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.