Sun's ability to fade colors

Why does the sun fade colors so quickly? Anything I’ve left in the sun for a decent amount of time loses color. Even in the winter, a sweatshirt in my car will fade from the sun quickly. Is it heat related or chemical or what?

Short answer:

It’s not really the heat, it’s the UV radiation. UV causes chemical changes in certain molecules; these chemical changes cause the absorption spectra to change, and the net visual effect is that the molecules lose (or sometimes just change) their color.

Some dyes do break down with heat, but not generally at the kind of temperatures you’d get from simply leaving them in the sun.

Largely, this is due to the particle nature of photons. Photons from the Sun’s light impact the pigment molecules and break them down (you can think of it as a form of erosion). Rich colors appear rich, because they are absorbing all but a very narrow range of photons and reflecting the color you see. The absorbed photons impart energy, which is converted to largely to heat and vibrational energy (the 266 nanometer (deep UV) wavelength is often referred to as the ‘photon jackhammer’), largely at the expense of the matter they are striking.

This is why white is such a prevalent choice with sailors. If you want your ropes, or sailcloth to last, it’s best to start with white, which reflects most of the Sun’s energy. Brightly colored cloth absorbs energy, bleaching the pigment and breaking down the cotton fibers. By the time the bleaching has made the cloth more reflective, the damage has already been done.

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Ok, I guess I get the previous explanation, at least well enough for now. But why does red seem to bleach out so much more than blue? Is it something to do with the color itself (ie. red things are absorbing more radiation in the higher freq.s) or is it something to do with the kinds of things that we use to make red dyes and inks?

While we’re on the subject, does anyone know what I can spray on my suede couches to prevent fading? I had a luffly saddle colored suede coach that is now bleached due to the sun. I knew it would happen, but I hate having the window shade down all day.

Does Scotch Guard protect against this?

Scotch Guard probably won’t help any, unless it claims on the can that it does (if it could protect against fading, don’t you think they’d advertise it?).

A spray sunscreen might work. Disclaimer: I wouldn’t try it on MY couch.

Or you could treat your windows with something that blocks UV while letting visible light through. They can do this to glasses; it should work on your windows too.