Brittle and fragile paint layers constitute a major issue in the preservation of easel and canvas paintings, and current restoration practice still lacks appropriate consolidants. One of the most concerning cases regards modern and contemporary paintings, often made by artists experimenting with mixtures and industrial formulations, which can produce instable and fragile layers even before natural aging and weathering. Commonly adopted synthetic adhesives show poor physico-chemical compatibility to the original paint components. Starch is a promising candidate for consolidant formulations, but dispersions obtained from bulk polymer grains yield superficial films that either provide superficial consolidation or alter the aesthetical properties of the paintings. Instead, we show here that colloidal dispersions of starch particles in the nano-size are able to penetrate paint layers and provide thorough consolidation of poorly adhered/cohered pigments without altering the appearance of the paints. In particular, Jin Shofu is a gluten-removed starch with low gelatinization temperature, two characteristics that likely facilitate the precipitation of stable starch nanoparticles (SNPs) in a non-solvent (ethanol) after alkali treatment. Besides optimizing the syntheses of SNPs, we investigated their effect on different metal-containing pigments (ultramarine blue, red ochre, and Prussian blue), representative of common paint colors used by artists, using an in-house testing system based on micro-sampling, coupled with macro and microphotography, and 2D Fourier Transform Infrared (FTIR) Imaging. One intriguing perspective in future research regards deepening the characterization of SNPs-pigment interactions, where the formation of amylose- and amylopectin-metal complexes (through hydroxyl groups in the polymers) might enhance the consolidating effect. Overall, SNPs represent a valid example where the formulation of colloidal systems from biopolymers and renewable sources can improve the resiliency of Cultural Heritage to degradation processes, favoring the transfer of works of art to future generations.

Colloidal Jin Shofu starch dispersions for the consolidation of paintings

ACS Fall 2021

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