Fig 1 Painting (left) is Still Life with Flowers and a Watch by Abraham Mignon (1660-1679). Image a and b are the same flower imaged with a camera and X-ray fluorescence respectively.1
When you encounter art pieces in a museum “physics” probably isn’t the first thing that comes to mind. However, recent conservation efforts dedicated towards centuries old art pieces have found a use for x-ray analysis. The use of x-rays allow the conservationists to create images of the art piece which highlight specific pigments and the elements which it consists of. This allows the conservationists to see what the human eye cannot, where light, the environment or interactions with other pigments have caused the original detail to be obscured. In this way through the use of physics the art piece can be restored directly as the original artist intended.
There are two common techniques through which this imaging is carried out. The first is X-ray fluorescence. This process occurs when an x-ray collides with an atom and an inner electron is dislodged. An outer electron of the atom falls in energy to take its place and the difference in energy is emitted as an x-ray. This new x-ray is detected and tells us the identity of the atom it hit. Each element has a unique x-ray which is like its fingerprint and can be used to identify it. Image b in figure 1 illustrates the details which can be observed using x-ray based imaging. In this case the element which is identified is arsenic.
The second technique which is carried out to image paintings is X-ray diffraction. This process involves firing x-rays at a powder crystal and detecting the x-rays which are reflected back. This process is perfect for use with paintings as pigments are made up of powdered crystals. The pattern which the reflected x-rays make is unique to a compound and can therefore let us identify the pigment. In this case the arsenic based pigment found was schultenite, [PbHAsO4], and mimetite, [Pb5(AsO4)3Cl].2 These pigments were not the original pigment chosen by the artist as they were created by chemical reactions and are transparent to the naked eye. Knowing this, the original pigment can be reverse engineered and was found to be orpiment, As2S3, which has a vibrant yellow colour and was the colour of the original rose. This pigment is toxic and unstable with other pigments, this explains why this rose aged so poorly.
[1] Nouchka De Keyseret, et al.,Reviving degraded colors of yellow flowers in 17th century still life paintings with macro- and microscale chemical imaging,Sci. Adv.8, eabn6344, (2022). DOI:10.1126/sciadv.abn6344
[2] R. Mark Wilson, X-ray imaging shows how a 17th-century painting lost its color, Physics Today, 75 (8): 19–20, (2022). https://doi.org/10.1063/PT.3.5058
