The article discusses the discovery and characterization of colourless glass particles in the red glazes of paintings from two different altarpieces attributed to a specific painter in Portugal. The use of local glass in these paintings is suggested based on archaeological findings in southern Europe, particularly in Portugal. The glass particles in the paintings were likely used for their transparency and potential siccative properties, although the latter has not been definitively proven. By potentially lowering the oil concentration in the paint, the addition of glass may have indirectly assisted in the drying of the glazes.
The composition of the glass particles in these paintings differs from soda-ash glass typically analysed in European paintings, indicating unique compositional differences. The presence of glass in the uppermost medium-rich glazes suggests a deliberate addition by the painter, aligning with advice from Portuguese painting treatises of the period.
The glass particles in the paintings underwent severe leaching of alkali components, leading to degradation that may explain the disruption of the glaze layers in these artworks. Further research is needed to fully understand the complex processes involved in the degradation of the glazes and their glass particles. The high moisture levels and uncontrolled environmental conditions in the churches where the works are preserved, combined with the glass composition and paint formulation, likely contributed to the degradation observed.
Overall, the article sheds light on the use of glass particles in late 16th-century oil paintings in Portugal, highlighting the potential roles and consequences of incorporating glass into artistic materials of that time.
