Spectrophotometric color measurement for early detection and monitoring of greening on granite buildings

Abstract

This paper addresses the detection and monitoring of the development of epilithic phototrophic biofilms on the granite façade of an institutional building in Santiago de Compostela (NW Spain), and reports a case study of preventive conservation. The results provide a basis for establishing criteria for the early detection of phototrophic colonization (greening) and for monitoring its development on granite buildings by the use of color changes recorded with a portable spectrophotometer and represented in the CIELAB color space. The results show that parameter b* (associated with changes of yellowness-blueness) provides the earliest indication of colonization and varies most over time, so that it is most important in determining the total color change. The limit of perception of the greening on a granite surface was also established in a psycho-physical experiment, as Δb*: +0.59 CIELAB units that correspond, in the present study, to 6.3 μg of biomass dry weight cm⁻² and (8.43 ± 0.24) × 10⁻³ μg of extracted chlorophyll a cm⁻².

Keywords:

• preventive conservation
• CIELAB color system
• biofouling
• greening
• monitoring
• phototrophic colonization

Acknowledgments

The present study was financed by the Xunta de Galicia (09TMT014203PR). The authors thank all the observers who generously agreed to collaborate in the psycho-physical experiment and CESGA building staff who allowed the authors to take the color measurements on the building façade.