How does nitrocellulose lacquer and common dyes used in guitars fade? This question has recently been asked of me by a member here and there seems to be interest on how and why this happens, so I put together a basic write up to help explain what goes on. Almost everyone here knows that 'light' does the fading. But how and why does that happen? Maybe I can shed some 'light' on the issue. First we have to understand some things about light and the electromagnetic spectrum, and polymers. Light: The electromagnetic spectrum is a continuous range of frequencies and energies, with visible light occupying a small portion of the whole. The sun puts out electromagnetic energy throughout the whole spectrum. What interests us is the visible spectrum and the ultraviolet range. In 1887, the photoelectric effect was discovered by Heinrich Hertz, as he was experimenting with radio waves. He was not particularly interested in the phenomenon, but he did notice that the effect was produced strongly by ultraviolet light and more weakly by lower frequencies. Light whose frequency was lower than a certain critical value did not eject any electrons at all. Why is this important? A light wave consists of electric and magnetic fields. The stronger the fields, i.e., the greater the wave's amplitude, the greater the forces that would be exerted on electrons that found themselves bathed in the light. Light excites electrons with enough energy to break atomic bonds, but light also enables us to see colors. We see those colors because they reflect light back to our eye from very specific parts of the visible-light portion of the electromagnetic spectrum. Unfortunately, what's not reflected is absorbed, and that's where the trouble starts. Light is energy, and when that absorbed energy equals or exceeds the so-called 'activation energy' of a molecule in a dye or pigment, the molecule becomes "excited," meaning, rendered available for chemical reactions. If your dye is chemically active, it means it's interacting with the environment and becoming chemically altered. A number of things, many of them destructive, can happen once a molecule gets excited. The extra energy may be converted to heat (infrared energy) or emitted as light (phosphorescence or fluorescence). It can break chemical bonds within the molecule, breaking the dye down. Generally speaking, organic materialsthose derived from plants or animalsare more susceptible than inorganic materials. For instance, natural dyes, which are organic, generally fade faster than pigments, which are usually comprised of inorganic minerals. The older red organic dyes are more susceptible to fading because of the organic chemical makeup and becuase they look red and thus absorb blue, which is the higher-energy light. The energy from light can also jump to another molecule. In one of the most damaging of such leaps, the energy transfers to an oxygen molecule, which can then react with other molecules to jumpstart chemical reactions. Oxidation takes place, bleaching out the color. But that's not all. There can be synergistic effects: with higher temperature and humidity, for example, reactions catalyzed by electromagnetic radiation can occur more rapidly. And there can be chain reactions: new substances formed as a result of photochemical reactions will have enough energy to also react with the original substance, launching a chain reaction of degradation. Something to keep in mind is chemical reactions initiated by light can continue even after the object is placed in the dark. The ultraviolet wavelengths of interest are: UVA: 320 - 400nm SKIN TANNING UVB: 280 - 320nm SKIN BURNING UVC: 100 - 280nm INDUSTRIAL GERMICIDAL USE. All types of UV can cause a photochemical effect within the polymer structure. It is the high energy UVC wavelengths that are produced by the Sun, and not by artificial lights except in industrial use that cause the most degradation to polymers. Polymers: Lacquer refers to polymers dissolved in volatile organic compounds (VOCs), such as nitrocellulose, and later acrylic compounds dissolved in lacquer thinner, a mixture of several solvents typically containing butyl acetate and xylene or toluene. A polymer is a long chain of molecules that repeats a pattern of atoms. These molecular groups are tied together through bonding of electrons. Like with dyes, one of the main problems of considering the effect of UV rays on polymers is the intensity related to: stratospheric ozone, clouds, altitude, the position of the sun height (time of day and time of year), and reflection. The main visible effects in polymer degradation are a chalky appearance and a color shift on the surface of the material, and the component surface becomes brittle. Often a fluorescent whitening agent (FWA) is added to the polymer. In natural light many polymer products can appear to have a yellow appearance. But by adding a FWA the UV light absorbed is then emitted in the blue region of visible light (400-500nm wavelength), instead of the yellow region. When we shine an UV flourescent light on a finish to check its consistancy, this is the blue light that we see from this additive. As the polymer breaksdown over time and exposure, the color returns to the yellow that we associate with 'old' nitro finishes. UV energy absorbed by polymers can excite photons, which then create free radicals. While many pure polymers cannot absorb UV radiation, the presence of catalyst residues and other impurities will often act as receptors, causing degradation. Only a very small amount of impurity may be needed for the degradation to occur, e.g. trace parts per billion values of sodium in polycarbonate will initiate color instability. In the presence of oxygen the free radicals form oxygen hydroperoxides that can break the double bonds of the backbone chain leading to a brittle structure. This process is often called photo-oxidation. However, in the absence of oxygen there will still be degradation due to the cross-linking process. As there is an interest in artificially aging guitars, a few things have to be considered. Many of the lacquers used in the 1950's are not of the same formulae as todays. Many new UV blockers, plasticizers, and stabilizers have been added. UV bulbs do not produce the same spectrum as the Sun, so the results will not be the same. As with the lacquers, todays dyes have more inorganic components to stabilize them than in the past. I hope that this has been helpful.