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Why Does Paint Dry?

2015 December 24. Thursday.

Interview with a Non-Expert

Eura Shin is a friend of mine who is pretty representative in terms of paint knowledge. I called and asked her to explain how paint dries:

Eura: Hello
Luke: Hi Eura!
Eura: Hey!
Luke: Have you ever painted anything? Like a wall, or a cat?
Eura: Yeah. Or a cat?
Luke: Well, you wouldn’t paint a cat; you’d paint a painting of a cat.
Eura: Oh okay yeah sure
Luke: So you know how the paint is wet at first, and then you leave it spread out on a surface for a few hours and it dries and is stuck there permanently?
Eura: (cautiously) Yes
Luke: Explain to me how that works.
Eura: Explain how paint dries?
Luke: Yes
Eura: Oh gosh, I’m guessing something to do with chemicals and some stuff evaporates and some stuff stays behind on there.
Luke: So it’s like you have the paint color mixed in with some liquid and…
Eura: Yeah
Luke: When you spread it on a surface the liquid evaporates off but the paint color stays behind?
Eura: Yeah
Luke: Then why doesn’t the paint become slop again when you pour water on a wall?
Eura: Isn’t paint based off of pigment with… I don’t know. [incomprehensible] I thought it had to do with the powder that you mix oils with and then the liquid goes away.
(At the same time) Eura: Possibly. Luke: Explain.
Eura: You have oil paints and water colors and stuff like that; doesn’t that have to do with… you start off with a powdery color and then you add in different things like oil or water, and that’s how you make the paint. And then the paint goes away, I mean the water goes away I guess. And that leaves behind the powdery stuff… Heheh I have no idea I’m just freeballing it.
Luke: Okay, thank you very much Eura!
Eura: [incomprehensible] You’re welcome!
Luke: Bye
Eura: Bye

Explanation

Latex paint works mostly like Eura described. The idea she was missing - why I can’t splash water on a wall and melt off the paint - is the binding agent. Peter Stotereau explains:

The solvent in the paint, usually the water, evaporates leaving the binding medium and the pigment (colour) behind. The binding medium forms a skin over the pigment after the solvent has evaporated.

Oil paint is completely different (same source):

Unlike water, the oil does not evaporate. These paints dry on the wall in a very different way, through a chemical reaction with the oxygen in the air. This causes the relatively small molecules in the oil to form much larger molecules which have a solid rather than liquid form (i.e. polymers are formed). In the sealed tin, oxygen is not available for this chemical reaction to occur, and the oil remains in its original, fluid (smaller molecule) form.

Most glues also use oxygen reactions, not just evaporation, to solidify. JB Weld (and any two part resin) uses two seperate chemicals that react with each other, but neither reacts with oxygen. The lid never gets stuck with JB Weld.

paint drying time lapse

I think this is latex paint:

Public Domain Dedication.