Fanal® pigments in the spotlight The characterisation of phosphorus tungsten molybdenum acid - PTMA - pigments

This study focuses on the identification and characterization of 31 Fanal pigments. The samples were provided by Technische Universität Dresden and derive from five product lines of Fanal pigments from the period 1924–1934, in the colours: yellow, yellowish green, green, blue, violet, pink, and red. Fanal pigments are made from synthetic organic dyes containing basic functional groups, mostly consisting of triphenylmethanes. Since their pioneering introduction on the market by the chemical company I.G. Farben in 1924 as ‘Fanal®’ pigments, the dyes were precipitated with the anions of the heteropoly acids phosphotungstic acid (PTA), phosphomolybdic acid (PMA) or a combination (PTMA). This study employs a diverse analytical toolkit, combining both non-destructive and destructive techniques, to set a precedent for the future identification of Fanal PTMA pigments in works of art. It focuses on confirming the chemical composition of Fanal pigments, as documented in the archives of I.G. Farben, and identifying markers unique to specific Fanal product lines. Micro-Raman spectroscopy identified nine pigments across the different Fanal samples. High-performance liquid chromatography with photodiode array and high-resolution mass spectrometry detection (HPLC-PDA-HRMS) revealed thirteen main colourants, including rare dyes such as Ceres blue I (C.I. 44065) and Indolenine yellow (C.I. 48010). In addition, varying amounts of dye compounds, closely resembling the primary dyes, differing by one or two methyl or ethyl groups, were identified. Thermally assisted hydrolysis and methylation pyrolysis gas chromatography mass spectrometry (THM-Py-GC-MS) evidenced the consistent presence of emulsifier 'Emulphor FM' in the Fanal 'supra' product line. Furthermore, some small molecules were identified as unreacted compounds from the colourant synthesis process. With scanning electron microscopy - energy-dispersive X-ray spectroscopy (SEM-EDX) various PTMA and PTA salts were distinguished and additional compounds like aluminium oxide and/or blanc fixe were identified. Notably, the presence of dye compounds complexed with PTA appeared to be exclusively correlated with samples from the first two 'alt' and 'neu' product lines; no pure PMA salt was found. All other pigments were PTMA salts. Large differences in the particle morphology between pigments of different product lines could be observed in the SEM-backscattered electron images. For each pigment the main dye used proved to be consistent across the Fanal product lines, except for the yellow dyes employed for Fanal yellow and yellowish green.