# Deterioration of White Tempera Mock-Ups Paints in a SO\({}_{2}\)-Rich Atmosphere
## 1 Introduction  
Over the last decades, part of the research studies carried out in the field of the conservation of cultural heritage has been focused on studying the influence of atmospheric contamination on the construction materials used in architectural heritage [1, 2, 3]. Monuments and historical buildings are frequently located in urban environments where air contamination coming from traffic, industrial and construction activities and domestic heating have negative consequences on the materials used in structures and decorative elements. It is known that, among cultural heritage materials, outdoor historical wallpaintings are also sensitive to anthropogenic pollutants, suffering, consequently, deterioration and subsequent loss [4]. Hence, it is important to characterize the decay forms and clarify the degradation mechanisms of such historical paintings when exposed to contaminated atmospheres in order to develop respectful conservation and restoration strategies [5; 6]. Even though SO\({}_{2}\) levels have been reduced over the last decades [7], sulfur dioxide continues to be the main pollutant gas involved in the deterioration of building stones [5] and the main agent responsible for the sulfation process of historical paintings [6; 8]. In this respect, the high porosity of the paint layers and their underlying mortars are susceptible to pollution-induced chemical decay. This deterioration also involves the action of particulate pollutants, as well as other atmospheric factors, such as moisture [1; 2]. Tempera is an old painting technique that consists of an emulsion where pigments are mixed with a water-soluble binder, typically of a proteinaceous nature, such as egg yolk, rabbit glue, casein (milk) or gums [9]. The tempera technique was replaced by oil painting in the Late Renaissance and Baroque periods; however, a remarkable revival of the tempera technique has taken place since the 20 th century [10]. Tempera wall paintings are widely spread along the south of Spain following the so-called a secco technique where, unlike in the buon fresco technique, pigments are fixed to the surface mixed with an organic binder [4; 10]. Tempera paintings are long-lasting since good examples from the ancient Egyptian period still stand [11]. Since the substrate in contact with water is a significant source of Ca\({}^{2+}\) and CO\({}_{3}\)\({}^{2-}\) ions, in contaminated atmospheres, oxides, sulfates, sulfites, oxalates, carboxylates and carbonates are commonly found as degradation forms of tempera paint materials [4; 6; 12; 13; 14; 15; 16]. Gypsum (CaSO\({}_{4}\cdot 2\)H\({}_{2}\)O) has been identified in wall paintings [12], in lime-rich mortars (i.e., made of calcite, CaCO\({}_{3}\)) exposed to medium/high SO\({}_{2}\) levels in south Italy [4], and in lime-based tempera mock-ups artificially aged in SO\({}_{2}\) rich atmospheres in the presence of moisture [6]. In the case of lead white (2 PbCO\({}_{3}\cdot\)Pb(OH)\({}_{2}\)), a pigment very susceptible to saponification by reaction with oil binders [14], blackening occurs when it is present in fresco paintings and subjected to SO\({}_{2}\) exposure. This is due to the formation of (i) PbO\({}_{2}\), due to the interaction with oxidizing agents, ii) or to the formation of PbS [15]. The exposure of paintings made with this pigment to sulfur pollutants and humidity results in the formation of Pb sulfides and Pb sulfates [16]. All these degradation products trigger colorimetric changes that can be increased by chemical modifications promoted by the interaction of the paint components with sunlight and atmospheric pollutants, as well as by physical changes that take place mostly due to pigment-binder interactions [17]. Regarding this interaction, recent studies have shown that the susceptibility to deterioration due to exposure to a SO\({}_{2}\) atmosphere depends greatly on the type of binder and its interaction with the pigment. Firstly, it seems that the susceptibility to deterioration of the binder varies depending on its composition.