Artisanal Lime Coatings and Their Influence on Moisture Transport During Drying

Abstract

Lime coatings such as whitewashes were originally used in historical buildings all across Europe and the rest of the globe, on lime plasters or directly on stone elements. Today, these coatings are increasingly used in conservation not only due to their unique aesthetic features but also for functional reasons. One of their main functional advantages is the ability to not hamper the drying of the substrate, which is very important because dampness is recurrent in historical buildings. The work presented here is aimed at improving the understanding of how and why lime coatings affect (or not) the drying of the porous building materials that usually constitute those substrates. We analysed experimentally the influence of one selected lime coating on the drying of five substrate materials with architectural relevance: one lime mortar and four stones, the well-known Ançã limestone, Maastricht limestone and Bentheimer sandstone, as well as a common Portuguese low porosity limestone. All the materials were characterized in relation to their capillary porosity and pore size distribution. Afterwards, the drying kinetics of the substrate materials, when coated or uncoated, was evaluated and compared. It was concluded that the lime coating not only does not hinder drying, but can even accelerate it. Indeed, at high moisture contents, the drying rate increases up to as much as 50%. This is likely to happen because the coating generates a larger effective surface of evaporation. In the article, we discuss the possible causes and implications of this phenomenon.