Studio Craft: The Principle of Fat over Lean

Ask the Expert: "Could you please explainthe Fat over Lean rule? I was taught that itreferred to the amount of oil you add to yourpaint on the palette. Does the same thingapply to acrylics?"

A: Simply put, the principle of "Fat over Lean" isa recommendation that initial layers should havea lower oil content than subsequent ones.Observance of this principle is a good way toachieve durable results and avoid cracking,crazing, wrinkling and other defects.

Adherence to "fat over lean" yields severalimportant advantages:

  • Ensures topmost layers are moreflexible than those beneath
  • Places slowest drying colors on top,avoiding movement between a dry skinand soft foundation
  • Lower layers absorb some oil from thenext ones so each subsequent layer can"key" onto the previous

Fat over Lean is not just about the amount of oiladded by the artist in the form of medium, butalso involves an awareness of the oil content ofeach color on the palette. Some pigments likeFlake White require relatively little vehicle toproduce a workable paint, while others are veryabsorbent and take up a lot of oil in the milling process.

It's not always possible to discern leanness byappearance alone- some very lean colors cantend to shed oil, which may collect at the top ofthe tube and give the impression that there is alot of oil present. Conversely, a very "fat"pigment may take up a lot of oil yet still have adry, waxy appearance.

The fat over lean principle does not apply toacrylics except as it relates to layering oil paintover acrylic gesso or paint (a dry acrylic film istreated as 'lean'). Regardless of the amount ofmedium added to acrylics, the resulting film canaccept more paint with any amount of mediumadded, or none at all. Adhesion between acryliclayers is very strong, and acrylics don't undergothe same complex chemical changes as oils doover months and years.

It's important to remember that the craft ofpainting has always been about pushing thelimits of the materials. Traditional "rules" like fatover lean are guidelines, not laws. It's admirableto place emphasis on permanence, but not atthe expense of creativity. Oil paint is a resilientmedium that allows for a good deal of variety intechnique and process, and while indiscriminatelayering may not be the best approach, there'sno need to worry excessively about the absoluteoil content of every stroke. General awarenessof the principle is enough for most artists to haveconfidence in the durability of their results whilestaying focused on making art.

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