"For painting, oil was added to the ground pigment and the two were ground together (usually on a stone slab with a muller) until a thick, soft paste was obtained."
The stone slab was often made of marble and called a painter's marble or stone.
As quoted from: Jo Kirby, Susie Nash, and Joanna Cannon, eds. 'the appendix: Pigment Glossary', in Trade in Artists’ Materials: Markets and Commerce in Europe to 1700. (London: Archetype Publications, 2010), 454.