Cutting-edge technology to protect Prague’s Charles Bridge tower

Conservationists have applied a special protective paint on one of the Gothic towers on Prague’s Charles Bridge, the city’s most famous landmark. The transparent nano-coating can decompose algae and mosses with the help of light alone, keeping the stone walls clean and protected.

The entrance to Prague’s famous Charles Bridge is guarded on both sides by monumental Gothic towers. The taller one of them, known as the Lesser Town Bridge Tower, has been regularly overgrown with mosses and lichen, making its sandstone walls look green rather than beige.

Karel Kučera  | Photo: Kristýna Vašíčková,  Czech Radio

Thanks to modern technologies, experts may have found a solution to the problem. A few months ago, they scraped the walls clean and painted them over with a special coating of transparent nano-paint with self-cleaning properties. So far, it seems to have done the trick, says Karel Kučera, from Prague City Museum:

“There was quite a lot of green growth in the form of cyanobacteria, algae and lichens on both of the tower’s galleries. Occasionally you can still see a trace of lichen. It’s a very indomitable organism and it creates sort of a green map on the stone, but otherwise it's stable, and nothing new has appeared.”

The special nano-paint uses photocatalytic reactions: when sunlight falls on the titanium dioxide in the coating, the microorganisms, including mosses, cyanobacteria and lichen, are decomposed.

Among the experts who worked on the development of the photoactive layer is František Peterka from the Technical University in Liberec, one of the first pioneers of photocatalysis applications in Europe:

“The paint can ultimately break down the microorganisms into small amounts of carbon dioxide and water. Especially when there's a lot of sunshine, because this coating works on the UV component of light, the material works well.”

It took many years of research before the special protective coating could be applied to one of the Charles Bridge towers and its development involved a number of scientists.

The Technical University in Liberec was also involved in deciding the final composition of the material, says Alena Ševců from the university’s Institute for Nanomaterials, Advanced Technologies and Innovations.

“We were given samples of photocatalytic paints with different compositions and we then determined which samples were effective in killing the microorganisms. We selected the best ones from dozens of samples.”

One of the tasks of the scientists was also to convince conservationists that the coating was safe to use and wouldn’t cause any damage to the historic masonry, says Mr. Peterka:

“At first we treated the monuments in the depository of the Prague City Gallery in Buštěhrad near Kladno. Then we had to bring in certificates to ascertain that our substances won’t harm the monuments, but rather the opposite.”

So far the photoactive paint has been mainly used to treat facades of apartment buildings. However, experts believe that in the future it will increasingly help to protect historic monuments. One of the structures that could soon be covered with nano-paint coating is the Baroque bell tower of Prague’s Saint Nicholas Church.

Authors: Ruth Fraňková , Tereza Janouškovcová
run audio