Estudio del rol de los agregados minerales en la formación, envejecimiento y conservación de películas pictóricas al óleo.

Abstract

Very few stone sculptures conserve their original polychromy nowadays so the exceptional condition presented by the oil paint film of the Tomb of Chancellor Villaespesa in the Cathedral of Tudela (Spain) here presented as case study suggested that both the chemical-mineral composition of the oil paint films and of the lithotypes chosen for the artists to be in contact with the polichromy, could be responsible for their unexpected durability. When the mechanisms governing these painted structures stability are not understood properly, conservation materials and methods and even environmental conditions can have a dramatic effect in their performance and compromise their stability in the long term. In this study, mock-ups simulating different oil paint layers on the sandstone of the case study have been prepared. Mock-ups with the same oil paint films on other substrates have been made for comparison purpose. After four years of natural ageing, the aggregation of lead and calcium carboxylates have been identified and characterized on some mock-ups by means of the combined use of UV-VIS light microscopy, SEM-EDX and micro FT-IR spectroscopy. pH and conductivity have been measured and monitorized both in the mock-ups and the case study. Saponification reactions, ionomer formation and carboxylate aggregation in lead white or minium oil paint films contributed to make them less acidic, less ionized, less hydrolyzed and less soluble after ageing sandstone or on pine wood at room environmental conditions for four years. The same given paint films became more acid and more ionized after ageing on a glass slide. Understanding the properties of such films, the chemical-physical mechanisms governing their adhesive and cohesive forces, and their surface pH is not easy. Mineral assemblages as well as the availability of ions in the substrate of a given oil paint film containing lead white or minium play a fundamental role in the eventual formation, migration and aggregation of lead and calcium carboxylates not only at their surface but also within the substrate-film interface. Furthermore, monitoring surface pH and conductivity as well as understanding solubility issues and the adhesion forces existing between soils, the paint layer and the substrate is needed when designing any conservation strategy to minimize risks and to preserve the balance acquired by these artworks in time.; Muy pocas esculturas petreas conservan su policromia original. El excepcional estado en el se mantienen las peliculas pictoricas al oleo del caso de estudio de esta tesis doctoral (el Sepulcro de Villaespesa de la catedral de Tudela, Navarra, Espana) sugiere que tanto la composicion quimico-mineral de la roca seleccionada por sus escultores como la composicion de las peliculas pictoricas al oleo con las que se policromo dicha piedra, podrian ser responsables de su inesperada durabilidad. En el transcurso de esta tesis doctoral se prepararon probetas con distintas peliculas pictoricas al oleo sobre la arenisca del caso de estudio. Para hacer un…