Experimental Research with Salt Crystallization in Historical Portuguese Azulejos

Abstract

The most serious form of degradation of historical azulejos is the spall of the glaze and it is widely accepted that the crystallization of salts dissolved in water circulating in the masonry is its main cause. This article reports the results of an accelerated aging test aimed at studying the physical deterioration of azulejos caused by cycles of dissolution and re-crystallization of sodium chloride (NaCl). Experiments were carried out on 26 Portuguese azulejos from the 17th–19th century, some of which with original manufacturing defects or with varying degrees of previous decay.

The results of 75 cycles over 18 months did not verify that the action of NaCl could trigger an accelerated loss of glaze in a healthy azulejo. Furthermore, the results lead to the induction that there must be other, eventually concurring, causes for the spall of glaze than a simple challenge with salts that crystallize by evaporation.