Sol-silicate versus organic paints: Durability after outdoor and ultraviolet radiation exposures

Sol-silicate paints and polyvinyl acetate paints with similar red and green colour tones were applied to concrete and brick specimens and underwent either natural outdoor exposure in a marine setting for 12 months or accelerated ultraviolet radiation exposure (total exposure 4620 h). Chromatic changes were regularly monitored via spectrophotometer measurements. After exposure, physical, chemical, and mineralogical changes were evaluated using a multianalytical protocol based on stereomicroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and contact angle and peeling test measurements.After both tests, all the samples underwent colour changes whose intensity was independent of the type of substrate and type of exposure, being more related to the nature of the chromophore than to the nature of the base paint (organic or sol-silicate). After the outdoor test, the detachment of the pictorial layer was occurred in all the samples, more intensely in the sol-silicate painted brick mockups; this detachment process was related to salt precipitation-dissolution phenomena, mainly gypsum from marine aerosol, which causes effects that are more adverse when the adhesion between the paint and the substrate is weak.Compared to conventional organic paints, the use of sol-silicate paints in works of art in public spaces could meet the durability expectations of artists, always performing a prior evaluation focused to the selection of substrates that provide good adhesion and located in environments free from sources of salts.