Micro‐Raman and SER spectroscopy to unfold Lefranc's early organic pigment formulations

Abstract

The introduction of synthetic dyes and pigments into commerce in the late 19th century was a stimulating time for experimental scientists in their laboratory formulations and for artists in their usage. Nowadays, the period of pioneering syntheses and employment of synthetic organic colouring materials is highly intriguing for scientific research and for conservation purposes in heritage science. In this work, a combination of spectroscopic and chromatographic methods has permitted the elucidation of the chemical composition of 16 early synthetic organic pigment powders from the Lefranc experimental laboratory collection dating 1890-1914. It can be hypothesized that the most successful of these formulations could have been finalized in paint tubes and released onto the market, meaning that they may even be found in artworks from the same period. Specifically, information from this crucial period of transition from the exclusive use of natural dyes/colourants (such as neoflavonoids and plant and insect anthraquinones) to the introduction of their synthetic counterparts (such as synthetic alizarin and purpurin, and acid stable cochineal, together with monoazo and diazo pigments) is confirmed through Raman/surface-enhanced Raman spectroscopy and high-performance liquid chromatography with diode array detection. By means of X-ray fluorescence and mid-Fourier transform infrared spectroscopy, a combination of traditional and modern inorganic substrates has been identified (alum, kaolin, barium sulfate and metallic salts), while Raman has provided additional data regarding the possible complexation of these pigments in the form of lakes

HPLC-DAD and HPLC-ESI-Q-ToF characterisation of early 20th century lake and organic pigments from Lefranc archives

“…Mixing eosin (Pigment Red 90) and phloxine B (Acid Red 92) is mentioned in the historical treatise The Dyeing of Paper Pulp by Julius Erfurt, a practical manual, dated 1901, for use by papermakers, paper stainers, students and others, and phloxine B was detected by FT-Raman in one of the samples of actual dyed papers included in the book to illustrate each recipe [27]. In this case, it can be hypothesised that phloxine B was present in the powder as its aluminium lake, known as Pigment Red 174.…”

“…Due to the poor solubility of organic lake pigments and synthetic pigments in any medium (including water or organic solvents), the most commonly used approaches for their detection and characterisation exploit Raman spectroscopy or, whenever possible, surface-enhanced Raman spectroscopy (SERS) [18][19][20][21][22][23][24][25][26][27][28], thanks to the availability of dedicated spectral libraries. Whenever sampling has been permitted, analytical pyrolysis coupled with gas chromatography-mass spectrometry (Py-GC/MS) [1,[29][30][31][32] has been successfully applied.…”

Recent advances in linear and non‐linear Raman spectroscopy. Part XI

“…Gabrieli and co‐workers used micro‐Raman and SERS to unfold Lefranc's early organic pigment formulations. In this work, a combination of spectroscopic and chromatographic methods permitted the elucidation of the chemical composition of 16 early synthetic organic pigment powders from the Lefranc experimental laboratory collection dating between 1890 and 1914 . Gao et al demonstrated the on‐site detection of phosgene agents by SERS coupled with a chemical transformation approach.…”

Applications of Raman spectroscopy in art and archaeology

“…The elucidation of the chemical composition of 16 early synthetic organic pigment powders from the Lefranc experimental laboratory collection dating 1890–1914 was performed by Gabrieli et al . It can be hypothesized that the most successful of these formulations could have been finalized in paint tubes and released onto the market, meaning that they may even be found in artworks from the same period.…”