Smalt is a glass pigment, a material relatively uncommonly used as pigment in western painting traditions. The majority of mineral pigments and their synthetic inorganic equivalents are crystalline with a structure at a molecular scale, constructed of an ordered framework of atoms. Glasses in contrast have disordered structure. They are amorphous and inherently unstable substances. As a consequence, smalt is perhaps more well-known for its fugitive nature than for its stunning blue colour when freshly made.
The pigment has a long history. Many synthetic inorganic pigments were discovered as by-products of metal purification or the manufacture of glasses. Blue cobalt glasses have been available since antiquity in Persia, Egypt and the Roman world and it is certainly in the Middle East that sophisticated coloured glassmaking technologies became established. Smalt as a pigment was probably first developed in this region; it has been detected in painting in Central Asia in the 11th to 14th Centuries, but we know more about its production and use in Medieval Europe from the sixteenth century onwards, it was a pigment widely used in 17th Century painting. The name is derived from the Italian smaltare, meaning ‘to melt’ and it was made by fusing cobalt oxide with silica glass and potash. Cobalt ore was mined in Saxony, Germany and despite the Italian name, Saxony was the main source of smalt.
Johann von Löwenstern-Kunckel writing in his Ars vitriaria experimentalis in 1689 tells us that to make smalt, the processed cobalt ore (cobalt oxide) was crushed and mixed with sand and potash and then heated in a furnace until molten. This mass of blue glass was then plunged into cold water which made it friable and it could then be crushed and sold as pigment. It is clear that the process was somewhat hit and miss; Kunckel tells us that the product was graded from pale blue to the favoured deep violet blues and priced accordingly.
Pigment particle size also has a strong effect on colour. Coarse-grained, deep blue streublau or strewing smalt could be sprinkled on surfaces, however it was gritty and difficult to work with as a pigment. The smaller the particle size, the pigment became less intense in colour and had a greater susceptibility to fading. In London’s National Gallery, Paolo Veronese’s series of paintings, Allegories of Love, illustrate figures cavorting against what should have been an azure, Mediterranean Summer sky; today they appear to be underdressed for a dull day in November. The smalt that Veronese used as his sky blue has now lost almost all of its colour.
The cause of the fading of smalt has been a subject of much debate in pigment chemistry circles. It has been suggested that it the result of a change in the oxidation state of the cobalt or more fundamentally, a reaction between oil media and the potassium and cobalt ions to form soaps. More recently, spectroscopic and other analytical studies by Ilaria Cianchetta and colleagues have demonstrated that due to the inherent instability of glass, potassium leaches out of the smalt particles over time.
Whilst this process itself does not impact the colour, its removal from the glass alters the internal structure and this in turn affects the co-ordination state of the cobalt ions. The consequent breakdown of an octahedral coordination (wherein the cobalt is bonded to six oxygen ions) to a tetrahedral coordination (where there are only 4 oxygens per cobalt ion) changes the wavelength of the reflected light from blue to yellowish grey.
You can explore the full Winsor and Newton oil paint range here.