Stillife with Fruit and Cheese

Stillife with Fruit and Cheese. Oil on gessoed panel. The addition of chalk to the paint gives the paint a slightly luminous quality. The old masters mixed their own paint fresh every day. The question is how did they incorporate chalk which is found in their paintings?

It must have been a technique that they borrowed from the Italian fresco painters, who added chalk to their paint before they applied it to the wet plaster.