FAT OVER LEAN IN OIL PAINTING MADE SIMPLE

The fat over lean rule doesn't have to be confusing.

Fat Over Lean Explained

Here is a simple explanation of what painting fat over lean means, why it's recommended, and how to do it.

In oil painting, there is a rule called the fat over lean rule. Some say "learn the rules so you can break them". This is not one of those rules. This rule prevents paint cracking. The fat over lean rule is a critical method in oil painting.

What Is The Fat Over Lean Rule?

Fat in the fat over lean rule means oil. Fat equals oil in oil painting.

The fat over lean rule can be confusing. Remember this rule is about fat and has nothing to do with the thickness of the paint.

Fat means more oil. Lean means less oil.

Oil paintings are traditionally painted in layers. Oil paint used in the first paint layer is traditionally thinned with a bit of solvent such as turpentine or odorless mineral spirits. Turpentine and odorless mineral spirits evaporate. The solvent thins the paint, then evaporates, leaving a thin paint layer behind that contains little fat. This makes the first paint layer a lean layer.

This first lean paint layer is often called an underpainting.

What Does Fat Over Lean Mean In Oil Painting?

Oil paint pigments are suspended in oil. Oil paints are manufactured containing a certain amount of oil.

There are two ways of thinning oil paints.

  1. Thin oil paints with solvent
  2. Thin oil paints by adding more oil

Oil in the paint makes the paint layer flexible once it dries. More fat equals more flexibility. The fat over lean meaning in oil refers to order in which paint layers are applied.

Imagine painting a piece of elastic. Once the paint dries, if you stretch the elastic, the paint is going to crack. The elastic is going to be more flexible than the paint layer.

Upper layers should be more flexible than bottom layers, never the other way around.

A paint layer that has been mixed with solvent becomes less flexible when it dries. A paint layer containing more oil becomes more flexible when it dries. The layers must be layered in order of flexibility to prevent cracking later on.

Apply paint layers containing the least oil first. The underpainting should contains little fat. Apply layers containing the most oil last. Traditionally, glazing layers contains the most oil, therefore glazes should be painted last.

Drying Time Paint Fat Over Lean

In addition to controlling the flexibility of the paint layers, painting fat over lean prevents the upper layers of oil paint from drying faster than lower paint layers. Upper paint layers drying faster than the lower layers can also cause paint cracking.

Color Saturation - Oil Painting Fat Over Lean

The fat over lean method in oil painting is also said to preserve color saturation.

How To Paint Fat Over Lean

Paint In Layers

We use this solvent thinned layer to draw out our composition and color in the shadow areas. The next layer might be painted with oil paint thinned with only a couple drops of oil. The next oil paint layer might be painted with oil paint thinned with a larger amount of oil. The final layer might be painted with glazes or paint thinned with the largest quantity of oil, therefore the fastest layer.

Fat Over Lean Technique

Applying the fat over lean technique is really quite simple.

Add no additional oil into your first paint layer. The oil paint used for the first paint layer is traditionally thinned with a few drops of turpentine or odorless mineral spirits. This thinning makes it easier to draw out the composition with a paintbrush. It also makes the first paint layer a rigid, inflexible lean layer.

Example Of Fat Over Lean Oil Painting Layers:

Leanest Paint Layer
- 1st paint layer = underpainting
- Oil paint thinned with solvent
- Contains very little fat or oil
Lean Paint Layer
- 2nd paint layer
- Oil paint straight from the tube
- Contains only the oil added by the paint manufacturer
Fat Paint Layer
- 3rd paint layer
- Oil paint mixed with a few drops of oil or painting medium
- Contains added fat or oil
Glazing Layer
- 4th paint layer
- Transparent mix of oil paint and oil
- Contains the most fat or oil

Think of oil paint colors straight from the tube as lean.

The oil pigments are suspended in a minimal amount of oil. The paint's label will often say the type of oil the manufacturer has used to produce the paint.

Oil Painting Mistakes To Avoid

The following oil painting mistakes can cause paint cracking to occur.

Do not let your underpainting contain more oil than the overpainting. Underpainting means the first paint layer. Overpainting means any layer that is painted on top of the first layer.

If using a painting medium, use the same type of painting medium in every layer of the painting. This doesn't mean that each layer must contain painting medium. Just use the same type throughout. Painting mediums have different composition and ingredients. These ingredients have different drying times. Using one type of painting in the lower layers and another type of painting medium in the upper layers can result in paint defects like cracking. Keep it the same throughout.

Conclusion:

For strong paint layers, follow the fat over lean oil painting technique. Using the fat over lean painting method helps to prevents paint cracking. The most common causes of oil paint cracking are:

  • Underpainting containing more oil than the overpainting
  • Using more than one type of painting medium within a single painting
  • Applying a top layer of paint thinned with a solvent

Remember That Fat Means Oil In Oil Painting

Fat layers contain more oil. Fat over lean refers to the flexibility of the paint layer. The most flexible paint layers should be at the top or applied last. Flexible paint layers contain the most fat or oil. Use little to no oil in the first layers of an oil painting. You can use more oil or painting medium in the upper layers of an oil painting. Following this simple rule is designed to prevent cracks from forming in your oil painting as it dries.

Remember, the fat over lean rule has nothing to do with the thickness of the oil paint. The oil painting fat over lean rule refers to the amount of oil that is mixed into the paint.

Author: Sonia Reeder-Jones