In situ evaluation by colour spectrophotometry of cleaning and protective treatments in granitic Cultural Heritage

- Colour spectrophotometry allows to reliably evaluate the cleaning effectiveness in granite.
- Colour spectrophotometry is a reliable tool to monitor the biological recolonization in granite.
- Colour spectrophotometry is not a reliable tool to evaluate the effectiveness of water repellents after chemical cleaning.
- Biocida was considered as the most efficient cleaning product along these two years.

This paper is focused on the reliability of the colour spectrophotometry as a non-destructive in situ technique to evaluate the cleaning effectiveness. A biofilm composed by filamentous green algae and cyanobacteria developed on a granitic building located in NW Iberian Peninsula was removed using different cleaners commonly used by professionals in stone conservation (distilled water, ethanol, benzalkonium chloride, Hyvar X, ethanol, sodium hypochlorite and the commercial products LimpiaFachadas1 and Biocida). Also, the durability of some water repellents applied after the cleanings was evaluate. The colour measurements expressed in the CIEL*a*b* colour space were performed before and after the cleanings and after the water repelling products application, at eight moments during two years (since June 2011 until June 2013) of exposition under a temperate humid climate.As general results, the colour spectrophotometry resulted in a reliable method to monitor the cleaning effectiveness, allowing to identify the most effective methods, but also the rate of the recolonization of the granitic surface during time. However, this technique was not recommended as re-growth indicator when water repellents were applied after the cleanings, due to the impact produced by the products on the colour of the stone.