Visualizing and Quantifying Rembrandt's Smalt Mixtures in The Night Watch in Three Dimensions Using μ-X-ray Fluorescence Tomography.

Smalt is one of the main blue pigments in Rembrandt van Rijn's The Night Watch (1642). Smalt is a cobalt-containing potash glass, ground to a powder to be used as an artists' pigment. In this study, three smalt-containing paint samples from this painting were studied using synchrotron radiation-based micro-X-ray fluorescence (SR-μ-XRF) tomography. To study the original intentions of the artist, the degradation phenomena taking place in the paint and the best preservation methods for the painting, a fuller understanding of the smalt-containing paint mixtures is desired. The smalt particles in Rembrandt's paints show a wide variety in size and elemental composition. Elements related to smalt are, e.g., cobalt, potassium, arsenic, bismuth, nickel, and iron. The heterogeneity is very high among the particles within one paint sample. Different clusters of particles can be established based on the intensity of bismuth. This variety could be caused by heterogeneity in the cobalt ore, the glass-making process, the use of multiple types of smalt or it might be the result of in-situ degradation. Next to the smalt, the volume percentages taken up by other components of the paint are semi quantitively estimated based on elemental information. This information can be used in follow-up studies to improve the accuracy of models and the representativeness of reconstructed paint mockup samples. SR-μ-XRF tomography proves to be a valuable tool for the identification and quantification of complex heterogenous smalt paints.