A colourless crystalline compound, C6(COOH)6, m.p. 288°C. Its molecules consist of a benzene ring in which all six hydrogen atoms have been substituted by carboxyl (–COOH) groups. It occurs naturally in some lignite beds as honeystone (the aluminium salt), and is made by oxidizing charcoal with concentrated nitric acid. It decomposes on heating to form pyromellitic anhydride, used in making epoxy resins. Condensation products of mellitic acid are employed in making a wide range of dyes.