CHROMA(Bio)

Spectral reflectance signature of colored subaerial biofilms as an indicator of stone heritage susceptibility to biodeterioration

CHROMA(Bio) is a is a project coordinated by the National Research Council – Institute of Cultural Heritage Sciences and funded by the European Union – NextGenerationEU – M4.C2.1.1. (PRIN 2022, Prot. 2022KTBX3M).

The project is based on synergic interdisciplinary research activities, thanks to the participation of experts in microbiology, chemistry, physics, geology, computer science, and mathematic.

The project

The hallmark of subaerial biofilms (SABs) on outdoor stone monuments is the color of their colonies. Because the SABs’ color is often deemed unpleasant, the conservation strategies traditionally encourage the SABs’ removal from the artistic surfaces without evaluating their overall impacts on the stone. The damages to the stone caused by the SABs removal are irreversible. Furthermore, common cleaning chemicals are harmful to humans and the environment and can lead to the proliferation of antimicrobial-resistant microorganisms. Finally, the colored SABs can have a neutral or even a protective role on the rock substrate. Thus, following the principles of minimal intervention and sustainable conservation of stone heritage (SH), a fundamental question is raised: “Should the SABs be removed from stone monuments?”

To answer this question, we are proposing the use of SABs’ color as a (bio)indicator for SH susceptibility to biodeterioration. Thus, the SABs’ reflectance spectra, which represent the color fingerprints, can be exploited to categorize the SAB’s impact on monuments based on their deteriorative, neutral, or protective roles.

For the first time, the color fingerprints of SABs will be measured and assessed throughout the full reflectance spectrum—as opposed to the simple color triplet—generating a set of data beyond the human eye’s perception. The SABs’ reflectance spectra contain large information meaningful for in-depth analyses beyond the traditional colorimetric methods.

CHROMA(Bio) rely on a synergistic partnership among microbiologists, chemists, physicists, geologists, computer scientists, and mathematicians to correlate the SABs’ spectral profiles to the tombs’ multidisciplinary investigation, which is essential to the definition of the SAB’s impact on the stone. A dedicated open-source database will be developed for the scientific community, stakeholders, and public to store, share and analyze the data. The mathematical correlation between the spectral profiles and SABs’ impact on the stone will be established through data-analysis techniques and predictive algorithms. A Graphic User Interface will be developed to allow the end-users to interact with the algorithm without requiring the input/knowledge of command codes. Thus, the workflow for a heritage professional would be: i) to collect the SAB’s spectral profile with the low-cost portable spectrophotometer proposed in this study, ii) to upload the spectral data in the widget, iii) to attend the output, which will be the effect of SAB on the stone: deteriorative, neutral, or protective. Overall, the results from this study will help predict the resistance and resilience of stone to biodeterioration and improve the development of comprehensive approaches to SH sustainable management. A sustainable SH management is essential to culturally related economic activities, socio-political development, urban sustainability, and environmental protection.