In-situ survey of the presence of soluble salts associated with decaying azulejos

Abstract

Pattern or figurative azulejos are commonly found decorating the interior of churches, palaces and religious buildings in Portugal, representing local productions from the late 16th century onwards. The history and micro-climatic background of each building determined that not every azulejo reached the present day in good condition. Restorers tend to associate azulejo decay to the action of soluble salts through cycles of dissolution and crystallization. NaCl is often mentioned and the suspicion of its presence justifies long desalinization processes during restorations. Whenever there are glaze lacunas and the ceramic biscuit is exposed, efflorescence is sometimes apparent at the surface. Other times there is no visible efflorescence, even in locations inaccessible to cleaning actions, and no signs of sanding associated to crypto-florescence. The aim of this research was the survey of in situ forms of degradation, and the sampling of efflorescence and decaying biscuits, aimed at: i) determining the occurrence of soluble salts even if unapparent; ii) determining analytically their nature; iii) establishing circumstantial relations between the occurrence of particular soluble salts and decay, to be later explored through laboratorial simulations. 29 properties throughout Portugal were visited (of which 14 inland and 15 in coastal areas) in which walls were lined with azulejos produced in Coimbra or Lisbon. The salt samples and small fragments collected in situ were analysed with a HITACHI 3700N Scanning Electron Microscope with microanalysis unit (SEM-EDS) at the Hercules Laboratory (Évora University). The communication will illustrate the results of the study, connecting some forms of decay to the action of soluble salts recurrently found associated with them such as NaCl, more abundantly found in near-shore properties, along with sodium carbonate (trona). Magnesium and calcium sulphate were more common in inland properties.