Raman imaging to quantify the thermal transformation degree of Pompeian yellow ochre caused by the 79 AD Mount Vesuvius eruption

Most of the wall paintings from Pompeii are decorated with red and yellow colors but the thermal impact of 79 AD Mount Vesuvius eruption promoted the partial transformation of some yellow-painted areas into red. The aim of this research is to develop a quantitative Raman imaging methodology to relate the transformation percentage of yellow ochre (goethite, alpha-FeOOH) into red color (hematite, alpha-Fe2O3) depending on the temperature, in order to apply it and estimate the temperature at which the pyroclastic flow impacted the walls of Pompeii. To model the thermal impact that took place in the year 79 AD, nine wall painting fragments recovered in the archeological site of Pompeii and which include yellow ochre pigment were subjected to thermal ageing experiments (exposition to temperatures from 200 to 400 degrees C every 25 degrees C). Before the experiments, elemental information of the fragments was obtained by micro-energy dispersive X-ray fluorescence (mu-ED-XRF). The fragments were characterized before and after the exposition using Raman microscopy to monitor the transformation degree from yellow to red. The quantitative Raman imaging methodology was developed and validated using synthetic pellets of goethite and hematite standards. The results showed almost no transformation (0.5% +/- 0.4) at 200 degrees C. However, at 225 degrees C, some color transformation (26.9% +/- 2.8) was observed. The most remarkable color change was detected at temperatures between 250 degrees C (transformation of 46.7% +/- 1.7) and 275 degrees C (transformation of 101.1% +/- 1.2). At this last temperature, the transformation is totally completed since from 275 to 400 degrees C the transformation percentage remained constant.