It is certainly possible to work with nothing but plain, unadulterated oil paint, but certain effects are easier to achieve with judicious application of other materials. These can include various mixtures of varnishes, balsams, solvents, and thickened oils; such mixtures are called mediums. Because all mediums weaken the final paint film somewhat, they should be used in very small amounts.
Here are three ways to use mediums:
1. An oily or resinous medium can be mixed directly with paint. If so, it is best to use the smallest amount that will achieve the effect you are looking for—generally no more than 20% of total paint volume, and preferrably much less. Use a knife to mix a bit of medium thoroughly into each blob of paint on your palette, or into whatever mixture you want to have the properties the medium imparts. Then paint normally. A good medium will make the paint handle more smoothly. Artists have various opinions about which mediums are best.
2. An oily or resinous medium can also be spread thinly onto the surface of the painting before applying paint (this is referred to as painting into a “couch” of medium). Wipe with a cloth or rub it in with the palm of your hand to get it as thin as you can. Don’t apply medium to areas where you will not be painting this session, since oil on the surface can eventually result in excessive yellowing. The couch method has the effect of lubricating the surface (which can make precise detail work easier) and reducing “chatter” (i.e., dragging and streaking of paint strokes). It also improves adhesion between layers, especially if the medium contains a balsam.
3. If you use a thin medium containing a high proportion of solvent, keep it in a small covered container next to you as you paint. Dip your brush in medium and mix it into the paint on your palette just before you apply it. Don’t make the paint watery; use just enough medium to make the paint more workable. Solvents can dissolve a lower layer of paint if you haven’t given it time to dry completely. Use solvents only if you have good ventilation and keep the medium container covered when you are not using it in order to limit evaporation.
Now go forth and smear colored goo on flat surfaces!