How does my paint change colour as it dries?
Question
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.
Answer
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.
Comments
I think the reason that flat paint dries darker is shading.
I think that when flat paint is wet, it reflects more light because it has a smoother surface, like a mirror. The surface tension of the water molecules (or the oil molecules) holds the surface of the paint mixture smooth. The smoother the surface, the more light reflection you will get.
As the paint dries, the water molecules evaporate. The dried product no longer has surface tension holding it smooth. The dried product has nooks and crannies. These rough nooks and crannies will cast shade.
Some of the light that goes inside a nook and cranny will bounce around the inside of the nook several times. Each time it bounces around, some of the light gets absorbed and doesn't come back out. The more nooks and crannies you have, the more light gets absorbed and the end result is that rough surfaces look darker.
THE PAINT DRIES DARKER than the color it was when it was wet.
I have beige walls. Sometimes I need to touch up a dusty spot where a picture used to hang. I don't want to redo the whole wall, just the dusty spot.
I take a chip off the wall. I go to the store and have them use a machine to match the paint to the chip. The salesperson shows me that the wet paint matches the chip perfectly. I paint the dusty spot and the "wet" paint DOES match the wall perfectly. I come back the next day and every inch that I painted is darker than the rest of the wall. THE PAINT DRIES DARKER than the color it was when it was wet. I added one cup of pure white to 3 cups of the bad paint. I touched up the botched area with the paint that was 25% lighter. When it was wet on the wall, it looked lighter than the rest of the wall. But when it dried, it was a perfect match to the rest of the wall.
A year later, I need to do another touch up. I learned my lesson. I learned to ask the paint store to mix the paint lighter. I still had my old paint can with the formula used to match the chip from the wall. I remember that I need to ask for a color that is 25% lighter than the formula I got from the chip. There is a formula on the can that shows how much of each color of dye went into making the paint. I tell them, "reduce the numbers for each color of dye by 25 percent to make the paint lighter." For example, if the red was 23 and the black was 11 and the green 16, I tell them, "Program the machine to give me red 17, black 8, and green 12. I take the lighter paint home. I redo the area that needs to be touched up. The wet area "looks" lighter than the rest of the wall for "now" (because it's lighter when its wet). The next day, after it dried, it had become a perfect match because it dried darker by 25%.
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