Specifically this symbol refers to Atramentum, or Iron Gall ink, Ferrous Tannate &Gallate, a formulation of an Iron salt, usually from Green Vitriol (Iron Sulfate) and Gallic Acid from Oak Gall, thickened with Gum Arabic and sometimes colored with a pigment. The ink produced is waterproof, penetrates the parchment, and is difficult to erase, but acidic, and degrades paper.