Terre de Cassel is the geological source of the pigment class that includes Vandyke brown and Rubens brown — named after the German regions where the bituminous earth deposits were found. Its use by Rubens and van Dyck gave it prestige that led to it being named after both artists. The Cassel deposits have been worked since the 17th century.

Deep, warm, and slightly problematic. Terre de Cassel carries the psychological weight of Old Master painting — the dark, rich browns of Flemish portraiture. Like all bituminous pigments, it embodies the tension between immediate beauty and long-term consequences. The paintings that used it most lavishly are now the most in need of conservation.

The confusion of names around terre de Cassel, Vandyke brown, Cassel earth, and Cologne earth reflects the pre-industrial art materials trade — pigments were often named inconsistently for the place of discovery, the artist who used them most, or the merchant who sold them. This terminological chaos creates significant challenges for art historians and conservators trying to identify which specific pigments were used in historical paintings.