Arsenic migration and water uptake in multilayer painting systems

Abstract

Orpiment and realgar are yellow and red arsenic sulphide pigments that have been used since antiquity in works of art until the 19thcentury, when their use was restricted due to their toxicity. With time, these pigments degrade to form colourless arsenic oxide. Different oxidation states of arsenic (+3 and +5) in the degradation products have been recently found. Moreover, they have been identified throughout the whole painting layout from the panel until the varnish, suggesting the migration of degradation products through the paint layers. Besides changing the aspect, they might change the stability of the painting or painted object. Furthermore, they might represent a potential hazard for conservators when dealing with the work of art. Since a painting is in equilibrium with its environment, there is always water in motion inside the painting. It is therefore believed that the degradation products are migrating via water. However, the migration mechanism is still not well understood. This thesis aims to provide a better insight into the diffusion process of arsenites (As+3) and arsenates (As+5) in materials commonly found in paintings. For this, two different approaches were followed:

In one hand, light aged painting reconstructions were analysed with FTIR microscopy and SEM-EDS to asses the effect of different grounds and relative humidity on the migration of arsenic. In all the samples, degradation products were found in the orpiment layer based on the As-O vibration, detected with FTIR microscopy. It was possible to identify arsenic in the ground layer close to the orpiment-ground interface for all samples with both techniques. However, due to the low intensity and the proximity to the interface of the arsenic found in the grounds, no conclusions could be drawn about its migration.

On the other hand, in-situ ATR-FTIR spectroscopy and EIS measurements were performed on a set of samples to analyse the water, arsenates and arsenites diffusion through oil and two varnishes (dammar and mastic). The diffusion coefficients of water in oil, dammar and mastic were determined with both techniques. It was found that the water diffuses faster in oil than in the two varnishes. Despite being similar, mastic was found to be less stable than dammar since a decrease in its barrier properties with time was found. The diffusion of arsenates and arsenites in dammar and mastic was studied by EIS and the diffusion coefficients were calculated for the first time. It was found that the diffusion of arsenites in the coatings is faster than arsenates, which is in accordance to the diffusion of these species in water. EIS resulted in a promising technique in the study of diffusion of ions in coatings. Although further research is needed to fully understand the migration mechanism of arsenic in painting systems, this thesis provides a useful insight and a methodology with which the water and ion transport in coatings can be studied.