Effect of proteinaceous binder on pollution-induced sulfation of lime-based tempera paints

Even though, atmospheric SO2 contamination has been reduced over the last decades in Europe, practical experience has shown that sulfation of lime-based materials (i.e., lime-based tempera paint, lime mortar and plaster, as well as limestone) is still of importance, affecting monuments exposed to polluted air in urban centers. In order to evaluate the effect of organic binders (i.e., rabbit skin glue or egg yolk) on the chemical weathering resistance of lime-based materials, tempera paint dosimeters (i.e., paints containing calcite or mixtures of portlandite and calcite) were subjected to long-term outdoor exposure and accelerated SO2-aging. SO2-aging caused important morphological changes of the paint surface on the nano- and microscale. However, sulfation was significantly delayed in the presence of the organic binder. Furthermore, paints containing portlandite and calcite transformed faster into calcium sulfite hemihydrate and gypsum than paints containing only calcite. Calcium sulfite hemihydrate formation onto calcite always preceded non-epitaxial gypsum crystallization after dissolution of the sulfite precursor phase. These results suggest that a passivating product layer will not form onto calcite, and so sulfation will continue under suitable environmental conditions until all calcite is transformed into gypsum. Nevertheless, the organic binder strongly affected the mineralogical evolution of paints containing portlandite, resulting in the formation of organic-inorganic hybrid materials similar to biominerals. These hybrid materials generally have superior weathering resistance and might explain the absence of any clear signs of sulfation after prolonged outdoor exposure. The selection of lime-based tempera paints for conservation interventions must be made considering the prevailing exposure conditions. In polluted dry environments where carbonation of portlandite will be significantly delayed, the use of calcite-based tempera paints might be preferable, while tempera paints containing portlandite would be more suitable in humid climates where carbonation is fast, resulting in the formation of weather resistant hybrid materials.