Using mediums makes oil colors thicker or thinner, glossier or more matte. This issue of the Gamblin occasional newsletter is about painting mediums.
Mediums are a more important aspect of painting for contemporary painters.
Research at the National Gallery in Washington, D.C. reveals that Rembrandt used no painting mediums at all. He really did not need mediums because his oil colors were made by hand. Handmade paints have a low pigment load so they are thin and do not require diluting. The Renaissance style of painting was based on the application of many thin layers of oil colors. Then that became an established technique.