Iron gall ink contains three ingredients, plus water: oak galls, ferrous sulphate, and gum arabic. I used a 1770 recipe that calls for two ounces of crushed oak galls soaked overnight in one pint of water, then strained into one ounce of ferrous sulphate. One-half ounce of gum arabic is added, and the mixture stirred until it is dissolved.
Oak galls grow when a gall wasp lays an egg into a puncture on the underside of an oak leaf. As the larva develops, the tree secretes tannic and gallic acids, creating a round formation known as a gall nut or oak apple. These are harvested and dried. The hole from which the wasp emerged is clearly visible on each gall nut. I harvested live oak galls during the New England spring.