The mineral pigments raw sienna and burnt sienna, known as bolar earths, mostly consist of iron (hydr)oxides; the two pigments, yellow and red-brown respectively, differ for thermal processing of the raw material. Sixteen bolar earth samples from a quarry in Monte Amiata and from the Accademia dei Fisiocritici Museum were investigated by XRD, SEM-EDS, TEM-EDS, thermal treatment, TG/DTA and DRIFT.
Raw sienna is yellow to brown and prevalently contains goethite. Burnt sienna is red and consists of hematite obtained by dehydration of goethite at around 270°C. Chemical analyses show dominant Fe2O3 (53.5–71.5 wt.%), with minor SiO2 (3.0–24.7 wt.%) and Al2O3 (0.8–7.1 wt.%). A very high arsenic content characterizes all the samples (with a mean of 5.6 wt.% As2O5). Goethite and hematite occur as finely dispersed nanocrystals, respectively 2–10 nm and 10–40 nm in size. They are enclosed in a minor amorphous silica matrix, with arsenic adsorbed on the iron (hydr)oxide nanoparticles. Due to its high arsenic content, the ochre used to produce raw and burnt sienna is an important example of an effective natural sink for arsenic.