New autogenous lime-based grouts used in the conservation of lime-based wall paintings

Abstract

The best material to be used in the restoration of a work of art is that most similar to the original. When grouting of lime-based wall paintings is necessary, lime-based grouts are the obvious choice. Compatibility is particularly important for grouting, as this is an irreversible operation. To set, lime-based grouts need air, specifically carbon dioxide, CO2. However, in the narrow space between two delaminating layers where the grout is injected, CO2 is not usually available in large enough quantities for this process to occur satisfactorily. To overcome this problem, extensive use has been made of hydraulic grouts, which set with water, and grouts containing synthetic or natural organic binders. A search was conducted for additives that would facilitate the setting of lime-based grouts, even at low CO2 concentrations. In this study, a group of substances that produce CO2 by reacting with water in an alkaline environment, esters of carbonic acid, were identified and tested to understand the kinetics of the setting process. Additives that would increase the fluidity of the grouting mixture, thus improving its injectability, were also investigated. The practical application of these grouting mixtures is illustrated by the recent restoration of Masaccio and Masolino's frescoes in the Brancacci Chapel in Florence.