gall ink on them that have no damage or degradation whatsoever from the iron gall ink. This understanding however should not be taken to ignore the potential issues of documents that use iron gall ink for the writing.
The earliest recipes on how to make oak gall ink come from Pliny the Elder and are vague at best. 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 thought to be written in the middle of the fourth century. Due to the ease of making iron gall ink and its quality of permanence and water resistance this ink became the favored ink for scribes in the European corridor as well as around the Mediterranean Sea. Surviving manuscripts from the Middle Ages as well as the Renaisance bear this out as the vast majority are written using iron gall ink the balance being written using lamp black or carbon black inks. Laws were enacted in Great Britain and France specifying the content iron gall ink for all royal and legal records to ensure permanence in this time period as well.
The popularity of iron gall ink traveled around 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 the use of their customers. It was not until the invention of more chemically produced inks and writing fluids in the latter half of the 20th century that iron gall ink fell out of common use.
The permanence and water-resistance of the iron and gall-nut formula made it the standard writing ink in Europe for over 1,400 years, and in America after European colonization. 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, iron gall ink is manufactured and used chiefly by artists enthusiastic about reviving old methods.
Fountain pens 
Traditional iron gall inks intended for dip pens are not suitable for fountain pens that operate on the principle of capillary action. Ferro gallic deposit accumulation in the feed system can clog the small ink passages in fountain pen feeds. Further very acidic traditional iron gall inks intended for dip pens can corrode metal pen parts (a phenomenon known as redox reaction/flash corrosion). These phenomena can destroy the functionality of fountain pens.
Instead, modern surrogate iron gall formulas such as blue-black bottled inks by Lamy (discontinued in 2012), Montblanc, Chesterfield's Archival Vaultor, Diamine Registrar's Ink, Ecclesiastical Stationery Supplies Registrars Ink, Gutenberg Urkundentinte G10 Schwarz (certificate ink G10 black) or Rohrer & Klingner "Salix" and (purplish grey) "Scabiosa" inks are offered for fountain pens. These modern iron gall inks contain a small amount of ferro gallic compounds, but are gentler for the inside of a fountain pen, but can still can cause problems if left in the pen for a long period. Manufacturers or resellers of modern iron gall inks intended for fountain pens sometimes advise a more thorough than usual cleaning regimen - which requires the ink to be flushed out regularly with water - to avoid clogging or corrosion on delicate pen parts. For more thoroughly cleaning iron gall ink out of a fountain pen sequential flushes of the pen with water, diluted vinegar (to flush out residual iron gall compounds), water, diluted ammonia (if needed to flush out residual color dye stains) then finally water are often recommended. The color dye in these modern iron gall formulas functions as a temporary colorant to make these inks clearly visible whilst writing. The ferro gallic compounds through a gradual oxidation process causes an observable gradual color change to gray/black whilst these inks completely dry and makes the writing waterproof.
Though not in mainstream 21st century use like dye-based fountain pen inks, modern iron gall inks are still used in fountain pens in applications that require permanence.
In the United Kingdom the use of special blue-black archival quality Registrars' Ink containing ferro gallic compounds is required in register offices for official documents such as birth certificates, marriage certificates, death certificates and on clergy rolls.
In Germany the use of special blue or black urkunden- oder dokumentenechte Tinte or documentary use permanent inks is required in notariellen Urkunden (Civil law notary legal instruments).
German regulation for Urkundentinte inks (1933) 
- In a liter of ink there has to be at least 27 g of tannic acid and gallic acid, and at least 4 g of iron content. The maximum iron content is not allowed to surpass 6 g / l.
- After 14 days storage in a glass container the ink is not allowed to have stained the glass or show sedimentation.
- Eight-day-old writings after washing with water and alcohol must remain very dark.
- The ink has to flow easily from the pen, and may not be sticky even immediately after drying.
See also 
- Indian ink
- Alizarine ink
- Pen and ink
- Ink, especially the section "Writing inks and preservation"
- Oak Marble gall
- Iron gall ink - Chemistry by Elmer Eusman (1998)
- Fruen, Lois (2002). "Iron Gall Ink".
- Mazzarino, Sara. "Report on the different inks used in Codex Sinaiticus and assessment of their condition". Retrieved 16 March 2013.
- Registrars' Ink
- Dienstordnung für Notarinnen und Notare (DONot), Abschnitt Herstellung der notariellen Urkunden § 29 (German)
- Buchheister-Ottersbach: Vorschriften für Drogisten. 11. Auflage von Georg Ottersbach (Volksdorf/Hamburg). Verlag Julius Springer, Berlin 1933 (German)
|Wikimedia Commons has media related to: Iron gall ink|
- Iron Gall Ink - Traveling Scriptorium - A Teaching Kit by the Yale University Library 21 March 2013
- The Iron Gall Ink Website