The pigment smalt is the most important blue pigment in Rembrandt’s paintings. It is also used in The Night Watch.
Smalt is a finely ground blue glass. It was made by melting quartz (sand), together with potash (as flux), and roasted cobalt ore (this gives it its blue colour) at around 1200°C. In the 17th century the cobalt ore came via Saxony (Germany) to the Netherlands, which was a large production centre.
Rembrandt used smalt not only for its blue colour, but he also mixed it with red to make a purple, and with yellow to make a green, and to enliven his brown mixtures. He also used smalt to thicken his paint and to make it dry faster.