Why does red fade so easily outdoors?
November 15, 2010 10:44 PM   Subscribe

On signs and other media shown outdoors, why does the color red seem to fade the fastest?

In a variety of places, such as signs, advertisements painted on buildings, posters, etc., it always seems like pure-red color is the first to fade, before any other color.

Is there any photochemical reason for this? Does red just absorb UV better than other colors just by being red? Or is there some specific dye or pigment frequently used for red that isn't light-fast?
posted by Hither to Science & Nature (7 answers total) 3 users marked this as a favorite
I've worked for years as a signmaker. The specific mechanism of action has been explained to me in the past, but I've forgotten the details.

Red does indeed fade much faster than anything else -- regardless of medium, (any type of) paint/vinyl/ink/coat -- and a common and quite effective work-around to this is to apply a UV coat. This would suggest that your hypothesis regarding UV is correct.

Another data-point that might help your search: certain shades of red are notorious within the industry for fading faster than others. Darker, burgandy-ish reds, for example, will fade even quicker than, say, a tomato-red, regardless of vinyl brand or type.
posted by jjjjjjjijjjjjjj at 11:09 PM on November 15, 2010

Red has a bad reputation for fading when used as a hair dye or a tattoo ink, but permanence in other media can vary. This page describes several common pigments and their relative permanence.

Good permanence: Cadmium Red, Pyrrole Red, Pyrrole Alizarin, Quinacridone Red/Rose/Magenta.

Poor permanence: Alizarin Crimson, Rose Madder, Napthol Reds.

As this page explains, some pigments are just more sensitive to ultraviolet light than others, and will darken or lighten with prolonged exposure.
posted by maudlin at 11:12 PM on November 15, 2010

The cyan (blue) dye is often phthalocyanine which is exceptionally stable among all colors of pigment. It's also quite strong - a tiny dab will turn a big blob of yellow paint green.
posted by scose at 11:23 PM on November 15, 2010

From this Nova page about preserving historical artifacts (emph. mine):
The Star-Spangled Banner is a case in point. Both the dyes and the wool of our country's most famous flag have been seriously light-degraded over time. And as expected, the red dye is more faded than the blue. "The red dyes are more susceptible to fading because they look red and thus absorb blue, and blue is the higher-energy light," notes David Erhardt of the Smithsonian Center for Materials Research and Education, who assisted the flag's conservation project.
posted by mhum at 12:11 AM on November 16, 2010 [4 favorites]

As an aside - this happens on housewares, too. Good luck ever finding an old Pyrex set with the red intact.

A good, interesting question - thanks!
posted by bibliogrrl at 6:35 AM on November 16, 2010

Um, ok. I hate to tempt flames because I can not find any documentation on this and I've been looking for a long time. BUT, I've painted two rooms in my house red, and if you've done the same, you probably know that it can take some extra coats to get full coverage. In one of my rooms, I needed 4 coats just to cover up the light streaks. I seem to recall touching it up even after that. (FWIW, the next red room was more of a brick red, and I didn't have such an extreme problem with it. Or maybe the paint was better.)

Anyway. At the time I talked to a lot of people about it, and somebody told me that red pigment molecules(?) are larger. So, as a layer of large rocks will let through more light than a layer of sand, a layer of red pigment molecules end up having a lot more space between them. And that's why it can take so many layers of paint to achieve full coverage.

So I'm just thinking, if that's the case, anything that's colored with red pigment would be more susceptible to fading. If a red pigment molecule fades to the same degree as a blue pigment molecule, the red still has all that space between molecules, and the red will just become more transparent, if you will, more quickly.

I hope I don't sound like a complete moron. I don't know anything about pigment, but I do "get science" and that explanation of the red paint has always stuck with me and makes sense. :) So the same idea might carry over.
posted by iguanapolitico at 9:48 AM on November 16, 2010

I am no kind of chemist, but I am a whiteboard enthusiast -- a context in which you WANT dyes to fade on command.

Green marker is notoriously quick to stain and difficult to erase. Unsurprising, then, that red -- its RGBIV opposite -- would be more transient? Not an expert answer, just a datapoint.
posted by foursentences at 11:11 AM on November 16, 2010

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