iron gall inks (tannic acid-based inks)

Ingredients

tannic acid, gum arabic, iron (II) sulphate heptahydrate, copper (II) sulphate pentahydrate, ultrapure Mili-Q water

Preparation Steps

Original Recipe Text

As per (p. 4), the inks were prepared with 31.4 gL−1 gum arabic and 49.2 gL−1 tannic acid. Copper (II) sulphate and iron (II) sulphate were added in proportions to achieve Cu(II):Fe(II) molar ratios of 0, 0.35, and 0.7. The 0 ratio formulation used 7.5 mmol of iron sulphate in 50 mL of solution. Reference ink Cu/Fe-GAL0 was prepared using gallic acid instead of tannic acid at the same mass. Inks were applied to aluminium foil to create dry samples and to Whatman™ paper No. 1 with a brush usually using a single stroke (p. 3).

Evidence

"As historical iron gall inks often contain other transition metal ions [21], such as copper, in addition to iron, we prepared inks with different ratios of copper to iron based on literature recipes [31]. All these inks consisted of 31.4 gL−1 gum arabic, 49.2 gL−1 tannic acid, 0.15 molL−1 of copper (II) sulphate and iron (II) sulphate."

Remarks

Iron gall inks can be distinguished by three strong Raman features at ca. 1475, 1343, and 603 cm−1, characteristic of the metal polyphenol complex (p. 5). The study notes that iron gall inks are corrosive, potentially attacking metal nibs and causing catalytic degradation (acid-hydrolysis and metal-catalysed oxidation) of the paper carrier, leading to discolouration and embrittlement (p. 3). Reflection FTIR only detected residual tannic acid at ca. 1714 and 1535 cm−1 on paper, as other bands are hindered by cellulose signals (p. 8).