The glow of blues and golds from Buffalmacco’s Universal Judgement (Giudizio Universale) fresco in the Monumental Cemetery of Pisa is inspiring – even more so now that it has been restored with a little help from pyrolysis-gas chromatography-mass spectrometry (Py/GC/MS).
Dr. Maria Perla Colombini, Professor of Analytical Chemistry at the University of Pisa, and colleagues recently reported that restoration of the Pisa frescoes was possible because Py/GC/MS analysis had determined the components of the hydrophobic polymer surface, a surface which resists traditional restoration techniques.
“During the last 30 years of art restoration, the Py/GC/MS technique has become widely used to analyze a range of organic art materials, such as proteinaceous binders, beeswax, resins, oils, and volatile pigments (arsenic and mercury),” says Dr. Silvia Prati of the University of Bologna. Strong and fast heating of a sample in the absence of oxygen for pyrolysis provides many advantages for analysis. Art restorers agree that Py/GC/MS is a better technique to use than GC/MS because an untreated sample can be analyzed, whereas in a GC-MS procedure you need to hydrolyze the sample for 1-24 hours, extract with several solvents, and then derivatize analytes with silyl compounds.
Py/GC/MS enabled the researchers to identify polymeric materials in the frescoe samples. “Only with the pyrolysis procedure {at 600°C} were we able to identify animal glue and casein in the samples through the identification of pyrole (animal glue) and indole (casein) in the pyrograms,” explains Ilaria Bonaduce of the University of Pisa. In addition, according to Prati, the identification of polystyrene and polifluorarate polymers, synthetic modern consolidants, in Giotto’s Murals was possible with pyrolysis, but difficult with GC/MS. Another advantage of the Py/GC/MS technique is the small sample requirement. Samples of just a few micrograms were used in the Buffalmacco fresco analysis. In addition, since the Py/GC/MS technique is four times faster than GC/MS, “you {can} obtain diagnostic information in less time and practically at the same price,” says Colombini.
According to Colombini, “The Py/GC/MS technique may replace the use of GC/MS since Py/GC/MS does not require any sample pre-treatment, but the quality of information….is pretty different.” Pyrograms are generally more difficult to interpret because pyrolysis produces molecule fragments that then undergo radical rearrangements, for example combination of radicals and cyclization of radicals.
Traditional GC/MS still has its uses – for example it can determine the distribution of amino acids in a sample, and was used for evaluating the proteinaceous degradation process in the Buffalmacco fresco analysis. Bonaduce and Colombini also note that the Py/GC/MS pyrogram is more difficult to interpret than results from a microwave-assisted saponification GC-MS procedure, although that method is more labour intensive.
New developments continue to be made. Pyrolysis in the presence of a derivatizing agent, such as tetramethylammonium hydroxide, which gives rise to saponification of esters, followed by methylation of acids and alcohols, now allows the detection of polar and volatile compounds. Colombini concludes, “Py/GC/MS has greatly evolved since the initial applications.”
Italian and other art conservationists continue to analyze a wide range of art using Py/GC/MS – from funerary paintings in Macedonia, frescoes, and siccative oils, to organic residues from Pompeii.
Article by Laura Ruth
