Shedding light on the composition and degradation mechanism of dyes in historical ink's collection (19th-20th century)

Man has always used writing to be able to communicate, express, and disseminate his thoughts. In time, many different coloured extracts of plant and animal origin have been used to produce inks; after the development of synthetic chemistry, artificial and synthetic dyes started to be widely exploited. The end of the 19th century marked great technological and industrial innovations in commercial production of artists' materials. To reveal ink formulations and build a database of red inks by different producers, we developed a multi-analytical approach and investigated a collection of writing inks produced in France in the late 19th - early 20th century. The materials used as binders, additives, dyes, and pigments have been investigated by high performance liquid chromatography coupled with diode array and tandem mass spectrometric detectors (HPLC-DAD-MS2), in situ derivatisation pyrolysis coupled with gas chromatography-mass spectrometry (Py(HMDS)-GC/MS), surface enhanced Raman (SERS, TLC-SERS) and X-Ray Fluorescence spectroscopies. Several dyes and pigments were detected, showing that the French ink's formulations of the early days of synthetic dye industry were based on rhodamine B and 6G, eosin Y, rose Bengal and methyl or crystal violet. Instead, as binder and additives only gum Arabic and shellac resin have been identified, respectively. Mass spectrometry also allowed us to detect possible by-products of the synthesis of ink's dyes and even early degradation products, that can be used for ink identification in historical writings and drawings. Our studies can pave the way to investigate inks in historical samples by introducing ultra-sensitive chromatographic and mass spectrometric methods in the array of analytical tools available to the chemist.