The following is an excerpt from The Artist’s Illustrated Encyclopedia, by Phil Metzger, regarding the important concept of understanding “fat” content in oils and pastels.
“Fat: Containing a high proportion of oil. Many artists choose to remove some of the excess oil from oil paints before they use it. They do this by squirting a mound of paint onto a sheet of paper, such as bond or newsprint, and letting the paint stand until the paper has drained some of the oil from the paint. The draining is easy to see because a ring of oil develops around the paint.
Fat-Over-Lean Rule, oil: You should not cover a layer of oil or alkyd paint with a less flexible layer. Always paint flexible “fat” layers over less flexible “lean” layers. To do the reverse would be like having a lean “skin” over a fat pudding. Any movemnet in the pudding underneath is likely to crack the skin.
Fat means containing a relatively high proportion of oil, such as linseed oil, and lean means relatively less oil. Oil or alkyd paints become fatter when you add to them such preparations as linseed oil, stand oil or various oil-containing mediums. Paint becomes leaner when you dilute it with turpentine or mineral spirits. If you paint without additives, using paint straight from the tube, you’re reasonably safe. If you paint a turpentine-thinned layer over a layer of straight paint, you violate the rule – in that case, the chances of damage (cracking) are much diminished if the fat underlayer is well cured.
Fat-Over-Lean Rule, pastel: Soft pastel may be easily applied over hard pastel, but not vice versa. When stroking hard pastel over a passage of soft pastel, the hard pastel tends to gouge and push aside the soft.
However, hard can be applied over soft for specific, limited results – for example, to sharpen details.”