Elemental composition
macro X-ray fluorescence scanning
Take a deep breath: macro-X-ray fluorescence scanning, or macro-XRF for short. This was the first scanning technique to be used in the major Night Watch project. The technique makes it possible to investigate which chemical elements are in the paint that Rembrandt used for his masterpiece. One reason why this is important is that it helps to decide on the best way of treating the painting so as to preserve it as well as possible for the future.
How is it done?
The scanner uses X-rays, just as a dentist or a hospital does. But this scanner goes a step further. It reveals not only the contrast between heavy and light elements but can demonstrate the presence of certain chemical elements in the paint. The entire Night Watch was scanned millimetre by millimetre in this way.
56 scans
In order to map the entire canvas its surface was divided up into sections measuring 60 by 80 centimetres. A total of 56 scans were made. Together they form a map illustrating the distribution of the chemical elements in the paint. The scans are black-and-white contrast images revealing the distribution of an element on or beneath the surface of the picture. The paler or whiter that part of the image the more there is of that element. The darker it is, the less there is of the element.
What do we learn from this?
Seventeenth-century painters had a limited palette of pigments at their disposal. Each pigment contains specific chemical elements. By detecting those elements it is often possible to conclude which pigment has been used. Mercury, for example, is only found in the red pigment vermilion. The presence of cobalt usually indicates the blue pigment smalt. Pigments that have the same elemental composition, such as various earth pigment, cannot be distinguished from each other. That requires supplementary detection techniques.
Underlying layers made visible
In addition to information about the pigments used, this technique also provides an insight into Rembrandt’s painting process. It can reveal underlying alterations, and can give an idea of the condition of the painting. For instance, later additions made by conservators (retouchings) stand out because of their chemical composition. Those retouchings often contain pigments that did not yet exist in Rembrandt’s day, or ones that he did not use.