Additives are essential to fully enjoy the qualities of extra-fine oil-based paint. There are four types of additives: Thinners, Driers, Mediums and Varnishes.
Thinners are most often petroleum spirits and oils. They enable you to modify the liquidity of the paint. Petroleum spirits are used during the first stages of production, particularly when working "fat over lean" in order to promote proper drying.
Mediums ensure the works cohesion to canvas, board and other surfaces, modify the gloss, allow highlighting in particular areas, facilitate impasto techniques and can modify the drying time. They are especially valuable when working Plein Air.
Driers, as their name suggests, speed up the drying time (more accurately, oil paint does not dry, it hardens).
Varnishes change the final appearance of the painting prior to any preservation of the work.