A historical ink dating back at least as far as the Roman Empire, iron gall inks were a staple in the western world into the early 20th century. These inks were traditionally made by mixing iron salt crystals with tannic acid from oak galls 2 —hence the name “iron gall.” Like pigment inks, iron gall inks are made of insoluble particles that become highly water resistant once embedded in paper.

Pure iron gall ink is clear when wet and turns dark gray over several minutes as it dries and oxidizes. To make them easier to see when wet and more appealing to read when dry, iron gall inks are mixed with dyes to give them color. A typical iron gall ink like Diamine Registrar’s Ink will start off as a light blue when wet and transform within minutes into a dark blue black. The dyes used to color the ink may run or fade when wet, but the dark gray lines that the iron gall particles make will remain sharp and clear.

In addition to being waterproof, iron gall inks tend to be dry-writing and very well-behaved, making them a good choice when using wetter pens or less fountain pen friendly paper.

So if iron gall inks are so great, why don’t we see more of them around? The main reason is that they’re acidic. If you open a medieval book written in iron gall ink, you may find pages filled with letter-shaped holes as if the letters were cut out of the page with a laser. Plenty of medieval iron gall writings have fared much better, however, and modern iron gall inks are far gentler than their medieval forebears. In some countries iron gall inks remain the preferred ink for signing legal documents due to their permanence. Still, you may want to stick with a pigment-based ink if you find the thought of your words crumbling away after a mere few centuries disturbing.

A more immediate concern with iron gall inks is the possibility of the ink corroding steel nibs. We’ve never seen a confirmed case of this happening ourselves, but we do recommend cleaning your pen regularly to be safe.