Cold Wax Medium alone is soft and brittle when dry.  Gamblin recommends a maximum of 25% Cold Wax (1:3 ratio of cwm:oil) if you want to paint on stretched canvas.  As we have all been taught, up to 50% Cold Wax (1:1 cwm:oil) is acceptable when working on a rigid surface:  plywood, hardboard or thick paper that will be framed.    If you add too much to your oil colors, it will diminish the strength and flexibility of your paints.