iron complexes of tannic acid, gallic acid, pyrogallol, and syringic acid (iron gallate)

Ingredients

tannic acid, gallic acid, pyrogallol, syringic acid, iron(II) sulfate

Preparation Steps

Original Recipe Text

The pigment is formed in situ by the complexation of Fe3+ ions (oxidized from Fe2+ vitriol) with phenolic hydroxyl and carboxylate groups of gallnuts extracts (p. 27937, 27944). The study analyzed specific models: GAOH-2Fe (where two Fe atoms interact with O atoms at positions 3, 4, and 5) and GACOO-1Fe (where one Fe atom interacts with the carboxylate group) (p. 27942).

Evidence

"The complexation of iron with these molecules leads to a strong change in color due to the deep restructuring of the polyphenol that can be clearly seen by Raman and FTIR spectra. Three main Raman bands appeared at 1450−1490 cm−1 (ν1), 1320−1345 cm−1 (ν2), and 400−650 cm−1 (ν3), which are characteristic of the metal complexes."

Remarks

This entry focuses on the chemical nature of the colorant within iron gall inks. The Raman marker bands at 1477 (ν1), 1344 (ν2), and 603 (ν3) cm−1 are characteristic of the TA-Fe complex (p. 27941). The ν3 band corresponds to the stretching modes of the new Fe−O bonds (p. 27942). The study also notes that iron complexation leads to significant fluorescence quenching, which facilitates the Raman inspection of historical manuscripts by reducing background interference (p. 27937, 27945). The pH decrease upon metal complexation is identified as a factor in the subsequent acidification of IGI inks and degradation of paper (p. 27944).