Non-destructive evaluation of ammonium oxalate treatment penetration depth using micro-SORS

The determination of the penetration depth of treatments is a crucial issue in conservation science for assessing treatments efficacy. Micro-spatially offset Raman spectroscopy (micro-SORS), that permits the non-destructive collection of Raman photons generated under the surface of a turbid material, here has been used to non-destructively retrieve the distribution of oxalates formed after ammonium oxalate (AmOX) treatment on plasters. Two sets of samples treated with AmOX have been studied, namely plaster mock-ups and plaster fragments collected from the painted façades of Palazzo Besta in Teglio (Sondrio, Italy). The evaluation of the distribution of the newly formed oxalates was carried out considering the slope of the oxalates/calcite Raman bands intensity decay trend with increasing micro-SORS defocusing distance. Micro-SORS outcomes were found to be consistent with those obtained with conventional micro-Raman performed on cross-section, confirming the extension of this innovative method to the evaluation of treatments efficacy.