Fat Over Lean: Essential Guide to Oil Painting Techniques for Artists

The "fat over lean" rule is a foundational principle in oil painting, yet it often leads to confusion among artists, both beginners and professionals. This rule, which dictates the proper layering of oil paints, is essential for creating durable, long-lasting artworks. Understanding and applying the fat-over-lean rule correctly can prevent common issues such as cracking, flaking, and instability in the final painting. This comprehensive guide will demystify the fat-over-lean rule, providing you with the knowledge and techniques necessary to ensure your artwork stands the test of time.

What is the Fat Over Lean Rule?

The fat over lean rule in oil painting refers to the practice of applying paint with a higher oil content (fat) over paint with a lower oil content (lean). The underlying science behind this rule is rooted in the drying process of oil paints. Lean layers, which have less oil, dry faster and are less flexible, while fat layers, with more oil, dry slower and remain to some degree more flexible. This difference in drying times and flexibility is crucial because applying a less flexible layer over a more flexible one can lead to cracking as the painting ages.

The rule can be broken down as follows:

• Fat Paint: Paint with a higher oil-to-pigment ratio or low pigment volume concentration (PVC).

• Lean Paint: Paint with a lower oil-to-pigment ratio or high pigment volume concentration (PVC).

By increasing the oil content in each successive layer, artists create a structurally sound painting that is less prone to deterioration over time.

Why is the Fat Over Lean Rule Important?

Adhering to the fat-over-lean rule is critical for the structural integrity of your oil paintings. If the rule is ignored, the upper layers of paint may dry faster and become more brittle than the underlying layers, leading to issues such as:

• Cracking: The most common problem is where the top layer of paint contracts more than the underlying layer, resulting in visible cracks.

• Flaking: Over time, brittle top layers can begin to flake off, compromising the artwork's longevity.

• Instability: The overall stability of the painting is compromised, potentially leading to a complete breakdown of the paint film.

Following the fat-over-lean rule ensures that each layer of paint dries at a consistent rate, allowing the painting to cure properly and maintain its intended appearance over time.

Pigment volume concentration (PVC) is a measurement of the volume of pigment in a paint film compared to the volume of the binder. It's calculated by dividing the volume of pigment by the total volume of pigment plus binder, excluding solvents and water. The point at which there is just enough binder to wet pigment particles is called the critical pigment volume concentration (cPVC).

If we think of our paint as a brick wall, with the bricks representing the pigments and the mortar between the bricks representing the oil binder, then we have an excellent analogy of the optimum amount of oil in our paints.

How to Apply the Fat Over Lean Rule in Oil Painting

Step-by-Step Guide:

Step 1: Begin with Lean Layers

Start your painting with lean layers, which contain less oil, such as paint from the tube, or are thinned with solvents. These layers dry quickly and provide a stable foundation for subsequent layers. Ensure that the solvent used evaporates fully, leaving behind a solid and lean base.

Step 2: Gradually Add More Oil to Subsequent Layers

As you build up layers, gradually increase the oil content. This can be done by adding a small amount of oil or medium with oil content to your paint. The increase in oil ensures that each new layer remains more flexible and dries more slowly than the layer beneath it. Alternatively, if you paint with very little or no oil or medium added to your paint layers, this is also an excellent way to preserve the fat-over-lean rule and also reduces the potential for yellowing of the paint.

Step 3: Final Layers and Finishing Touches

The final layers should have the highest oil content, ensuring maximum flexibility and durability. Be cautious with the amount of oil added; too much can lead to excessive gloss and yellowing over time. If necessary, use a medium designed to add oil without drastically altering the paint's consistency, such as a paste medium.

Practical Tips:

Drying Times: Allow each layer to dry thoroughly before applying the next. This prevents issues like alligatoring, where the top layer cracks as it dries faster than the underlying layer.

Mediums: Be mindful of the mediums you use. While they can enhance the paint's properties, they also alter the oil content. Ensure that you adjust the amount of medium to maintain the fat over lean balance.

