fat over lean definition

Fat over lean is a painting rule that, when followed, prevents oil paintings from cracking over time.  'Fat' oil paint refers to paint that comes from a paint tube with nothing mixed into it.  In order to make oil paint even fatter, it can be mixed with oil.  When paint is mixed with turpentine, it is said be 'lean'.  Lean oil paint has a lesser drying period than fat oil paint.  Because of this, a layer of paint should, in general, be fatter than the layer going on top of it.  Not following this painting rule can make a painting prone to cracking.  It should be noted that thick paint is not the same as fat paint.  Even if a painter used paint diluted with oil, it would be considered fat paint.