Smalt has been used since 1500’s, a finely ground blue glass (a mixture of cobalt and potassium). It is a very transparent pigment, which is easily overwhelmed in mixtures due to its weak tinting strength. Depending on how fine the powder is ground depending how dark it is.

Dark smalt is larger particles (200 mesh) the finer particles make a lighter colour (400 mesh). Historically an affordable blue compared to the alternative Lapis Lazuli from Afghanistan.

Smalt works better in water based mediums (acrylic and watercolour) rather than oil were it may fade due to the oil reacting with potassium particularly in a damp environment.