Mastering the Art of Oil Painting
Advanced Medium Management
The 'Fat Over Lean' Rule Revisited
You already know the rule: fat over lean. But the reason behind it is a matter of chemistry. Oil paint doesn't dry like watercolor; it doesn't just evaporate. Instead, it cures through a chemical reaction called oxidation. The oil molecules absorb oxygen from the air and link together, forming a solid, durable polymer film. This process is called polymerization.
The key is that this process never truly stops. The paint film continues to change for decades, becoming more brittle over time. A "fat" layer, with a higher ratio of oil to solvent, creates a more flexible, slower-drying film. A "lean" layer, with more solvent, has less oil and forms a less flexible, faster-drying film that is also more brittle in the long run.
Placing a fast-drying, rigid layer on top of a slow-drying, flexible one is a recipe for disaster. The top layer will cure and become rigid while the layer beneath it is still shifting and curing. This tension is what causes cracking and delamination over time. You must build your painting from the least flexible layer to the most flexible.
Anatomy of a Medium
A painting medium is more than just a thinner. It's a tool to control your paint's properties. Most traditional mediums consist of three components: an oil, a solvent, and sometimes a resin.
Oil: This is the binder. It determines the flexibility, drying time, and finish (gloss or matte) of the paint film. Linseed oil is the workhorse, drying strong and relatively quickly, though it can yellow. Stand oil, which is linseed oil that has been polymerized by heating, is thicker, dries slower, and yellows less, making it excellent for creating smooth, enamel-like surfaces.
Solvent: This is the thinner. It makes the paint more fluid by reducing its viscosity. Odorless mineral spirits (OMS) or turpentine are common. Solvents evaporate from the paint film, leaving the oil and pigment behind. A higher solvent ratio makes a layer leaner.
Resin: This adds gloss and durability to the paint film. Traditional resins like Damar can become brittle and yellow over time. Modern alkyd resins are a popular alternative, as they increase gloss, speed up drying time, and create a very tough, flexible film.
By adjusting the ratios of these components, you can create a medium tailored for any task, from a quick-drying underpainting to a luminous final glaze.
Crafting Custom Mediums
Creating your own mediums gives you precise control over your paint. Start with simple recipes and keep your layers in mind. The goal is to gradually increase the oil content in each successive layer.
| Layer | Purpose | Recommended Medium | Why it Works |
|---|---|---|---|
| Underpainting | Block in shapes, establish values | 1 part Linseed Oil, 2 parts OMS | Lean and fast-drying, creating a stable base. |
| Middle Layers | Build form and color | 1 part Linseed Oil, 1 part OMS | A balanced medium, slightly fatter than the underpainting. |
| Glazing/Details | Add luminosity, fine details | 1 part Stand Oil, 1 part OMS | Fat and slow-drying, providing a flexible top layer and more working time. |
This is a basic progression. For faster drying, you can introduce an alkyd resin. An alkyd medium like Liquin or Galkyd acts as both the oil and resin component. You can use it straight from the bottle for middle layers or thin it with a small amount of OMS for underpaintings. Because alkyds dry quickly and form a strong film, they are very forgiving, but the fat over lean principle still applies: don't put a lean layer of pure alkyd over a very oily, slow-drying traditional layer.
Understanding these materials on a chemical level demystifies the painting process. It transforms you from a painter following rules to an artist making informed decisions. By managing the properties of your paint, you ensure your work not only looks the way you intend but also stands the test of time.
The curing process of oil paint is primarily due to:
What is the primary reason for applying paint in 'fat over lean' layers?