Multianalytical Approach to explain the darkening process of hematite pigment in paintings from ancient Pompeii after accelerated weathering experiments

[EN] In this paper, recently excavated fresco painting fragments from the House of Marcus Lucretius (Pompeii) and not exposed to the atmosphere since the eruption of the Mount Vesuvius were subjected to a controlled SO2 atmosphere and high relative humidity. These experiments were conducted in order to simulate under accelerated conditions the possible deterioration of the hematite pigment and plaster. The mineralogical transformation of the polychromy and plaster was monitored using mainly Raman spectroscopy, a non-destructive technique, but also infrared spectroscopy (FT-IR) and scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS). After different exposure cycles to SO2, it was confirmed that hematite red pigment (Fe2O3) can be reduced into magnetite (Fe3O4), which provides the darkened colour to the pigment. While Fe(III) from hematite is reduced into Fe(II) or mixed Fe(III) and Fe(II), the SO2 can be oxidized (SO3) and hydrated to experience a subsequent wet deposition (H2SO4 aerosol) causing also the transformation of calcite into gypsum. Finally, it was assessed that high concentrations of SO2 can also cause the sulphation of hematite pigment promoting its transformation into paracoquimbite/coquimbite (Fe2(SO4)3$9H2O). Moreover, in some areas of the deteriorated painting fragments, non-expected iron(II) sulphate and sulphite species were also identified

In situ time-lapse synchrotron radiation X-ray diffraction of silver corrosion

Several heritage systems have been studied using state-of-the-art synchrotron techniques. The cultural heritage value of silver is documented in museum collections across the globe. However, the silver surface is not as chemically stable as that of other precious metals, and is susceptible to corrosion by atmospheric gases. It is therefore of special interest to clarify these surface reactions by using in situ, time-lapse chemical and structural analysis in controlled ambients in order to develop strategies to reduce or even prevent the atmospheric attacks. In order to study the initial corrosion processes of silver in the presence of corrosive gases in situ time-lapse X-ray diffraction experiments were performed on the XMaS beamline at the European Synchrotron Radiation Facility, Grenoble. Highly pure silver samples were weathered with synthetic air containing 500 ppb of both H2S and ozone, at relative humidity (RH) levels, and XRD patterns were tracked every 10 min over a total weathering time of 24 h. The time-lapse Synchrotron Radiation (SR)-XRD data show that pure silver exposed to those atmospheres starts to form crystalline corrosion products after only 10 minutes. Silver sulfates, silver oxides, intermediates and mixed species are formed on the sample surface over the duration of the experiment. The data collected using a newly combined environmental cell/gas flow set up introduces a set of highly useful tools for scientists who wish to study time-lapse gaseous corrosion at ambient temperature and pressure

The corrosion process of sterling silver exposed to a Na2S solution: monitoring and characterizing the complex surface evolution using a multi-analytical approach

Many historical 'silver' objects are composed of sterling silver, a silver alloy containing small amounts of copper. Besides the dramatic impact of copper on the corrosion process, the chemical composition of the corrosion layer evolves continuously. The evolution of the surface during the exposure to a Na2S solution was monitored by means of visual observation at macroscopic level, chemical analysis at microscopic level and analysis at the nanoscopic level. The corrosion process starts with the preferential oxidation of copper, forming mixtures of oxides and sulphides while voids are being created beneath the corrosion layer. Only at a later stage, the silver below the corrosion layer is consumed. This results in the formation of jalpaite and at a later stage of acanthite. The acanthite is found inside the corrosion layer at the boundaries of jalpaite grains and as individual grains between the jalpaite grains but also as a thin film on top of the corrosion layer. The corrosion process could be described as a sequence of 5 subsequent surface states with transitions between these states