Why does paint change colour as it dries?

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Seymour, John

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Why does paint change colour as it dries?
« on: 17/05/2010 09:30:03 »
Seymour, John  asked the Naked Scientists:
Hello there!
I have been listening to podcasts of the Naked Scientist for several months now. I truly enjoy the slightly whimsical approach to edu-tainment.
I apologize for being late with this, but I have only this morning listened to your March 14 show. I would like to offer up a somewhat different explanation as to why paint changes color as it dries. I have quite literally spent a few days watching ink dry… measuring the color of it with a spectrophotometer periodically so as to understand the physics of the colour change.
Try this kitchen science experiment on a cold day: Buy a can of flat black spray paint. Follow the directions for spray painting and paint a flat surface at room temperature. Then try this again where the paint can, the surface, and the air are all at something like 50 degrees F.
What you will see is that the room temperature sample will be matte (i.e. the specular reflection will have a Lambertian distribution) just like you would want it to be. The sample that was painted below the requisite temperature will surprisingly be glossy.
How does that happen?
The difference between glossy and flat paint is in the vehicle (the stuff that carries the pigment and evaporates). A flat paint will have a very volatile vehicle. In this way, the particles of pigment will have little time to form a smooth surface. As a result, the surface of flat paint is (ironically) rough. Flat paint is matte because it is not flat. A glossy paint will have a vehicle that evaporates relatively slowly. In this way, the surface dries to a smoother finish. When the flat paint is applied at a cool temperature, the evaporation occurs more slowly, so that the pigment has time to dry to a smooth surface.
Why does this change the colour?
When light hits paint, there is always a certain amount of light (maybe 5% as predicted by Fresnel's law and the index of refraction of the paint) that is reflected specularly. Specular light is light that is reflected directly from the surface in a billiard ball fashion, without interaction with the bulk of the paint. Note that,  the index of refraction does not depend a great deal on the wavelength of light, so the specular light has pretty much the same colour of the incident light, i.e. it is kinda white.
Now, when you look at an object with a smooth surface (that is, a glossy object), you will tend to orient the object in such a way that the specular reflection is not apparent. Even if the specular reflection is apparent, we will tend to not think of this gloss as part of the colour of that object, but rather think of it as a specular highlight.
On the other hand, when you look at an object with a rough surface (that is,  a matte object), the specular reflection is Lambertian. The specular light spreads out in all directions equally. We cannot orient the surface so as to exclude the specular reflection, and out brain cannot exclude a specular highlight from its interpretation of the colour. Thus, we have effectively mixed the colour of the bulk paint with that of the illumination so that it looks brighter and less saturated.
One might think that 5% of the specular light would have very little impact on our perception of the colour of an object. Fechner's law, however, states that all our senses (including our sensation of light) are logarithmic in nature. Thus, the eye is very sensitive to a small differences in the amount of light reflecting from a dark surface. A very rich black paint may have a reflectance of only 1%, so a tiny amount of change is readily apparent.
This same phenomena explains why images printed on a glossy magazine are much richer in colour than those printed on newsprint. Glossy paper has a coating of clay and calcium carbonate that is polished so the surface is smooth. Newsprint is basically just raw paper fibers without this polished coating, so surface is very rough at the microscopic level.
John Seymour
Principal Engineer - Research
A subsidiary of Quad/Graphics
Sussex, Wisconsin USA

What do you think?
« Last Edit: 17/05/2010 09:30:03 by _system »