Elucidation of the Fe(III) Gallate Structure in Historical Iron Gall Ink

Abstract

Synthetic, structural, spectroscopic and aging studies conclusively show that the main colorant of historical iron gall ink (IGI) is an amorphous form of Fe(III) gallate• xH 2 O (x = ∼1.5−3.2). Comparisons between experimental samples and historical documents, including an 18th century hand-written manuscript by George Washington, by IR and Raman spectroscopy, XRD, X-ray photoelectron spectroscopy, and Mossbauer spectroscopy confirm the relationship between the model and authentic samples. These studies settle controversy in the cultural heritage field, where an alternative structure for Fe(III) gallate has been commonly cited.P rior to the 20th century, historical iron gall ink (IGI) was by far the most common writing material of the western world, and a plethora of recipes from which to produce the ubiquitous dark, brown-black ink can be found starting from at least the Middle Ages. This ink has been used to pen many of the most important documents and drawings in human history, including unique, hand-written works such as Thomas Jefferson's original draft of the Declaration of Independence, Abraham Lincoln's first draft of the Emancipation Proclamation, and Beethoven's original scores. While the virtue of IGI lies in its relative permanence, the great vice of this medium lies in its well-known tendency to degrade paper and parchment substrates. 1−7 Despite its historical importance, there is little consensus on the chemical structure or composition of the iron-gallate complex, the main species responsible for the color of the IGI. We describe here a series of synthetic, structural, spectroscopic, and aging studies, which unequivocally demonstrate that the primary colorant in IGI is an amorphous form of an octahedral Fe(III) gallate metal organic framework structure that has previously been described by Wunderlich 8−10 and Feller. 11 Unlike the majority of prior studies, we use authentic IGI precursors to prepare both crystalline and amorphous forms of the IGI precipitate and study the crystal-to-amorphous transition by way of XRD, thermal gravimetric analysis (TGA), IR and Raman, Mossbauer, and X-ray photoelectron spectroscopy (XPS). Spectroscopic comparisons with historical documents prove the relevance of the synthetic crystalline and amorphous forms of the model IGI materials to those found in the authentic manuscripts.

Rust‐Mediated Continuous Assembly of Metal–Phenolic Networks

“…The metal–ligand interactions in GA/R‐Fe III films were further probed by resonance Raman (RR) spectroscopy (Figure n). The RR spectrum revealed bands in the high‐frequency region around 1470, 1320, and 1230 cm −1 , which can be attributed to the skeletal modes of a substituted benzene ring . Bands in the low‐frequency region of 650–400 cm −1 can be attributed to the FeO vibration (ν FeO ) due to galloyl–Fe III interactions .…”

“…The RR spectrum revealed bands in the high‐frequency region around 1470, 1320, and 1230 cm −1 , which can be attributed to the skeletal modes of a substituted benzene ring . Bands in the low‐frequency region of 650–400 cm −1 can be attributed to the FeO vibration (ν FeO ) due to galloyl–Fe III interactions . In addition, bands at 1578 and 1430 cm −1 arising from asymmetric and symmetric stretching of the carboxylate (COO) group (ν as and ν s , respectively), can be assigned to (COO)/Fe III interactions .…”

Iron Gall Ink Revisited: In Situ Oxidation of Fe(II)–Tannin Complex for Fluidic‐Interface Engineering

“…Two main players in the ink are tannins (e.g., tannic acid (TA) and gallic acid), from oak galls, and Fe(II) cation, which form a soluble Fe(II)–tannin complex in aqueous solution. The darkening and indelibility of the ink, rather pale initially upon mixing, is made achieved by the air oxidation of Fe(II) to Fe(III) ion in the Fe(II)–tannin complex, followed by (semi)‐infinite self‐assembly of Fe(III)–tannin complexes . Although the coordination complex formation of Fe(II) with tannins is known to facilitate the oxidation of Fe(II) by lowering the oxidation potential of Fe(II), the air oxidation step—rate‐limiting step in the ink preparation—is adequately slow enough not to cause significant flocculation of big, nonadhesive Fe(III)–tannin flocs (also known as flakes) in the ink.…”

Drawing with Iron on a Gel Containing a Supramolecular Siderophore

“…The pattern should be erasable by reducing chelated Fe 3+ to Fe 2+ . Although iron complexes of gallic acid are venerable inks, and phenolic‐Fe 3+ complexes are used for writing, our strategy is unique in that the HA chelator is imbedded in the “paper” and color is generated by applying FeCl 3 . While polymeric gels containing HAs are known, there are no examples of supramolecular hydrogels with HA groups.…”