The existence of historical pigments databases is important to speed up cultural heritage research. Knowledge of their chemical composition and their manufacture contributes to the study of art history and helps develop accurate conservation-restoration strategies. In this study, a total of nineteen pigments, among which we find silicates (Egyptian blue, natural and synthetic blue ultramarine, green earth and chrysocolla), oxides (natural and synthetic hematite, red and yellow natural ochres, and chromium green), carbonates (natural and synthetic azurite, natural and synthetic malachite, and white lead), sulphides (natural and synthetic cinnabar, and orpiment) and acetates, (verdigris) have been characterized by Fourier Transform Infrared-Spectroscopy in Attenuated Total Reflection (ATR-FTIR) and Diffuse Reflectance (DRIFT) modalities. Considering the latter, there is still a great deal of uncertainty in the interpretation of the different IR vibrational bands. Therefore, a comparative study between these two techniques has been carried out to highlight the potential of DRIFT spectroscopy as a portable and non-destructive technique that allows the differentiation and characterization of historical pigments in the field of cultural heritage. Before performing FTIR analysis, pigments were analysed using X-ray diffraction (XRD) to detect impurities and/or additives in the pigments. Differentiation between natural and synthetic pigments was possible due to the identification of impurities in natural pigments, and manufacture-related compounds or additives in synthetic pigments. Results obtained in this study have proven DRIFT to be a very useful analytical technique for in situ characterization of heritage materials. This study serves as an initial step in clarifying the challenges and uncertainties associated with interpreting spectra obtained through the DRIFT modality. However, the use of other complementary analytical techniques is required.

The chromatic reintegration of wall paintings, also known as inpainting, is a common practice in the conservation and restoration of Cultural Heritage (CH). The main objective of the treatment is to make the narrative content of paintings with lacunae (i.e. missing paint) more artistically and iconographically legible. Very few scientific studies have addressed the suitability and durability of the materials used in this kind of procedure, despite the importance of this question. Even less information is available regarding wall paintings, which are more susceptible to deterioration than other artworks due to their partial or complete exposure to extreme environmental agents (relative humidity, ultraviolet radiation, etc.). To this end, we have carried out a review of this subject bearing in mind that chromatic reintegrations should also comply with the principles of recognizability, reversibility and compatibility established in the mid-20th century. In this review, the influence and behaviour of pigments, binders (e.g. gum Arabic, mastic or urea-aldehyde resin, etc.) and protectors are discussed, together with the latest modern restoration practices based on ‘mineral painting’, in which silicates are used as binders. Overall, this field of study deserves more attention as the materials being used have not yet been thoroughly evaluated. These materials should be further assessed in terms of their visual and chemical-mineralogical compatibility with the original painting, their durability and stability in the long term under adverse weather conditions, and their reversibility.

Paint loss is one of the main forms of deterioration in historical wall paintings, generally restored by the application of chromatic reintegration. In the specific case of outdoor exposed paintings, it is essential to find a binder that will withstand diverse weather conditions. Since chromatic reintegrations have to be compatible with the original painting, fresco paint mock-ups were manufactured and compared to chromatic reintegrations made with an aqueous colloidal dispersion of silica nanoparticles as binder. The physical compatibility was studied by colour spectrophotometry and measurements of static contact angle, gloss, and roughness values, together with a peeling test, stereomicroscopy, and polarised light microscopy. They were also characterised from a mineralogical, chemical, and molecular point of view using X-ray diffraction, X-ray fluorescence and Fourier-transform infrared spectroscopy. The microtexture was studied by scanning electron microscopy with energy-dispersive X-ray spectroscopy. Chromatic reintegrations showed similar roughness and lower gloss values than frescoes, and the nano-silica binder ensured the natural breathability of the wall. Overall, the chemical nature of pigments was highly influential. The reintegrations with silicate-based pigments were more homogenous, with hardly any fissures, while those carried out with sulphide- or oxide-based pigments were severely cracked. The use of verdigris is discouraged due to the lack of affinity between the binder and the pigment.

Hematite- and cinnabar-based paint mock-ups prepared with either rabbit glue or egg yolk binder were artificially aged in an SO₂-rich atmosphere, as a model system for investigating the deterioration of tempera paints exposed to an industrial atmosphere. The overall research aim was to identify the type of degradation occurring in tempera paints and the different alteration mechanisms related to the physical, mineralogical and chemical characteristics of the paint. Tempera mock-ups were prepared by mixing binder (egg yolk or rabbit glue) and pigment (cinnabar of different particle sizes or hematite) and were then exposed to SO₂ for 2 months in accelerated aging tests. The colour, gloss, reflectance, roughness and micro-texture of the surfaces of the mock-ups were determined before and after the tests. In addition, chemical and mineralogical changes were determined by X-ray Powder Diffraction (XRPD), Attenuated Total Reflection-Fourier Transform Infrared spectroscopy (ATR-FTIR) and Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM-EDS) analysis. Colorimetric changes were confirmed, mainly in the cinnabar-based paints containing egg yolk, and in the hematite-based paints containing rabbit glue. Neoformed mineral phases have not been detected by XRPD, but precipitation of gypsum on the exposed surfaces has been confirmed by SEM. For cinnabar-based paints, the amount of sulfate-rich deposits was higher on egg yolk mock-ups than on rabbit glue samples, though the opposite was observed for the hematite-based paints. This confirmed the influence of the binder composition and pigment-binder tandem in the susceptibility to SO₂ deposition. Pigment particle size did not have a clear influence on the physical and chemical changes in the tempera mock-ups during the ageing tests.