*2.6. Preparation of the Cross-Sections*

The samples for the cross sections were taken by the restorer, using a micro scalpel, cutting triangular millimeter sized fragments containing all the layers composing the painting. Then, they were placed into cells of a silicon rubber mold on an already hardened resin layer. At this point, the embedding resin was prepared under the fume hood using some mL of unsaturated orthophtalic polyester resin (purchased from G. Angeloni, Venice) and few drops of hardener (methyl-ethyl-ketone peroxide). The components were stirred until the mixture was completely homogenous. Eventually, the resin was poured into the cells and left to dry for two days. Once the resin cured, the polishing procedure started. The cross sections were polished manually using silicon carbide paper with decreasing grain sizes. Different grit size were used (P120-P500-P800-P4000-P6000-P8000-12000, of an average size from 130 to 4 µm) and progressively replaced into the lapping machine, equipped with a magnetic rotating disk and water cooling system, from the coarsest to the finest one. The sections were pressed onto the abrasive papers for some minutes and frequently observed at the stereomicroscope to check the level of polishing. This procedure ended when the samples were reached from the long side of the resin blocks.
