Decay of red pigments on a wall painting adorning the church of ‘San Francesco Dei Capuccini’ in Racconigi (Italy): archaeometric survey and restoration intervention

An archaeometric survey allowed to understand the causes of an atypical ongoing decay, manifested by a 19th century fresco of an unknown artist titled ‘The divine adoration of King St. Louis IX’, held in the Church of ‘San Francesco dei Cappuccini’, in Racconigi (Northwestern Italy). The wall painting suffered of a diffuse darkening, principally affecting the red garments and flesh of some of the portrayed religious characters. The adopted multi-analytical approach (consisting of X-ray powder diffraction, micro-Raman and FT-IR spectroscopies) pointed out that the causes of the manifest darkening were represented by multiple, strictly intertwined factors. In particular, a partial but progressive blackening of the red pigments had occurred, due to an incipient conversion of hematite (Fe2O3) into magnetite (Fe3O4). Such a transformation, possibly confined in a very thin, superficial and discontinuous film on the pigmented layers, had been favoured by an ongoing sulfation – and consequent alteration of calcite into gypsum due to exposure to gaseous sulfur compounds (SO2 and SO3), produced after the burning of oil in votive lamps, candles and incense positioned directly below the artwork. In addition, these combustions caused a significant amount of particulate (soot and powders) to cumulate on the fresco surface, thus contributing to generally enhance the dimming of the whole painting. These results gave way to a restoration intervention, which removed most of the gypseous efflorescences and particulate from the artwork surface, thus rendering at best its original chromatic aspect and brilliance. While performing these operations, the signature of the presumed artist – identified as Dionigi Faconti, a renowned Italian painter of the 19th century – and the date of realization of the artwork (1852) were recovered.