Outdoor durability of nano-sized silica-based chromatic reintegrations. Influence of exposure conditions and pigment composition

Outdoor exposed wall paintings are highly vulnerable to decay from environmental factors such as solar radiation, humidity, and soluble salts. These factors can also affect the chromatic reintegrations in wall paintings. However, little research has been conducted on their durability. Therefore, a novel approach has been carried out by exploring the use of an aqueous colloidal dispersion of nano-sized silica as a binder in chromatic reintegrations under different environments. Thus, the binder was mixed with pigments (silicate-, oxide- and sulphide-based ones) to evaluate the influence of their chemical nature on the durability. Also, the paint mock-ups were exposed for one year to outdoor Csb- and Dsa-type environments (Köppen-Geiger climate classification system), in which moisture, temperature, rainfall and proximity to the sea differ considerably. Colour and gloss were measured before and after exposure. The paint samples were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and optical and scanning electron microscopies. After exposure, colour stability was highly influenced by the chemical nature of the pigments. A calcium-rich layer developed on all the mock-ups, together with other deposits, whose extent was highly influenced by the exposure conditions. Oxide-based pigments provided the best results, followed by the blue silicate-based ones. In contrast, sulphide-based and green earth-based pigments suffered substantial paint loss and colour changes. The different performance of the paintings was due to the affinity of the pigments with the binder and their predisposition to form bonds with hydroxyl (–OH), silanol (Si–OH) and siloxane (Si–O–Si) groups.