# Oil, Fat Media and the Rule of Fat Over Lean

Painting I · Colour, Perception and Materials · https://tryals.app/learn/painting-i/oil-fat-media-and-the-rule-of-fat-over-lean

## Drying Is a Chemical Reaction

An oil film does not dry by evaporation. It dries by **taking up oxygen** and polymerising into a solid network.

Drying speed tracks unsaturation, measured by its **iodine value**:

| Oil | Typical iodine value | Behaviour |
|---|---|---|
| Linseed | about 170 to 200 | Fastest drier; yellows most |
| Walnut | about 140 to 150 | Slower; yellows less |
| Poppy | about 130 to 155 | Slowest; least yellowing |
| Olive | about 75 to 95 | Does not dry at all |

### Fat over lean

**Each layer must contain more oil than the one beneath it.** Lean layers dry faster and set hard. Fat layers dry slowly and stay flexible.

A lean layer over a fat layer splits as the bottom moves. This produces **traction cracks**. **Thick over thin** follows the same mechanical principle.

Studios often mix **stand oil**, damar varnish, and Turpentine to adjust fatness.

### Studio Safety

Solvents like turpentine and white spirit have flash points near 40 °C. Both evaporate quickly and pose health risks.

Pigment toxicity depends on the **route of exposure**:

* Lead pigments are toxic if swallowed or inhaled.
* Cadmium dust is hazardous when airborne.
* Never sand paint without a mask.

### Varnish

A protective varnish must be **removable**.

Apply varnish only when the paint is dry **through**. This takes at least six months for standard layers.

> **Common pitfall:** adding oil to every stage. Excess oil causes severe wrinkling and darkening. Always build from lean to fat.

## Practice questions

7 of this lesson's 14 practice questions, with answers. The full set is in the app.

### 1. Sort each decision by whether it obeys the rule of fat over lean or breaks it.

**Answer:**

- Obeys fat over lean: Each successive layer a little thicker than the one beneath it, A final glaze carrying more medium than any layer below it
- Breaks fat over lean: A thin scumble of stiff, solvent-thinned paint over a still-soft impasto, A lean retouch over a passage finished two weeks ago in a fat medium

**Why:** The last item is the one that catches people, because it feels like a small repair rather than a layer. It is a layer, it is leaner than what it sits on, and it will craze first, which is why retouching is done with a medium at least as fat as the passage being repaired.

Page: https://tryals.app/practice/painting-i/oil-fat-media-and-the-rule-of-fat-over-lean/sort-each-decision-by-whether-it-obeys-the-rule-of-fat-over-lean-or

### 2. A student lays a rich, oily underpainting and then glazes over it with paint thinned only with turpentine. Within a year wide cracks have opened across the glaze. Why does the rule of fat over lean predict exactly this?

A. Glaze solvent evaporated pre-curing, leaving unbound pigment that cracked under strain
B. Lean top layer dried faster over a shifting base, splitting the rigid skin as it shrank
C. Rich base blocked oxygen to the glaze, stopping the cure and embrittling the upper film
D. Excess turpentine dissolved underlayer binder, causing glaze to lose grip and flake off

**Answer:** B. Lean top layer dried faster over a shifting base, splitting the rigid skin as it shrank

**Why:** The leaner layer sets first and ends up brittle, while the fat layer beneath goes on drying and shrinking for months. The skin cannot follow that movement and tears, which is what traction cracks are. Reversing the order gives a flexible top that follows whatever the hard layer beneath does.

Page: https://tryals.app/practice/painting-i/oil-fat-media-and-the-rule-of-fat-over-lean/a-student-lays-a-rich-oily-underpainting-and-then-glazes-over-it

### 3. Complete the account of fat media.

**Answer:** An oil film cures by taking up **oxygen** and polymerising, so it gains mass rather than losing it. The measure of how unsaturated an oil is, and therefore how readily it dries, is its **iodine** value. The rule that each successive layer must carry more oil than the one beneath it is called fat over **lean**. A finishing layer that is by definition removable is a **varnish**.

**Why:** The last blank is a definition rather than a description. If a final layer cannot be removed without taking paint with it, it is not functioning as a varnish however it was sold, which is the whole argument for the synthetic resins that were developed to stay soluble for decades.

Page: https://tryals.app/practice/painting-i/oil-fat-media-and-the-rule-of-fat-over-lean/complete-the-account-of-fat-media

### 4. Which of these are sound studio practice with solvents and pigments?

A. Ventilate the space and keep solvent containers closed when not in use
B. Keep oil-soaked rags in a closed metal container, because they can self-heat
C. Never sand or spray dried paint films without respiratory protection
D. A pigment is safe to handle in any way as long as the tube is not opened while eating

**Answer:** A. Ventilate the space and keep solvent containers closed when not in use; B. Keep oil-soaked rags in a closed metal container, because they can self-heat; C. Never sand or spray dried paint films without respiratory protection

**Why:** The fourth is not a rule at all. What matters is the route: inhalation of dust or vapour, ingestion from hands, absorption through skin. Naming the route tells you the precaution, which is why sanding a dried cadmium film is dangerous while brushing it out of a tube is not.

Page: https://tryals.app/practice/painting-i/oil-fat-media-and-the-rule-of-fat-over-lean/which-of-these-are-sound-studio-practice-with-solvents-and-pigments

### 5. Oil dries by oxidative polymerisation rather than solvent loss. What follows mechanically from this when building up a painting in multiple passes?

A. Upper layers must absorb oxygen before the lower layers dry
B. Upper layers must remain more flexible than the substrate
C. Upper layers must seal off the lower layers from atmospheric air
D. Upper layers must evaporate more rapidly than the underpainting

**Answer:** B. Upper layers must remain more flexible than the substrate

**Why:** Oxidative cross-linking creates an evolving, flexible network over many months. Trapping a brittle film over a slowly curing, flexible base produces structural failure, while drying oils gain mass from ambient oxygen rather than simply evaporating.

Page: https://tryals.app/practice/painting-i/oil-fat-media-and-the-rule-of-fat-over-lean/oil-dries-by-oxidative-polymerisation-rather-than-solvent-loss-what

### 6. Arrange these layers of an oil painting in order from the leanest to the fattest, as the rule requires them to be applied.

**Answer:**

1. A thin imprimatura wiped out with solvent
2. A monochrome underpainting with very little added oil
3. The first opaque colour layers, with a small amount of medium
4. Later modelling layers with proportionally more oil
5. Final glazes carrying the most medium of all

**Why:** The order is also a timetable, because leaner layers dry sooner. That is the practical benefit hidden inside the structural rule: working lean to fat means each layer is touch-dry before the next arrives, so the sequence protects the film and the working method at the same time.

Page: https://tryals.app/practice/painting-i/oil-fat-media-and-the-rule-of-fat-over-lean/arrange-these-layers-of-an-oil-painting-in-order-from-the-leanest-to

### 7. An ordinary oil painting should not be varnished until the film is dry through. How many months is the usual minimum given for that?

**Answer:** 6 (within ±2)

**Why:** Conventionally about **six months**, and longer for a thick film. The figure is a rule of thumb rather than a measurement: what actually matters is the thickness, the pigments and the room it dried in, so a heavily impasted picture can still be moving after a year while a thin one is stable in three months.

Page: https://tryals.app/practice/painting-i/oil-fat-media-and-the-rule-of-fat-over-lean/an-ordinary-oil-painting-should-not-be-varnished-until-the-film-is