Rapid chromatic alteration of Street Art: Mechanisms of deterioration of the painting materials of the 20 years of Freedom and Democracy mural

Over the last few years, contemporary mural paintings have gained significant importance as both artistic and social expressions within urban environments. The study focuses on the characterization of the faded painting materials from a case study in Milan, 20 years of Freedom and Democracy, and its aesthetic, morphological and chemical alterations. The goal is to study the phenomenon of chromatic alteration and understanding causes and mechanisms. In this work, a diagnostic study based on non-destructive and micro-destructive investigation methods is presented, improving and integrating the knowledge about Street Art deterioration. The colour fading of this mural was clearly evident from the available online photographic documentation, which allowed the reconstruction of the evolution of its appearance over time since its realization. The analysis revealed that colour fading occurred within the first two years. A wide range of analytical techniques were employed to gain deeper insights into the qualitative and semi-quantitative composition of the materials. These included hyperspectral imaging, microscopy observations and molecular and elemental spectroscopies of collected paint samples (μ-FTIR ATR and reflectance mapping, μ-Raman spectroscopy, FE-SEM observation and EDXS mapping). Colour fading was found to affect less than 10 μm of the surface. The deterioration of the polymeric binder promotes the loss of pigments and therefore the enrichment of white fillers on the surface, the so-called chalking effect, in agreement with other studies on Street Art murals. Furthermore, the photochemical instability of specific pigments (PR112, C.I.:12370 and PY83, C.I.:21108) was demonstrated, leading to the dramatic colour changes observed on the real surface.