Nanostructured systems for the consolidation of matte and porous modern paintings

The preservation of brittle and fragile paint surfaces is undoubtedly one of the most significant issues in contemporary and modern art, both for their technical and optical properties, as well as their aesthetic values. Artists' unfettered experimentation with painting techniques and additive-rich paint formulations have led to artworks with weak powdering surfaces exacerbated by severe climatic conditions and outdoor pollution. As a matter of fact, current conservation practice lacks adequate strengthening methodologies, and traditional consolidants can alter the optical properties and water permeability of painted surfaces. Herein, a new starch-based nano-structured consolidant was synthesized to boost penetration into the porous paint layers and avoid aesthetic alterations; the large surface area of the starch nanoparticles (SNPs) is abundant with -OH groups, promoting adhesion to pigments. The SNPs were formulated through a bottom- up approach, where gluten-removed Jin Shofu wheat starch was gelatinized and then precipitated in a non-solvent. Wheat starch's low gelatinization temperature is most likely crucial in favoring disassembly in alkali and re-assembly in non-solvent. The synthesis conditions can be fine-tuned to produce amorphous SNPs with an adequate polydispersity and dimensions of around 50 nm. Once re- dispersed in an aqueous phase, SNPs form nano-sized gel-like fractal domains by the organization of smaller units in polymer-rich and deficient regions as observed with Cryogenic Electron Microscopy. Additionally, by means of infrared spectroscopy, X-ray diffraction, and calorimetric measurements, the amorphous nature of the SNPs was revealed. Short and mobile gluten-removed starch chains, obtained after gelatinization, assemble quickly by non-solvent precipitation, forming disordered, rather than crystalline, nano-structures. Finally, aqueous and hydroalcoholic SNPs dispersions were satisfactorily tested as consolidant on artificially aged painted mock- ups that mimic degraded modern and contemporary painting surfaces. The efficacy of the consolidation was assessed using an in-house protocol: the SNPs penetrate across the paint section and significantly boost pigment cohesiveness, maintaining the painted layer's original optical properties, while bulk starch dispersions merely accumulate on the paint surface generating superficial rigid and glossy films. Overall, SNPs are an excellent example of how biopolymers and renewable sources can be used to create colloidal structures that can improve the resiliency of Cultural Heritage to degradation processes, favoring the transfer of works of art to future generations.