# The Influence of Raw Materials on the Stability of Grisaille Paint Layers  
[PERSON]  
1 Department of Conservation and Restoration, NOVA School of Science and Technology,  
2829-516 Caprica, Portugal 12 Research Unit VICARTE-Glass and Ceramics for the Arts, NOVA School of Science and Technology,  
2829-516 Caprica, Portugal 2 Research Unit VICARTE-Glass and Ceramics for the Arts, NOVA School of Science and Technology,
for example, in the Eraclius recipe for the Jewish glass [5], and later, as in the 19 th-century treatise _Guide du verrier_ by [PERSON], is described the use of the mineral minium as lead source [6]. The lead is going to reduce the melting temperature of the glass [7], which explains the continuous use of high lead-based glasses in the grisailles formulation, as it is needed that grisailles have a lower melting temperature than the support glass in the way of not deforming it during the firing treatment of the paint layer. Iron and/or copper oxides, obtained by firing these two metals, were the main coloring agents described throughout the centuries [2,4]. However, firstly proposed by [PERSON] in the 16 th century [8] and later described by [PERSON] [6], iron-based pigments, such as hematite, substituted these burned metals. Manganese is also punctually mentioned as a grisaille colorant in the 17 th century, _Ars Vitraria Experimentalis_ by [PERSON]. The recipes also describe vehicles and temporary binding agents as the materials that give the necessary plasticity to the mixture before being fired, allowing it to be applied on the glass support. The vehicles usually described are the common ones used in glass painting, such as gum arabic and water, wine, vinegar, egg white, and oils such as lavender oil [2]. Other materials, including various compounds, are also mentioned. The role of these compounds on the grisailles is uncertain, as it is not described in the historical sources. However, they can be understood as additives that can work as opacifiers and/or fluxes. For example, the use of the pigment lead white, described by [PERSON], can also help lower the melting temperature [2]. From the analytical results on historical grisaille compositions described in the literature, it was possible to confirm the use of the same raw materials described in the historical recipes. The grisailles generally present a high quantity of lead in their composition [9, 10, 11], which confirms the use of high lead-based glass in their production. [PERSON] et al. [12] analyzed the stained-glass windows of the Sainte Chapelle in Paris (France) and detected that the vitrified matrix had high lead contents, low silica, and small traces of alumina, lime, and potash [12]. Regarding the coloring agents, it is also possible to confirm the recurrent use of iron and copper oxide, being the hematite (\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)) and tenorite (CuO), the main compounds identified in historical grisailles used individually or in combination, agreeing with the historical recipes [9,11,13]. When used in combination, mixed compounds can be formed as the cuprospinel (CuFe\({}_{2}\)O\({}_{4}\)), identified in the grisailles from the cathedral of St. Michael and Gudule in Brussels (Belgium) [14] and in the grisailles from the cathedrals of Avila and Segovia (Spain) [11]. All the changes in the raw materials can impact the grisaille paint layer's stability. As stated by [PERSON] [8] and further developed by others [1,9], the grisaille alteration can depend on its chemical composition and production methodology. Additionally, the firing conditions, the ratios between the different components, and the compatibility between the grisaille and the substrate glass can also influence the grisaille's durability [1,15,16]. For example, [PERSON] et al.