Unlocking the secrets of historical violet hues: A spectroscopic and mass spectrometric investigation of logwood ink recipes

The spread of logwood bark (Heamatoxylum campechianum L.) in Europe as a dyestuff dates back to the 16th century, while its integration into ink formulations only began in the 19th century (logwood inks). Renowned for their broad spectrum of achievable colours and their non-corrosive impact on paper, logwood inks have been used by many artists, for instance by Impressionists in their drawings. This work investigates the gap in the literature related to the chemical composition of logwood-based inks, specifically focusing on the comprehensive characterisation of dye components and the influence of recipes on the inks molecular profile. Several 19th-20th century formulations of logwood inks, differing in inorganic salts, additives and pre-oxidation time, were selected and reproduced according to historical recipes. Colourimetric measurements enabled us to evaluate the chromatic differences of logwood-based inks produced following historical methods. Instead, the organic component was investigated on reference mock-ups by analytical methods based on micro-Raman spectroscopy and liquid chromatography coupled to diode array detection (HPLC-DAD) and high-resolution tandem mass spectrometry (HPLC-ESI-Q-ToF) after suitable sample treatments. Raman analysis revealed significative shifts in vibrational modes of dyes depending on the inorganic salts used in the recipes, providing a unique fingerprint able of distinguishing alum-, alum/copper-, iron-, and chrome-based logwood inks. Additionally, HPLC-DAD and HPLC-ESI-Q-ToF enabled us to expand the dataset of molecular markers available for logwood inks identification, by tentatively assigning the structure of compounds never or seldom reported in the literature (G-compounds and hematein derivatives). The potential synergy of spectroscopic and chromatographic techniques was also explored, allowing us to achieve valuable insights on the oxidation pathway of hematoxylin to hematein (both C.I. 75290) in relation to the ink recipes. The proposed method is particularly valuable in forensic and heritage science, where the composition of inks can offer crucial insights into the origin and authenticity of documents or artworks.