## 5 Discussion
In the flesh tones, colorless powdered glass was mixed with lead white, vermilion, bone black, hematite, red lake and dolomite particles, the latter probably intentionally added as a filler or extender, as seen in other mixtures. According to the FTIR and FTIR-ATR results, as well as the GC-MS data, the presence of oil binders is attested in almost all the samples. Only on the SF samples (_Visitation_ altapiece panel) and on those containing azurite, egg binder was detected. However, we should mention the identification of drying oil in the aged samples using GC/MS is very difficult due to the oxidization/polymerization of the unsaturated fatty acids [56; 65] and the conservative history of the artworks has to be considered [5]. The widespread presence of casein and glue in the _Martyrdom of St. Sebastian_ and the _Tobias and the Archangel Raphael_ could be attributed to the use of these materials in the restoration interventions of the past rather than a voluntary use of them as a binder [66; 67; 68]. Past restoration interventions might also be the cause of the frequent occurrence of wax on the _Martyrdom of St. Sebastian_ (GSM samples). The presence of a colorless powdered glass type containing manganese in [PERSON]'s painting layers, hypothesized through non-invasive exams [6], was confirmed in the painting layers of the _Visitation_, the _Martyrdom of St. Sebastian_ and in _Tobias and the Archangel Raphael_. According to the results, [PERSON] seems to have used this glassy material mainly with the red lake, as it was detected in red glaze layers, as well as in flesh and pink underpinit layers. The addition of powdered glass was also observed in verdigris-based layers, a technical expedient scarcely documented in the literature. Oil binder was attested in all of these painting layers. The use of glass and nut oils is also attested in [PERSON]'s _Virgin and Child_ of the National Gallery, London, dating from 1488 [62] and in the _Music_ painted by [PERSON] dated from the 1470s and probably realized in Urbino [58]. The occurrence of colorless powdered glass particles was attested in the latter author's _Communion of Apostles_, not only in the red lake-based layer, but also in a blue pictorial layer: an occurrence that seems quite rare for that time. While the presence of red lake in this blue layer containing natural ultramarine could explain the glass, the minimal number of red lake particles suggest that the glass was not solely due to the red lake and might have been used to maintain layer transparency, thereby taking advantage of the intricate and rich underdrawing [69]. The comparison between the elemental composition of powdered glass particles in [PERSON]'s paintings and the _Communion of the Apostles_ showed quite similar values, indicating they both used a soda lime Al-poor glass, which aligns with the Italian type rather than the German or Dutch types. This composition is related to the use of river pebbles as a possible source [58]. Starting already from the 14 th century, Venetian glassmakers used quartz pebbles instead of silica sand to reduce coloring contaminants such as iron, alongside a Levantine source of flux, soda-lime coastal plant ash [59]. In light of these results, with regard to the glass powdered function, it is likely that the colorless powdered glass was used as a filler to create overlapped translucent layers, without significantly modifying the refractive index of the binder, i.e., giving more body to the layers and providing transparency that other fillers would not have allowed. Recent studies on the drying properties of the powdered glass in red lake layers showed that they did not produce significant differences in drying time, but they noted that the glass additions made the madder lake easier to grind and paste with the oil medium [70; 71; 72]. Regarding the size of powdered glass, in the [PERSON] paintings, they measure up to 45 \(\upmu\)m as attested in the Italian artworks ([70] and references therein).