The chemistry team has been busy this past month sampling and preparing cross-sections for analysis from The Triumph of David. As mentioned last time, x-ray fluorescence (XRF) is a technique that allows us to get a broad overview of what inorganic elements are present in a particular area of a painting. Usually, XRF can be incredibly useful in getting an idea of what pigments may be present in the area being analyzed. However, since the x-rays can penetrate all the way through to the canvas of the painting, we have no way to tell in which paint layer the detected elements are present by XRF alone. In order to determine which layer the pigment is located, we need to take a sample and prepare a cross-section. Cross-sections are beneficial for our research in that they not only allow us to visualize the different layers of paint present in an area, but also, with new developments in microscopy, we can determine which inorganic materials and binding materials are present in each layer as well.
Sampling from a 17th century painting isn’t quite as scary or taboo as it might seem at first. While the removal of anything original to the painting is not ideal, sometimes a sample is necessary in order to get the most accurate idea of the make-up of the painting. Conservators and scientists will take micro-samples, often no bigger than the period at the end of this sentence. We micro-sample from areas of pre-existing loss: meaning areas where the paint has already started to flake away. In the case of The Triumph of David, there were innumerable areas of loss from which we could sample. Obviously, we couldn’t and didn’t want to sample from every area of loss; so, we chose our sampling locations partially based on where the XRF gave intriguing results and partially in areas of interest to the conservators–namely, areas that appeared to be heavily overpainted or areas where it was difficult to distinguish pentimenti from later restoration campaigns. The micro-sample is then embedded in a plastic and cut and polished until the cross-section is exposed as in the figure above. This single exposed cross-section can be examined using a vast array of microscopic techniques which will be expounded upon in future blogs.
This sample in particular was taken from the red cloak on King Saul, and the image above was taken at 80x magnification. We took a sample in this location because we were curious to see if Saul’s cloak had been repainted and also to see if the red pigments matched other red pigments seen throughout the painting. As shown in the full size cross section above, there are obviously very different paint layers present in this one cross-section. The bottom two tan/brown layers are called ground layers. A ground layer is present in any painting and serves as a preparatory layer, creating a uniform surface on which to paint. Layer 1 appears to be made with less finely ground particles than layer 2, as you can see from the bigger particles present in layer 1. This means that layer 1 provided an initial ground layer to size the canvas, whereas the second layer created a uniform paint surface. Layer 3 is the first intentionally pigmented red paint layer that provides the coloring for Saul’s cloak. Finally, Layer 4 is the 1956 re-varnish present on the entire painting that the conservators are actively removing. Varnish serves the purpose of bringing more saturation to a painting as well as providing protection for the paint underneath. It is this varnish that has degraded over the years and has accumulated dirt which can be seen in the cross-section.
However, not all cross sections are this straight forward to interpret. A good example is the cross-section, shown to the right, taken from a soldier’s red cloak, an area that may have been re-painted more than once during previous restoration campaigns. Cross sections, especially with a painting of this age and indefinite history, can be incredibly complex and often yield more questions than answers. Frequently, it becomes difficult to tell which layers are original and which are from subsequent restoration attempts. Cross sections like the one above require more than just a visible light micrograph in order to fully interpret the image. Techniques that go beyond what our eyes are capable of seeing, such as fluorescence microscopy and scanning electron microscopy, are incredibly useful as they can help us understand the binding materials and inorganic pigments used in each layer. So stay tuned as we keep unraveling the mystery of Pietro da Cortona’s The Triumph of David.
Until then, Ciao!
-Kristen, Amanda, and Anthony.