A painter does a detailed underpainting with oil paint straight from the tube (rich in oil), then applies a thin solvent-thinned layer over it the next day. What problem will likely develop over time?
ANo problem — using less oil in upper layers is always safe
BThe solvent in the upper layer will immediately dissolve the underpainting
CThe fat lower layer dries more slowly than the lean upper layer, so the upper layer becomes rigid while the lower layer is still moving — causing cracks
DOil paint never cracks, so layer order doesn't affect the final result
This violates the 'fat over lean' rule. The underpainting is fat (full oil content, slow-drying) and the upper layer is lean (solvent-thinned, fast-drying). When the upper layer dries and becomes rigid first, the still-wet, still-moving fat layer beneath it tries to contract as it eventually cures — and the rigid upper film cracks under the stress. The rule 'fat over lean' ensures each lower layer is stable before the layer above it dries.
Question 2 Multiple Choice
What distinguishes a glaze from simply mixing two colors on the palette?
AA glaze uses a palette knife rather than a brush to apply color
BGlazing produces optical color mixing — light passes through the transparent layer, reflects off the opaque layer beneath, and passes through the glaze again, creating luminous color that looks different from the same colors mixed directly
CA glaze means adding white to a color to make it more transparent and lighter
DGlazing and palette mixing produce identical results; glazing is just a slower way to achieve the same color
A glaze is a thin, transparent layer applied over a dry opaque layer. The optical effect is different from direct mixing because light actually travels through the glaze twice (in and out), interacting with the opaque layer beneath. A transparent red glaze over dry yellow doesn't just look like orange — it looks like a luminous orange with depth, because the light path through two separate layers creates a different visual effect than two pigments mechanically mixed on a palette.
Question 3 True / False
Oil paint dries by evaporation, just like watercolor or acrylic — it just takes longer.
TTrue
FFalse
Answer: False
Oil paint cures through oxidation — a slow chemical reaction in which the oil molecules react with oxygen in the air to form a hard film. This is fundamentally different from evaporation. The practical consequence: a surface that feels 'touch-dry' may still be chemically soft underneath for weeks or months. Understanding this prevents mistakes like varnishing too early or assuming a painting is structurally stable just because it no longer feels tacky.
Question 4 True / False
In oil painting, 'fat over lean' means that each successive layer should contain progressively more oil than the layer beneath it.
TTrue
FFalse
Answer: True
'Fat' means higher oil content (slower drying); 'lean' means lower oil content (faster drying, often achieved by thinning with solvent). Building lean-to-fat ensures each lower layer dries and stabilizes before the layer above it. Early layers are thinned with solvent; later refinements and glazes use paint at full consistency or enriched with medium. This structural principle has kept oil paintings intact for centuries.
Question 5 Short Answer
Explain why the 'fat over lean' rule exists in oil painting. What physically happens if you violate it?
Think about your answer, then reveal below.
Model answer: 'Fat over lean' exists because layers with more oil take longer to cure than leaner layers. If you place a lean (fast-curing) layer over a fat (slow-curing) layer, the upper layer dries and hardens while the lower layer is still soft and shifting. As the fat lower layer eventually finishes curing and contracts slightly, the rigid upper film cannot flex with it — and it cracks. By building lean-to-fat, you ensure each layer is stable and done moving before anything is applied on top of it, creating a structurally sound paint film.
The rule is not arbitrary — it reflects the physics of oxidative curing. Painters discovered it empirically centuries ago by noticing which studio habits led to cracked paintings. Understanding the mechanism (not just the rule) lets you reason about edge cases: for example, why you can paint wet-into-wet in a single session (all paint at the same curing stage) even if some layers are fatter than others.