The pigment was available in different grades, from pale gray to deep blue, with the color being dependent on the particle size and cobalt content. Smalt was widely used in the seventeenth century as an inexpensive substitute for the costly ultramarine. One of the unique features of Rembrandt’s late works is the use of large amounts of smalt: a potash-silica glass colored blue with cobalt.

As a result Rembrandt’s late works differ radically from the polished works of most Dutch painters from this period. Clearly, Rembrandt did not comply with the change in taste that took place in the second half of the seventeenth century, when fine handling of the paint and smooth surfaces were more highly appreciated than this rough manner. Some of the technical aspects-described in the seventeenth century as ‘the rough manner’-are also to be found even in his earliest works.

Rembrandt’s late works, which date from the 1650s and 1660s, are characterized by their loose, sketchy appearance, lively brushwork and unusual surface roughness. In conclusion, this study confirmed earlier assumptions that Rembrandt used substantial amounts of smalt in his late paintings, not for its blue color, but to give volume and texture to his paints, to deepen their colors and to make them dry faster. Optical coherence tomography (OCT) was used as an additional tool to reveal differences in translucency and texture between the different paint reconstructions. Most significantly, the texture of the paint was strongly influenced by adding smalt, creating a more irregular surface topography with clearly pronounced brushstrokes. The drying tests show that the paints with (cobalt-containing) smalt dried five times faster compared to those with glass (without cobalt). It was also found that the admixture of opaque pigments reduced the translucent character of the smalt.

However, in mixtures with high percentages of smalt, or when combined with more transparent pigments, it was concluded that the smalt did produce a cooler and darker paint. Results show that the influence of (the originally blue) smalt on the intended color of the paint of the Homer is minimal. Paint reconstructions were carried out to investigate the effect of different percentages of smalt on the overall color, the drying properties, translucency and texture of the paint. Macroscopic X-ray fluorescence imaging (MA-XRF) assisted by computational analysis, in combination with SEM-EDX analysis of paint cross-sections, provides new information about the distribution and composition of the smalt paints in the painting. This paper reports on the use of smalt in complex pigment mixtures in Rembrandt’s Homer (1663), Mauritshuis, The Hague. Since most of these smalt-containing paints have discolored over time, we wanted to find out more about how these paintings may have originally looked, and what the role of smalt was in his paint. One of the unique features in his late paintings is his abundant use of smalt: a blue cobalt glass pigment that he often combined with organic lake pigments, earth pigments and blacks. As part of the NWO Science4Arts REVISRembrandt project (2012–2018), novel chemical imaging techniques were developed and applied to the study of Rembrandt’s late experimental painting technique (1651–1669).

Cobalt blue paint first made