Iron gall ink

Iron gall ink is a historic ink made by combining iron salts with tannic acid extracted from oak galls, producing a liquid that starts out relatively pale and gray-blue when first applied but darkens significantly as it dries and oxidizes on the page. Some modern calligraphy inks are made in this traditional style or marketed as iron gall style formulas that mimic the look without the exact historic recipe.

The darkening-as-it-dries property is part of the appeal for some letterers, giving finished work a rich, deep black or near-black tone that develops over the following minutes and hours rather than appearing instantly. Traditional iron gall ink can be mildly acidic and, in some historic formulations, corrosive to steel nibs over long exposure, which is why many modern versions are reformulated to be gentler on equipment.

When choosing an iron gall style ink, check whether it is described as fountain-pen safe if using it in a cartridge pen, since acidity and particle content vary between brands and some are dip-pen only. A common mistake is judging the final color immediately after writing, before the ink has had time to oxidize and reach its true finished shade, which can lead to unnecessary reworking of a piece that would have darkened on its own.