A slimy, dark invader has been spreading across the surfaces of our stone heritage. From the milky-white marble of the Jefferson Memorial in DC to the reddish-brown sandstone of Angkor Wat in Cambodia, this greenish-black biofilm (a mixture of algae and cyanobacteria) has found an ideal home for colonization. With plenty of water collecting in porous surfaces and ample sunlight that can be converted to food, outdoor stone can serve as the perfect domicile for biofilm to thrive. Once the biofilm has secured its seat at the table, more uninvited guests arrive to the party: micro-fungi, lichens, and bryophytes. These biological guests are more than just aesthetically-unpleasant invaders; the chemical and physical changes caused by biological weathering can ultimately lead to stone deterioration.
So, how can we selectively remove the biofilm without affecting the stone surface? One strategy is to use its own food supply—green light absorbed during photosynthesis—against itself. The necessary weapon for the job? Lasers. No, this isn’t science fiction. If we irradiate the surface with a strong dose of green light, the components of the biofilm will absorb this light and be propelled off the surface. If the stone doesn’t absorb green light, then it theoretically can be left unaffected by the irradiation. This concept is shown in the cartoon in Figure 1.
This is the method used by Mascalchi et al. to clean a 19th century, biofilm-coated statue, the Speranza by Odoardo Fantacchiotti. The researchers compared the effect of 532 nm and 1064 nm laser light using fluorescence imaging (or more specifically, pulsed amplitude modulated (CF-PAM) imaging) to map chlorophyll fluorescence and found that 532 nm was more effective for cleaning this biofilm. Although laser cleaning of stone is a relatively common technique, Mascalchi et al. were the first to perform a validation study using this technique. After fine-tuning a methodological protocol, they cleaned a biofilm- and black crust-coated statue using only laser cleaning. The steps of their protocol are illustrated in Figure 2. They also present evidence that their selected parameters leave behind a smoother stone surface than typical mechanical-chemical cleaning methods, suggesting that the laser cleaning method was gentler than mechanical-chemical methods. A smoother surface should also help to discourage future biofilm colonization. If you would like to learn more about their parameter optimization, validation method, and the cleaning of Speranza, you’ll find many more details in their article.
Although laser cleaning can be a gentle, effective method for cleaning statues and structures with no chemical waste, a careful analysis of the stone substrate before and after cleaning is good practice. Not only should the treatment effectively remove biofilm, grime, or soil without damaging the stone surface underneath, but one should be cautious of what impurities are left behind from the soil. For instance, a common problem resulting from laser cleaning is future yellowing of the surface, caused by iron that might not be removed during laser cleaning.
Has anyone reading this post used laser cleaning for a treatment before? Would you like to share what you liked about the method? Or perhaps you would like to express critiques or concerns? If so, be sure to comment below!
Lastly, before you go, if you’d like to see laser cleaning in action (and learn a little bit more about it!), check out this wonderful video from Harvard Art Museums: