How does my paint change colour as it dries?

14 March 2010


Hello Naked Scientists!

My wife and I are painting the ceilings in our house, and the ceiling paint we are using goes on light blue, and dries perfectly white. At the paint store, they put in a little package of powder and shook the can in the paint shaker before we brought it home. How on earth does it work?

Owen and Chrystal, Keswick, Canada.


Chris - Now that's a fantastic question. It's the same science behind why clothes look a bit darker when they're wet than when they're dry.The reason that this happens is because when you have paint in the tin, the paint is mixed with some kind of solvent - usually water or oil or something, which makes the paint easy to spread onto the surface so you get a nice even coat. When you paint the paint onto the wall, it's got all that solvent in it. The solvent then evaporates off - dries - and this leaves behind just the particles of paint on the wall.Now, the particles, if we take white paint as an example, are usually titanium oxide; they're very, very white. These particles are roughly the same size as the wavelength of light, which is why they reflect and scatter lots and lots of wavelengths of light back at you, which is why you see a white surface.But when the paint is wet, those particles are surrounded by little droplets of water or oil (the solvent). And so, when light goes in, it doesn't see these tiny particles of roughly the same size as the light wavelength; instead it gets subjected to a bit of refraction through the fluid and that buries it deeper into the wall surface, rather than reflecting it back out at you. So, if it's darker of course, what must be happening is less light is being scattered back towards you than being absorbed and that's why it looks darker. Once that effect goes away (when the solvent evaporates) and you've just got the particles there, you're scattering more light back at you, so the paint looks brighter.

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