smalto

Italian smalto meant 'enamel' or 'melted glass,' from Frankish *smalt or Old High German smalzjan, 'to melt.' The pigment called smalt was made by fusing cobalt oxide with molite glass, then grinding the result into a fine powder. The deeper the blue, the coarser the grind—but coarser particles were harder to work with in paint.

Smalt production began in earnest in the 1500s in Saxony and Bohemia, where cobalt ores were plentiful. The Schneeberg mines in Saxony became the center of European cobalt and smalt production. Miners had long cursed the cobalt-bearing ores because they were toxic and ruined silver smelting. The discovery that these nuisance ores could produce blue glass turned a mining waste product into a commodity.

Every major European painter of the 1500s-1700s used smalt: Titian, Vermeer, Rubens, Rembrandt. But smalt had a fatal flaw. In oil paint, the blue faded over time as the glass particles lost their interaction with the binding medium. Vermeer's skies, originally deep blue, have turned gray in several paintings. Conservators can detect the original smalt with X-ray fluorescence, revealing colors the human eye can no longer see.

Prussian blue's invention in 1704 and synthetic ultramarine's development in 1826 made smalt obsolete within a century. The pigment that colored the European Renaissance was replaced and forgotten. Today smalt is produced only by a handful of historical pigment specialists. But it persists in Vermeer's grayed skies—invisible proof that the blue was once there.