Oil paint can take a long time to dry. When it's thinned with turpentine the turpentine evaporates quickly and the paint "dries", but sometimes that layer can still be smudged. What is it about oil and oil paint that makes it stay "wet" and liquid for a long time and then - voila! It starts to thicken and then dries to a hard film.
Think about the liquids you use everyday; water, isopropanol (rubbing alcohol), possibly even solvents like acetone and ethyl acetate. Most of these liquids evaporate fairly quickly. Even a drop of water doesn't take days and days to dry. When these everyday pure liquids dry *poof* they're gone.
2. Typical solutions and their drying
Of course it's possible to take something solid, for example salt or sugar, and dissolve it in a liquid like water. When the liquid water evaporates, solid salt or sugar is left behind. In contrast, when oil paint is dissolved or thinned with turpentine or another solvent, the solvent evaporates and the pasty paint is left behind -- and then the paint slowly hardens over time. Step back and think about this - oil paint has an unusual property.
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| Solid particles (gray balls) dissolved in a solvent (blue "water molecules"). Instead of particles you can also imagine that the gray balls are molecules of something that would be a solid without the water dissolving it. |
