A problem encountered in the study of paintings is distinguishing the medium in which they were created, and delineating layers which may include different media of mixtures of media. This was the subject of a paper presented at the Research and Technical Studies session.
It is not easily possible to distinguish between oil paintings and tempera (egg-based) paintings by eye, or using many analytical methods. The authors discussed the benefits and drawbacks to three main types of analysis that are used within paintings conservation: cross-sectioning, Fourier-Transform Infrared (FTIR) spectroscopy, and thin layer chromatography (TLC). FTIR, for example, cannot distinguish between egg proteins and glue, and the results can be masked by pigments or colorants. None of these methods, as discussed, can be definitive when it comes to mixtures of media such as tempera grassa.
The author also considered the effectiveness of other common methods, such as GC/MS (Gas Chromatography – Mass Spectrometry). The main drawback to this is that results cannot be compared across different experiments if the methodology varies even slightly.
The combination of these drawbacks in common methodologies led the authors to pursue Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS), a high-resolution technique that is better at separating and identifying fragments which are different but have similar masses. It also allows for the presence of specific compounds to be ‘mapped’, giving a helpful visual of layers and levels. Using this method, they were able to map for amino acids, identifying the presence of animal glue in a mixture. Practically, this was shown to differentiate between a gesso-size ground and the glue layer which was determined to have been purposefully added.
The talk concluded with a reminder that this technology, as with most, works best in conjunction with other methodologies. While this is an important point to remember, the potentials of this technique are exciting. I’m very interested to see the potential that this technique has for three-dimensional objects with multiple painted or gilded layers. I hope that someone pursues this, and that the technique is able to be harnessed across conservation disciplines.