Experimental study on the salt weathering resistance of fired clay bricks consolidated by ethyl silicate

Fired clay elements are widely diffused in architectural heritage, for both structural and ornamental purposes, and are susceptible to severe decay forms, hence their conservation is a very urgent task, requiring appropriate consolidants to restore their cohesion and mechanical strength. As fired clay bricks and terracotta artefacts contain silicate fractions, allowing ethyl silicate (ES) to chemically bond to the substrate, ES is supposed to be suitable for their consolidation and indeed it is used in some conservation works on this kind of material. However, its behaviour and performance on fired clay elements has not been systematically evaluated in the scientific literature, mostly when durability is concerned. This aspect is particularly relevant as diverging data can be found in the literature about the impact of ES on substrate durability, to such an extent that it was found to either increase or hamper materials resistance to decay, depending on the specific case in exam. In this study, the behaviour towards salts crystallisation of fired clay bricks treated by ethyl silicate was investigated, specifically focussing on the role of the application procedure, an aspect which is usually left to the conservatorists’ skilfulness but which is however of paramount importance for the treatment outcome. In particular, ES was applied by 5 and 10 brush strokes, to assess how the different amount of product absorbed and its different distribution inside the porous substrate influence the final resistance of bricks to salt weathering. As a matter of fact, salt crystallization is commonly considered one of the most frequent and severe decay mechanism affecting bricks on site. Despite both treatments providing significant increases in mechanical properties and a limited modification in samples microstructure, durability was found to be dependent on the treatment conditions. Particular attention should be devoted to the amount of product applied, as higher product absorption, despite generally leading to better efficacy, could cause higher microstructural modifications, possibly hampering materials durability.