Italian scientists have used hydrated lime nanoparticles to re-adhere the ancient paint of Mayan murals. After only a week, the degradation process had not only stopped, but also partly reversed. They published about this in the magazine 'Chemistry, a European Journal'.
The murals in the Mexican Maya ruin Calakmul suffer from exposure to the elements. The images are applied to limestone with natural paint. Over the years, the paint has slowly started to peel off. This is because the limestone (CaCO3 ) from the walls, under the influence of sulfur compounds from the air, is converted to the much softer gypsum (mainly CaSO4 ). This washes away on contact with water, so that the paint adheres less and less well to the wall and the image is eventually lost.
Previous attempts to protect the murals with polymers or hydrophobic coatings have presented problems. They formed cross-connections or even attacked the limestone. Italian scientists from the Center for Surface Chemistry at the University of Florence tried the now nanoparticle slaked lime. And with success!
Slaked lime (calcium hydroxide or Ca(OH)3 ) reacts with CO2 from the air to (the hard) limestone (CaCO2 ) and fills the holes in the wall. The only by-product is some water. The paint can even re-adhere to this new limestone.
To be able to use slaked lime for this, it is essential that you use nanoparticles. Larger particles (300+ nanometers) do not penetrate the paint layer during spraying and cover the image with a white haze. Then you are even further from home. However, the nanoparticles penetrate deeply enough to be effective. But if there are a lot of soluble sulfates in the mural, the calcium hydroxide prefers to react with them, which in turn produces useless plaster. To prevent this, the scientists also added nanoparticles of barium hydroxide. That reacts even faster with the sulphates, so that the calcium hydroxide can do its work quietly.
The mix of barium and calcium hydroxide nanoparticles was sprayed onto the murals using organic solvent propanol and a difference was visible after just one week. The degradation had stopped and the flaking had partly disappeared. Now the murals are stable; however, the question is how long this effect will last. The scientists themselves are already working on ways to use nanoparticles in the conservation of ships, paintings and sculptures.