Common Misconceptions About the Fat Over Lean Rule

Despite its simplicity, the fat-over-lean rule is often misunderstood. Here are some common misconceptions:

Myth 1: Bodied Oils Are Always Fatter

Some artists believe that thicker or "bodied" oils are inherently fatter than other oils. However, the thickness of the oil does not directly correlate with its fat content. What matters is the oil-to-pigment ratio in the paint.

Myth 2: Certain Pigments Are Fat or Lean

It is a common misconception that some pigments, like lead white or alizarin crimson, are inherently fat or lean. In reality, it is not the pigment itself but the amount of oil in the paint mixture that determines whether it is fat or lean.

Myth 3: Solvents Are Fattening

Solvents like turpentine or mineral spirits do not add fat to the paint. Instead, they evaporate, leaving behind a leaner layer. Therefore, their use should be carefully balanced with the addition of oil to ensure the correct application of the fat-over-lean rule.

The Role of Mediums in Fat Over Lean

The use of painting mediums introduces additional complexity to the fat-over-lean rule. Mediums often contain solvents, resins, or additional oils, which can alter the oil content of the paint. For example, pre-mixed mediums like Winsor & Newton’s glazing and blending medium can make determining the exact amount of oil added to the paint challenging.

It’s important to note that solvents like turpentine or mineral spirits do not add fat to the paint because they evaporate entirely, leaving no residue in the paint film. On the other hand, resins and oils in mediums do contribute to the fat content.

Artists need to understand the composition of the medium they are using to apply the fat-over-lean rule correctly. This can often be done by consulting the safety data sheet (SDS) for the medium, which typically lists the solvent and resin content. For instance, if a medium contains 45% solvent, the remaining 55% would be a combination of oil and resin, both of which contribute to the fat content.

Practical Application of the Fat Over Lean Rule

To effectively apply the fat-over-lean rule, consider the following steps:

Start with Lean Paint: In the initial layers, use paint straight from the tube or thinned with a solvent. This ensures a lower oil content.

Increase Oil Content Gradually: As you build up layers, gradually add more oil or a medium containing oil to your paint. Each layer should be slightly “fatter” than the one beneath it.

Avoid Over-Thinning: Avoid over-thinning the top layers with solvents, as this would reduce the oil content, thereby increasing the pigment volume concentration and promoting faster drying, violating the fat-over-lean rule.

Think in Terms of Pigment Volume Concentration (PVC): This concept refers to the ratio of pigment to binder (oil) in the dried paint. At the critical pigment volume concentration (cPVC), there is just enough oil to bind the pigment without leaving excess oil in the paint film. Paint at this stage is flexible, semi-glossy, and less prone to issues like sinking in or cracking.

Simplifying the Fat Over Lean Rule

While the fat over the lean rule is foundational in oil painting, misconceptions and oversimplifications can cause confusion. By understanding the basic principle—that each successive layer should contain more oil than the last—artists can ensure their work remains durable and free from structural issues.

Think of the rule as akin to building a brick wall. The bricks (pigments) need just the right amount of mortar (oil) to hold them together without being too weak or too rigid. With careful planning and execution, the fat-over-lean rule becomes a straightforward guide to creating lasting, beautiful oil paintings.

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Advanced Techniques and Considerations

Pigment Volume Concentration (PVC):

Understanding PVC is crucial for mastering the fat-over-lean rule. PVC refers to the ratio of pigment to binder in the paint. Paints at or near their critical PVC are ideal for maintaining flexibility and durability. As you increase the oil content, the PVC decreases, aligning with the fat-over-lean principle.

Working with Unconventional Mediums:

If you wish to incorporate unconventional mediums like cold wax into your oil painting, it is important to do so with caution. Cold wax does not dry in the same way as oil and can create issues with flexibility and adhesion. Small amounts can be used without violating the fat-over-lean rule, but excessive use can lead to structural problems.

Expert Tips:

• Glazing: When glazing, ensure that each subsequent layer contains slightly more oil than the last. This helps maintain transparency while adhering to the fat-over-lean rule.

• Impasto: For impasto techniques, consider using a medium that adds body to the paint without drastically altering the fat content, ensuring that the paint remains flexible.