Have you ever thought about the challenges faced by art conservation scientists? No, neither had I. But in order to better understand art, what it’s made of and how it’s conserved or restored, efforts have to be placed in analysing each and every stroke, pigment and material used. However, an avid Da Vinci expert cannot simply ask the Louvre to take a sample from the Mona Lisa because they want to better understand what pigment and paint binder is used that differentiates the pale aspect of her upper breasts from the darker hues of her lower neck.
This is where a type of chemical imaging technique called near-infrared reflectance imaging spectroscopy comes in. One of its main advantages is that it is non-destructive and non-invasive; it is an in situ analysis tool, which means you do not need to take a sample of the matter you wish to analyse. As such, this technique is fast becoming an important tool in cultural heritage science.
A paper published last summer showed the accuracy and relevance of using near-infrared reflectance imaging spectroscopy in the field of conservation science. Early Renaissance paintings were analysed and scientists discovered that the painter, Cosimo Tura, had used two different paint binders – animal skin glue and egg yolk tempera – for different parts of the paintings. Animal skin glue binder mapped to blue areas of the painting whereas the binder in the red and brown parts corresponded to the egg yolk tempera. In addition, they noted that the binders co-localised in particular parts of the painting, namely the faces, displaying the complex layering of materials to render the faces.
Much like any spectroscopic method, near-infrared reflectance imaging spectroscopy studies the interaction between matter and radiated energy, in this case a near-infrared beam. The method relies on the fact that chemical bonds within molecules, such as O-H, N-H and C-H, absorb energy at a particular wavelength within the near-infrared spectrum and start to ‘vibrate’ in a process known as molecular vibration – some stretch, some scissor and others rock. Since it is known how particular chemical bonds within types of molecules vibrate, the measurement can identify what types of molecules are in the sample, and in what quantity – detailing the materials used. This story is a beautiful example of the inter-disciplinary nature of science today. On the one hand we have chemical engineers developing intricate techniques for identifying and mapping organic components and on the other we have cultural heritage scientists applying these methods to restore and better understand how paintings several hundred years old came about.
Dooley KA, Lomax S, Zeibel JG, Miliani C, Ricciardi P, Hoenigswald A, Loew M, Delaney JK. Analyst. 2013 Sep 7;138(17):4838-48. doi: 10.1039/ c3an00926b. Mapping of egg yolk and animal skin glue paint binders in Early Renaissance paintings using near infrared reflectance imaging spectroscopy.