## 4 Results
### Colorless Powdered Glass  
The occurrence of colorless powdered glass type containing manganese in [PERSON]'s painting layers was first suggested by Energy Dispersive X-Ray Fluorescence due to Mn presence, especially in red, pinkish and flesh hues [6].  
According to PLM and ESEM/EDX results (Figures 6 and S8, Tables S2-S4), the powdered glass particles were detected in layers containing red lake (in red, flesh and pinkish hues) or verdigris (in green hues).  
The glass particles measured up to 30 um in the painting layers of the _Visitation_, 30 um in the _Martyrdom of Saint Sebastian_ and 45 um in the _Tobias and the Archangel Raphael_. ESEM/EDX analysis showed an elemental pattern composed of silicon, calcium, sodium, aluminum, magnesium, potassium, iron and manganese, which clearly distinguish the glassy material. Table 1 reports the average value related to different glass particles found in the samples, expressed as weight % oxides. The data could be affected by systematic errors since the analyzed area is not exactly that of the glass particle, but also the surrounding area; for this reason, the larger particles were mainly considered. Moreover, red lake could contain other elements (lead, aluminum, potassium and phosphorus) which could interfere with the results. In fact, the Al content of glass particles can be overestimated in the red lake-based layers, ranging from 1.9 to 5.5% while in the green hue, it is 2%. The weight % oxides of glass powdered particles in [PERSON]'s samples (Table 1) indicate the use of common glass [58; 59]. The glass particles can be classified as sodic type as they contain Na\({}_{2}\)O with an average higher than 10%. Moreover, the average amount of K\({}_{2}\)O (2.09%) indicates the possible use of plant ash [58].  
The glass composition (Table 1) related to [PERSON]'s paintings was compared with that occurred from previous research in the _Communion of the Apostles_ panel by [PERSON] [11].  
Micro-stratigraphic investigations carried out on CA2 and CA7 samples of the _Communion of Apostles_ of [PERSON] (Table S1) revealed the presence of oil-based priming applied on the gypsum ground layer [10]. The red mantle of the kneeling apostle on the right (sample CA2) consists of an underpaint (40 \(\upmu\)m thick) composed of vermilion, lead white, red lake and powdered glass particles with sizes ranging up to 20 \(\upmu\)m. The overlayer glaze is composed of red lake and powdered glass particles (Tables S1 and S5, Figure 1c). The [PERSON]'s blue robe sample (CA7) is made of a layer (40 \(\upmu\)m thick) of ultramarine blue,  
Figure 6: _Martydom of Saint Sebastian_ panel. Sample GSM6: (**a**) cross-section, PLM micrograph, UV light, detail of glass particles in the red lake-based glaze layer; (**b**) ESEM-BSE micrograph, detail of glass particles. [PERSON] and the Archangel [PERSON] canvas. Sample GSSI: (**c**) cross-section, PLM micrograph, visible light; (**d**) ESEM-BSE micrograph of glass particles found in the underlayer based on green copper acetate. _Communion of the Apostles_ panel. Sample CA2: (**e**) cross-section, PLM micrograph, UV light, detail of glass particles in the red lake-based glaze layer; (**f**) ESEM-BSE micrograph of glass particle in the glaze layer.  
lead white, rare grains of red lake, and powdered glass particles measuring up to 20 \(\upmu\)m. The glass powdered composition is related to soda lime and Al-poor glass (Table 1).