organic groups. This provides a new route to lightweight periodic mesoporous materials." Stein's group has demonstrated the unique surface properties of its hybrid materials by a sorption experiment using an immiscible mixture of water and chloroform, he says. 'The sample initially floated on top of the aqueous phase. When the mixture was shaken, selective uptake of chloroform by the mesoporous solid and wetting by the organic solvent resulted in the solids sinking to the bottom of the flask as
Benefits flow both ways. As Baglioni and his colleagues have worked to develop better methods to restore some of the region's art treasures, particularly frescoes, they also have been able to study in detail the chemical changes that take place over time on stone andfrescosurfaces. Thesefindings,they believe, may enable conservators to protect the surfaces from future degradation as well as to develop better methods to restore damage that has already occurred. In the fresco technique, an artist applies a paste of calcium hydroxide to the surface to be painted. The painting is done on this wet surface, where, over the next day or two, the calcium hydroxide reacts with carbon dioxide in the air to become calcium carbonate, the same chemical substance as marble and limestone. This process traps the paint pigments within the structure of calcium carbonate crystals. n Florence, it seems a natural pair- director of the University Consortium for Last month, Baglioni, research feling. Much of the artistic and cultur- the Study of Large Interface Systems low Luigi Dei of CSGI, and their colal heritage of this central Italian (CSGI), a group of physical chemists leagues reported results of their investicity is quite literally written in stone, in from six Italian universities seeking ways gations of the way potassium nitrate the form of exquisite Renaissance fresco to use supramolecular and colloidal sys- crystals grow in porous stones of artistic paintings that adorn the city's famous tems in industrial and other applications, interest, such as Carrara marble and churches and other buildings. Frescoes including art conservation. two types of sandstone used in the slowly deteriorate under the buildings of Florence [Langbest of circumstances, and wamuir, 15,8915(1999)]. ter, salts, and pollution speed Detail of the fresco "La up the process. Leggenda della Vera Croce" With much art in need of before and after restoration restoration, Florence is also a using the Ferroni-Dini method. city of art restorers, who are A close-up (top left) shows continually facing new probthe powdery surface coating lems and searching for better of calcium sulfate that is ways to restore and protect the converted to calcium carbonate and consolidated to city's cultural heritage. "We restore the fresco (top right). have to do something to help The fresco was painted in the them," says Piero Baglioni, 15th century by Piero delta professor of colloid and interFrancesca in the San face chemistry at the UniversiFrancesco Church in Arezzo, ty of Florence. Baglioni is also near Florence. they absorbed the denser organic solvent. We observed similar sorption behavior using MCM41 containing vinyl surface groups. In contrast, this behavior was not observed for pure silica, although it is hydrophobic." The Minnesota researchers are now planning to study in further detail the ability of UOFMN materials to absorb organic compounds. "Applications could involve removal of small amounts of hydrophobic organic solvents from aqueous systems or 'dry' organic reactions within the mesopores," he says.^
The Right Chemistry For Fragile Frescoes
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science/technology Crystallization of nitrate salts is one near Florence, give very encouraging way to salvage a seriously degraded fresco was to carefully detach it from of the most serious causes of degrada- results. tion of works of art on stone, the reEmulsion chemistry is also being ap- the wall and re-adhere it to a polymer searchers point out. Pressure that plied to help clean wax and other organ- support. Currently, many frescoes are builds up as the salt crystals form and ic materials from oil paintings, Baglioni restored using a method developed in press against the walls of pores within notes. For him, though, the "real fun" 1970 by Enzo Ferroni, then professor of the stone weakens and fractures the comes in working with frescoes and oth- chemistry at the University of Florence, stone much the same way frost forma- er stone surfaces because many of their and restorer Dino Dini. The two-step tion breaks up soil. Although this crys- surface properties are not yet well technique involves applying a saturated solution of ammonium carbonate to the tallization process has been studied ex- understood. surface of the fresco followed by tensively in very simple porous treatment with a barium hydroxmaterials like silica gels and poide solution. The first step rerous glasses, those studies can't forms calcium carbonate and probe used to predict how the produces ammonium sulfate, which cess will occur in building stone. is water soluble and can be As the potassium nitrate cryswashed away. The second step tals form in the stone, the reproduces barium sulfate and carsearchers find, the size distribubonate crystals that partially fill tion of the pores shifts toward the voids created by formation of larger pores, possibly because the smaller calcium carbonate the growing salt crystals quickly crystal structure and helps to seal off the entrances to smaller consolidate the painting. pores within the stone. Finegrained types of stone, such as Although the method works marble, lose more of their poroswell, it does leave a certain ity through this process than amount of barium sulfate in the coarser stones do. restored fresco, Baglioni points out. An ideal method would reThe chemists are now trying store the artwork to its original to adapt magnetic resonance chemical composition as well as techniques originally developed to its original appearance. The for brain tomography to enable Florentine chemists are now exthem to actually watch the pore perimenting with ways to use stastructure change within buildble dispersions of calcium hying stones or frescoes themdroxide in place of barium hyselves. "In the Langmuir paper, droxide for the consolidation we studied the pore distribution step of fresco restoration. by classical techniques," Baglioni explains. "But NMR is much It's not easy to form stable dismore powerful because you can persions of calcium hydroxide in follow in real time what is going water, but the chemists have deon in the pore and how the pore veloped a method of dispersing distribution develops and chang- A wail decoration from the fresco "Cappeilone degli the base in short-chain aliphatic es." Larger pores mean the stone Spagnoli" in the church of Santa Maria Novella in alcohols such as 1-propanol. The is more fragile and more easily Florence before restoration (top) and after restoration alcohol has the additional advanof only the right portion by using a dispersion of broken. tage of a lower surface tension calcium hydroxide in 1-propanol. The fresco, by Andrea than water, allowing the disperUnderstanding the changes da Firenze, was painted in the 14th century. sion to better penetrate the surtaking place within the surface layers of stone or frescoes may lead to Over time, frescoes degrade be- face of the fresco. Over time, reaction better ways to prevent them. The cause their calcium carbonate is trans- with carbon dioxide in the air converts chemists are investigating a way to in- formed into calcium sulfate. Moisture the calcium hydroxide to calcium carhibit the formation of calcium nitrate and the presence of nitrates and other bonate in the same process that proand other salt crystals within frescoes salts can increase the rate of degrada- duced the fresco originally. So far, the chemists have tested their and stones with organophosphonate tion. The unit cell in the crystal lattice antiscaling agents. These agents pre- of calcium sulfate is nearly twice as calcium hydroxide dispersion technique vent formation of the nuclei needed for large as the unit cell of calcium carbon- on some painted decorations in the crystal formation and greatly slow ate, so as the fresco degrades, its sur- church of Santa Maria Novella in Flordown the crystallization process, face erupts in a powdery bloom of cal- ence. Before using it on the figures in the Baglioni explains. Preliminary tests in cium sulfate that carries with it some fresco, "they let us try it on this wall paintwhich the chemists introduced their of the pigment that was once trapped ing," Baglioni says. The technique seems antiscaling agent at the parts-per- in the calcium carbonate lattice. Brush to work, he says. "We're getting basically million level during the restoration of away this powder, Baglioni explains, the same result as when we use a dispersion of barium hydroxide." some 19th-century wall paintings in and the painting goes with it. Rebecca Rawls the San Francesco Convent in Fiesole, Until about 30 years ago, the only 3 6 JANUARY 24, 2000 C&EN
