Safer solvents for restoring frescoes
By Davide Castelvecchi
From Science News, May 19th, 2007; Vol.171 #20 (p. 310)
By suspending small amounts of solvents in nanoscale droplets, chemists have found an environmentally safer method of cleaning centuries-old frescoes and saving them from the unintended consequences of previous restorations.
The preservation of historic frescoes often involves firming up the paint and slowing its degradation by oxygen, light, and air pollution. In the 1970s, synthetic resins seemed like an ideal fix. Conservators began coating frescoes with protective layers of these acrylic polymers. However, the use of the synthetic chemicals created unforeseen problems, says Piero Baglioni, a chemist at the University of Florence.
For one thing, the polymers obstructed microscopic pores within the paint, preventing the natural perspiration of the underlying walls. This accelerated the accumulation of damaging salts, such as sulfates, under the coating.
Furthermore, within 2 decades, the protective layers themselves began to degrade. They often turned yellow from photooxidization, and they tended to shrink, creating stresses on the underlying paint, says Baglioni’s collaborator Rodorico Giorgi.
Conservators began using solvents to remove the polymers. But the solvents were toxic materials such as aromatic compounds, which could be hazardous to the user. Moreover, the solvents couldn’t clear the paint’s pores, according to Baglioni. “It’s impossible to remove these resins using a normal solvent,” he says.
In the 1990s, Baglioni’s team began replacing pure organic solvents with less toxic, water-based microemulsions of the aromatic compounds. That meant using surfactants to suspend the solvents in microscopic droplets of water. These water-based microemulsions cut the concentrations of hazardous solvents by at least 95 percent.
Microdroplets of solvents could easily get inside paint pores, where they would gobble up the resins. Conservators could then absorb them with a wet poultice.
The microemulsions were effective at removing not only synthetic polymers, but also organic materials such as soot and wax from burning candles, Giorgi says.
Using a new class of surfactants and new processing techniques, the Florence team has now brought down the concentration of hazardous solvents to less than 1 percent. Their new emulsions contain droplets as small as 10 nanometers. This increases their surface area per unit volume and enables conservators to use less-toxic concentrations of the solvents. The team describes its results in the May 22 Langmuir.
As an alternative to synthetic resins for saving frescoes, the Florence team favors the use of nanoparticles of calcium hydroxide, or slaked lime. These can also penetrate the paint’s porous surface, providing a natural way of integrating them with the original, calcium carbonate–based paints.
Ramón Carrasco, an archaeologist at the National Institute of Anthropology and History in Campeche, Mexico, says that he has enlisted the help of the Florence team for preserving frescoes in the Mayan ruins of Calakmul, in the Yucatán. “We were very careful not to use synthetic resins,” says Carrasco. “They prevent the original materials from breathing.”