Smalt was the primary blue pigment in Northern European painting from the 15th to 18th centuries — a cheaper alternative to ultramarine when lapis lazuli was unavailable or too expensive. Many Vermeer paintings that now appear grey-green in blue passages were originally a rich blue — the smalt has become transparent over centuries as the oil medium has aged around the glass particles.

Historically deep and now hauntingly faded. Smalt carries the melancholy of a colour that has aged badly — paintings that were once vivid blue are now grey. This transformation embeds a meditation on impermanence into the history of Dutch Golden Age painting. The grey passages in old Vermeer paintings were once as vivid as ultramarine.

The gradual discovery that many Old Master paintings have dramatically changed colour due to smalt degradation has transformed art historical understanding. What we see in museums is not what was painted — the greys in Vermeer's skies and Rembrandt's landscapes were once vivid blues. This realisation requires a fundamental reimagining of the visual culture of 17th century Northern Europe.