Synthesis of yellow and red ochre: a preliminary multi-analytical, micro-invasive investigation on recipes and recognition criteria

This research aims to underline distinctive features to detect synthetic ochres on paintings, being the use of synthetic red widespread and well documented since Prehistory, while synthetic Mars yellow has been commercialized since the eighteenth century. Mars yellow was synthesized from Fe(NO3)3 and KOH reaction, according to ancient recipes. Hematite was synthesized by heating Mars yellow and other natural yellow ochres, simulating a soft, less sophisticated, and a hard, more modern, heating. The natural products were characterized by X-ray diffraction. Micro-Fourier transform infrared spectroscopy (μ-FTIR) and scanning electron microscopy (SEM) were used, respectively, for a compositional and morphological comparison between natural and synthetic pigments. The synthetic pathway was also reproduced and followed through differential thermal analysis (DTA and TG). XRD remarked the different origin of precursory samples. SEM showed typical morphological features of Mars yellow that are partially retained even after heating, leading to the recognition of both Mars pigments—red and yellow. DTA and TG highlighted that the synthetic process sensibly differs from Mars yellow to natural ochres. Mars yellow showed a double-peak dehydration with minima at 278 and 304 °C, while a single-step process was recorded for natural ochres. Distinctive features of Mars products were for the first time characterized thanks to this combined methodology. Finally, μ-FTIR proved to be very effective to discriminate the soft heating from the hard one. In particular, hydroxyl and FeO6 bands in the fingerprint region were the most useful for this task. Graphic abstract: [Figure not available: see fulltext.]