Fading Away
by Peter Tyson

Saving the National Treasures homepage

Color photos, old newspapers, the Declaration—we've all seen materials damaged by long-term exposure to light. But what's actually happening?

The Declaration of Independence took a lot of abuse in the first decades of its life. It was rolled and unrolled countless times. It was tossed in a trunk or stuffed in a linen bag and whisked away every time the British Army got too near. And for 35 years, beginning in 1841, it was hung on a wall in the Patent Office in Washington, D.C., across from a sunny window.

The harm this last insult caused is apparent in a quote from the Public Ledger in 1876, when the Declaration was taken off that wall and transported to Philadelphia for the centennial. "Its aspect is ... faded and time-worn," the Ledger noted on May 8 of that year. "The text is fully legible, but the major part of the signatures are so pale as to be only dimly discernible in the strongest light ... and some are wholly invisible, the spaces which contained them presenting only a blank."

Light is likely not the only reason the Declaration has become so faded. In the early 1820s, an engraver made a facsimile possibly by laying a piece of moist paper over the Declaration to soak up some of the original ink. But light streaming through that Patent Office window surely further reduced the legibility of our nation's most precious handwritten document.

And fading of color or ink is only the most obvious injury that light can cause. In the presence of oxygen or moisture or temperature changes—or, worst of all, all three—light can trigger chemical reactions that can degrade the very substance of the parchment, paper, textile, or other material from which items of artistic or historical interest are made. In fact, considering the number of ways that light-induced chemical reactions can wreak havoc, it's amazing that such items survive for any time at all.

A bad reaction

Light is paradoxically both our best friend and worst enemy when it comes to enjoying our most beloved works of cultural heritage. For it is light that makes visible the rich reds, oranges, and yellows of a Turner sunset, say, or the deep blues of a Picasso blue-period painting, or for that matter the dark brown ink gracing the Declaration. 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, pigment, or ink—or in the paper or other material it graces—the molecule becomes "excited," that is, rendered available for chemical reactions.

"From a preservation standpoint, that's exactly what you don't want," says Paul Messier, a Boston-based conservator of photographs and works of art on paper. "If your object is chemically active, it means it's interacting with the environment and becoming chemically altered. Depending on the rate of those interactions, you've got a recipe for poor preservation and a short life span."

Why? Because any 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, creating smaller molecules and thereby weakening the paper or parchment by shortening the long fibers that make them strong.

Or it can 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. Such oxidation is the bane of museum curators, fine-art owners, and all of us who want to make things last, whether they are national treasures like the Declaration or a newspaper clipping of a personal milestone.

Adding insult to injury

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.

Moreover, some kinds of light are more problematic than others. Light toward the blue end of the visible spectrum, for example, is higher frequency—and thus higher energy—than light at the red end. Thus it packs a greater punch. And ultraviolet radiation is more energetic still. In fact, it's the most damaging type of electromagnetic energy in our everyday environment. Even relatively low-energy infrared radiation can damage materials by heating them and thereby helping to speed any chemical reactions already under way.

A 50-watt incandescent light bulb spews out 100 billion billion photons a second, and 95 percent of those are not helping us to see.

Conservators are concerned not only with frequency but with the intensity of light hitting artworks or other valuables—daylight, for instance, is typically brighter or more intense than artificial light. And then there's exposure time. As the Declaration shows so clearly, light damage is cumulative. It's also irreversible (except, as one expert reminded me, in PhotoShop). Finally, and arguably most unfair of all, chemical reactions initiated by light can continue even after something is placed in the dark.

Substance abuse

The range of our possessions at risk may be wider than you think. It includes the media used to write, draw, and create photographs, such as dyes, inks, pigments, varnishes, and oils, as well as the materials they're used on, like paper, textiles, furniture, feathers, fur, horn, and bone.

Some of these materials are at greater risk from light damage than others. Generally speaking, organic materials—those derived from plants or animals—are more susceptible than inorganic materials. For instance, natural dyes, which are organic, generally fade faster than pigments, which are usually comprised of inorganic minerals. But even organic materials vary in their stability: materials made of parchment, which is a specially prepared animal skin, are less vulnerable than, say, silk and wool.

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.

Light housekeeping

What to do? Well, for starters, most art on display, whether in a museum, gallery, or one's own home, is not being enjoyed at any given time. "It's got all these photons smashing into it, and so for every minute while it's being illuminated that nobody's looking at it, that's all wasteful damage," says Steven Weintraub, a conservator at Art Preservation Services in New York. "There are all kinds of efficiencies [we can adopt] in terms of UV filters, reducing the amount of light in general, and shutting off lights when nobody is around to look at it."

One thing is to remove all those wavelengths we can't see anyway from the light striking a valued object. A 50-watt incandescent light bulb spews out 100 billion billion photons a second, and 95 percent of those are either infrared or ultraviolet radiation, both of which are invisible to us. All those unnecessary photons slamming into our watercolor or framed photo at 186,000 miles per second are not helping us to see; they're only helping to harm the item.

The National Archives filters light on the Declaration to exclude higher-energy blue wavelengths, for instance. "You're looking through multiple layers of glass at a very low light level, and that light has had the harmful portion of the visible spectrum removed from it as well as the ultraviolet," says Kitty Nicholson, a conservator who worked on the Charters of Freedom project (see A Conservative Approach). "So it's a very special light that both enhances the viewers' visibility and also protects the document."

The National Archives also houses the Declaration in an oxygen-free encasement filled with the inert gas argon, with carefully controlled moisture and temperature. Some private institutions have gone further still, storing vulnerable materials in such oxygen-free enclosures in temperatures near or below freezing, in the dark. But that would be pushing it for many cherished objects. As Messier says, "What good is material that you can never see? It's all about figuring out the balance."