What chemical is produced in high concentrations by Oak galls and historically used to create ink?

• A. Citric acid
• B. Quinic acid
• C. Tannic acid
• D. Lactic acid

Answer: (C)

Oak galls are rich in tannic acids, a type of polyphenolic compound produced by the plant in response to insect attack. These tannins were historically harvested to make iron gall ink, a durable pigment formed when tannic acid reacts with iron salts and a binder. The resulting ferric tannate ink darkens as it dries and binds well to parchment and paper, which is why it was widely used for centuries. Tannic acid from oak galls directly links the plant tissue you see in a gall to the traditional ink-making process. The other acids listed occur in different contexts—citric acid from citrus fruits, lactic acid from fermentation, and quinic acid in various plants—but none are specifically associated with oak galls or the classic ink method.