Smalt was an important pigment in European oil painting, particularly in the sixteenth and seventeenth centuries. Its origins probably lie in the blue pigment used by the ancient Egyptians, known as us as "Egyptian blue"; both pigments are made from glass that has been coloured blue and both are also used as glazes on ceramics.
Being a glass, smalt is transparent. The particles of smalt are vitreous, meaning that they have highly reflective surfaces. As particles of smalt are ground finer, the surfaces are broken and fractured increasing its light scattering effect and lowering its color saturation. For this reason, smalt cannot be ground as finely as many other pigments, so it must be used rather course in paint.
Smalt is made by heating cobalt oxide with glass. Potassium, in the form of potassium carbonate, is added to the glass as a flux, making it flow more easily. Once combined, the molten glass is poured into water. The temperature shock causes it to shatter into small pieces, which are then ground to yield a coarse pigment.
Smalt is known to fade in paintings, but the degree to which this occurs varies even within the same painting. The reason for the fading has only been understood quite recently. Some assumed that the fading resulted from leaching out of the cobalt. However, Professor Jaap J. Boon and his collaborators recently found that the potassium concentration is crucial to fading. Where the ratio of potassium to cobalt is 1:1 or higher, the color has not faded.
Potassium is strongly alkaline and their findings suggest that a certain level of alkalinity is required to maintain smalt's blue color. Drying oil creates a slightly acidic environment. The small proportion of fatty acids present in the oil may react with the potassium. Moisture must be present for this reaction to take place, but potassium is known to attract moisture and there will be occasions when the amount of moisture in the air is likely to be high.
The conclusion is that smalt made with a higher proportion of potassium to cobalt will not fade. This is good news for artists whishing to use smalt in their work.