Fat over Lean…

One of the most frequently asked questions by students of oil painting is for an explanation to the “fat over lean rule”. The terminology is somewhat antiquated today, but the phrase persists. To put it simply, fat over lean is not a real rule, it’s a ‘rule of thumb’ for painting in layers; alla prima painters need not pay it any attention. The idea is to use a successively higher oil content as the painting progresses (fat = more oil, lean = less oil). And more crucially, to use as little oil as possible in the underlying layers. The point being, more oil means a longer drying time and more flexibility in the paint layer. A slow drying layer underneath a fast drying layer can cause a number of problems, most notably cracking in the dry overlying layers due to microscopic shifts in the underlying layers as they dry.

The trouble with the phrase, and perhaps the reason so much confusion exists today, is that oil alone is a far less common medium than it was when the phrase was coined. Traditionally, oil paint was made up of pigment and oil alone. The choice of mediums was also very limited. Today, many different things come into play which simply did not exist in the past. Is Liquin fat or lean? Is Galkyd leaner than stand oil? Is walnut oil fatter than linseed oil? Despite what some painters will inevitably insist, these questions really don’t have positive answers because the “fat over lean” principal did not take them into account.

In practical terms, “fat over lean” is about permanence. Do it wrong and your layers will be more prone to craquelure, delamination, or some other hazard. But remember, it’s a ‘rule of thumb’, not a real infallible rule. Simple common sense will avoid most of these hazards. Oil paint takes ages to dry completely. A painting dry to the touch may be wet underneath for months. If you’re using dryers (alkyds or other resins), the significance of this concern is somewhat diminished.

Quick drying colors, such as earths (siennas, umbers, ochres…) and lead (lead white principally), will help to combat these problems when used in the underpainting.

 

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