system
Given the following post, respond as if you're commenting on Reddit:
human
How do oil paints dry?

Acrylic based paints dry by water evaporating, and oil paints dry by "oxidation." What is being oxidized, why does it cause the paint to become less wet, and why does it take so long?
gpt
Oil paint is a colloid; the pigment is suspended in a liquid-in this case, a hydrocarbon oil dissolved in a solvent that makes the paint more fluid and easier to move around and manipulate. 

After application, the solvent evaporates, leaving the pigment and oil. The oil gradually oxidises and hardens. As this involves absorbing oxygen (oxidising) rather than evaporating water, it can only happen at a certain rate, independent of environmental factors like temperature. (Other factors can be manipulated to increase the speed of oxidation, usually by mixing other chemicals like Liquin, a commercial product) Oxygen is also absorbed only through the top layer of the paint, meaning that it has to oxidise before lower layers can do so. This increases the time taken as well.