Shirin Afra and Laura Speranza
Abstract
For the last few years, the research of the Laboratory for the Restoration of Ceramic and Vitreous Materials of the Opificio delle Pietre Dure has been oriented towards solving one of the most complex problems in the conservation of ceramic works placed outdoors: the compensation of the lacuna. Especially with regard to glazed surfaces, our goal, in addition to protecting the materials from the infiltration of water and other pollutants, is to restore the brightness and vividness of the original colors and, at the same time, ensure the reversibility of the restoration products.
In the past, materials such as epoxy or polyester resins and coatings that guaranteed the perception of brightness, which is typical of glazing, were chosen. However, this was at the expense of easy reversibility. These solutions, although realized with materials that seemed to be resistant to atmospheric agents, have proven to be unsatisfactory over the years, revealing a behavior that is harmful to the work as well as being aesthetically unappealing. These materials in fact tend to turn yellow and harden excessively, making their removal risky for the integrity of the original surface. Their disposal is also problematic since they are polluting materials of different nature and therefore cannot be differentiated.
The research to find an alternative to epoxy and polyester resins began in 2012 with the restoration of the majolica of the Russian Orthodox Church in Florence and deepened in 2014 on the occasion of the restoration of the “Dieci Putti in fasce” by Andrea Della Robbia, placed on the facade of the Ospedale Degli Innocenti in Florence. On that occasion, the Opificio Delle Pietre Dure carried out some tests on different types of acrylic plasters.
Five years after the restoration, we wanted to perform a monitoring to study the actual behavior of the fillers used over time. We came to the conclusion that acrylic materials are far preferable to epoxy resins, because they are easily reversible but require constant maintenance. In fact, the surface is subject to deterioration and the pictorial retouching tends to change color. Moreover, the question of eco-sustainability was left unresolved.
These considerations drove us to test natural mineral fillers and colors, which in theory promised outdoor durability and eco-sustainability. But how could we achieve the brightness of the glazed surface with a lime mortar, which is opaque by nature?
We therefore started a comparative experiment. We wanted to compare the aging of acrylic plasters with fillers based on natural hydraulic lime and silicate colors. In order to restore the shiny appearance of these completely natural materials, we created a “porcelain mortar” inspired by the Roman marmorino already in use during the Imperial Era in Ancient Rome.
With this paper we would like to show you the results of the tests, the difficulties and the application solutions, for an aesthetically appealing, ecological and long-lasting result.