Effect of a SO2 Rich Atmosphere on Tempera Paint Mock-Ups. Part 1: Accelerated Aging of Smalt and Lapis Lazuli-Based Paints

José Santiago Pozo-Antonio,Teresa Rivas,Carolina Cardell,Diana Barral,Amelia Dionísio

doi:10.3390/min10050427

José Santiago Pozo-Antonio, Teresa Rivas + Show 3 more

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https://doi.org/10.3390/min10050427

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Journal: Minerals	Publication Date: May 10, 2020
Citations: 5	License type: CC BY 4.0

Affiliation: Universidade de Vigo, University of Granada

Abstract

The behavior of historic tempera paints exposed to pollutant gases is an important issue when developing conservation strategies. In this work, binary tempera paint mock-ups that were made with either smalt or lapis lazuli pigments mixed with either rabbit glue or egg yolk binders were exposed to an SO2 accelerated aging test in order to find out more about the forms and mechanisms of alteration resulting from pigment-binder interaction. To this end, spectrophotometry, hyperspectral image analysis, and profilometry were used to study macro-scale, physical changes taking place on the surface of the paints, affecting color, gloss, reflectance, and roughness. Likewise, chemical and mineralogical changes were evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (ATR-FTIR), polarized light microscopy (PLM), and scanning electron microscopy with micro-analysis (SEM-EDS), which was also used to visualize micro-texture changes in the paints. The smalt-based tempera showed a higher degree of deterioration than the lapis lazuli-based tempera, in particular a notable whitening related to the precipitation of sulfate-rich salts and to binder and pigment chemical alterations. Moreover, whereas aged egg yolk-based paints showed visible color change due to damage to the oily binder and the pigments, the most evident change in rabbit glue-based paints was binder loss. The alteration suffered by the pigments varied in line with their composition; thus, smalt (blue cobalt-containing glass) grains were more sensitive to SO2 exposure than lapis lazuli-(Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)]-crystals. In the smalt grains, the SO2 test caused K+ leaching from the glass matrix, which was detected by means of K/Co ratios, but the lazurite crystals (main component of lapis lazuli) were unaffected (regardless of the binder used in the tempera). The most likely source of the crystallized sulfate rich salts were the impurities that were detected in association with the natural lapis lazuli pigment, i.e., calcite and diopside. Indeed, the precipitation of efflorescences is the main cause of the optical changes found in the smalt- and lapis lazuli-based tempera, in addition to the physical-chemical damage to the binders. The information reported here would be useful for preventive conservation, as well as for art restorers, who are planning work on paintings in which paints of this kind were used.

Highlights

The SO2 levels in some cities still surpass the limits recommended by the World Health Organizations (WHO), despite the fact that SO2 concentration in the atmosphere has been reduced in Europe in the last two decades due to European guidelines [1]
Note that the information that was provided by Kremer did not fully coincide with the composition identified by the authors, as reported for other pigments in previous works [22,23,35]
The authors found that the LAP pigment was composed of lazurite, calcite, and diopside, proving that this is a natural pigment, while that reported by the manufacturer was haüynite, known as haüyne, with empirical formula Na4 Ca2 Al6 Si6 O22 S2 (SO4 )Cl0.5

Summary

Introduction

The SO2 levels in some cities (e.g., in the Mediterranean area) still surpass the limits recommended by the World Health Organizations (WHO), despite the fact that SO2 concentration in the atmosphere has been reduced in Europe in the last two decades due to European guidelines [1]. The role of the deposition and oxidation of SO2 in the formation of gypsum (CaSO4 ·2H2 O) rich crusts on the surface of stones has been established in both sedimentary (carbonate) and silicate rocks [5,6,7]. Gypsum-rich crusts have been studied on building materials that have been used in numerous cultural heritage monuments [9,10]. Sulfation mechanisms have been described in decorative materials and mural paintings in a range of historical buildings, such as houses in Pompeii, which are exposed to medium/high SO2 levels due to the site’s proximity to Naples and to active volcanoes in southern Italy [2,3]. In “Villa Sora” (Torre del Greco near Pompeii), Cotte et al [2]

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