Q. Although I understand the fat-over-lean painting method, I’m not sure what’s considered “fatter.” Is oil paint straight out of the tube considered fatter than tube paint to which linseed oil, stand oil or other medium has been added?
A. “Fat-over-lean” has been a long-standing guideline of sound painting technique for artists who build up their oil paintings in successive layers of underpainting and glazes, referred to sometimes as indirect painting. This rule of thumb works on the principle that the more medium or “fat” in an oil paint layer, the longer it will take to dry. Thus, if you apply a lean paint layer over a fat layer, your lean layer will dry first. The underlying fat layer will then continue to move slightly in the drying process and frequently crack the upper leaner paint layer as it does so. In addition, dry lean layers of paint may have trouble adhering to rich, greasy underlayers, resulting in cleavage (separation) between the two paint layers and the potential loss of the upper paint layers.
When you add a medium such as linseed oil or stand oil to oil paint squeezed from the tube, you increase the “”fat”” content of the paint. Therefore, your paint with additional medium is fatter than your oil paint squeezed straight from the tube. This fatter paint shouldn’t be used for your underlayers.
Keep in mind that this method is for a painting technique that’s built up in discreet layers with some drying time between applications. If you’re creating a painting in one session–alla prima–using a wet-into-wet technique, the areas of paint become blended into one another while wet, diminishing the problems of fat and lean regardless of any additive mediums you use.
Heather Galloway is a freelance conservator living in northern Ohio.