The Binder Is the Medium
Pigments travel between media almost unchanged. What changes is the binder, the substance holding pigment particles together and anchoring them to a surface.
| Medium | Binder | Dries by | Characteristic |
|---|---|---|---|
| Watercolour | Gum arabic | Evaporation | Transparent |
| Gouache | Gum arabic plus chalk | Evaporation | Opaque and matt |
| Egg tempera | Egg yolk emulsion | Evaporation, then curing | Fast-drying, hard |
| Acrylic | Polymer dispersion | Evaporation and coalescence | Flexible, water-resistant |
| Buon fresco | Wet lime plaster | Carbonation | Highly permanent |
Refractive Index
Here is a core studio principle. A particle's opacity depends on how its refractive index compares to its surroundings.
- Air is 1.00.
- Linseed oil is about 1.48.
- Chalk is around 1.57.
- Rutile titanium dioxide reaches 2.70.
In air or water, chalk scatters light strongly and appears white. In oil, chalk is index-matched and turns nearly transparent. By contrast, titanium white differs from oil by 1.2 and covers aggressively.
Specific Media Rules
Buon fresco is painted directly into wet lime plaster. The plaster cures by carbonation, locking pigment into the wall. Work must finish before drying occurs. Plaster is therefore laid in a giornata, a single day's patch.
Acrylic dries by coalescence. Polymer particles fuse into a solid film as water evaporates. This process requires a minimum temperature, usually around 9 degrees Celsius.
Common pitfall: Over-thinning acrylic with water. Unlike watercolour, acrylic polymer is a particle suspension. Adding too much water prevents the particles from forming a continuous film. The dried paint will rub off as loose powder. Always use an acrylic medium for heavy dilution.