Identification and cleaning of an onion-based residue in a historical layer of Giacomo Balla's Ritratto d'uomo / Eugenio Riva.

The restoration of Giacomo Balla's "Ritratto d'uomo" / Eugenio Riva provided new insights into the long-term effects of historical cleaning treatments and their impact on the painting's surface. This study documents the identification of an onion-based residue, likely resulting from a mid-20th-century restoration attempt. A combination of FTIR ATR spectroscopy, SEM/EDS, UV imaging, and DNA analysis confirmed the organic nature of the layer, with genetic sequencing detecting Allium cepa fragments, further supporting its attribution. To remove this aged residue, enzymatic cleaning with amylase proved to be the most effective and controlled method, ensuring the selective removal of the layer without affecting the underlying paint. The comparative evaluation of polar solvent alternatives highlighted their differential action on the phenolic and carbohydrate components of the residue. Further analyses (XRF and SEM/EDS) determined the structural composition of the painting, revealing the stratigraphy of the layers and identifying the pigments used in Balla's palette.