How is pigment different than light?
We see color when light bounces off a surface, reflecting the colors back to our eyes, with the surface absorbing all other colors we do not see. This ability for us to see a color is limited by the colors available in the light source. The colors we see can be measured in 'wavelengths.'
Under daylight, we can see the colors of the 'Visible Light Spectrum.' Under red light, only the red colors are reflected, all others appear gray, brown, or black. This is called 'color bias' in lighting terminology
When illuminating works of art, it is desirable to have a balanced source of light that will reflect all colors, as you would see them in natural daylight.
Special Effects with Color
Combine all colors in paint and you get black : Combine all colors in light and you get white.
The Primary Colors in paint are: Yellow, Blue, Red
The Secondary Colors in paint are: Green, Orange, Purple
Colors that are opposite on the color wheel are called 'Complimentary.'
Value is the term used to describe the range of dark to light, often shown as 'grey scale'.
Saturation is the intensity or purity of the color.
If you place opposite colors of full saturation, next to each other, you achieve 'vibrating boundaries' or the experience that the colors are vibrating where the colors meet. See Color Phenomena for more.
Albert Bierstadt, Mount Corcoran, c. 1876–77
Corcoran Gallery of Art, Washington, D.C.
It is a huge responsibility for museums to conserve and preserve paintings such as the one below illustrating the exploration of the Western United States in 1876.
How do we protect works of art from light damage? Why no flash photography allowed?
Museums around the world struggle with preserving their collections by hiding them away or displaying them for viewing by the public. Many works are 'rotated' in the galleries, limiting the time of light exposure. Exposure to and duration of natural and artificial light are evaluated in combination with the materials used to create the work. The flash from your camera or cell phone not only distract other museum visitors; they can damage the art. This is because they are super-bright, and even though it is only a quick flash, over time, these repeated little moments have a cumulative effect and can add up to big damage including fading, cracking, flaking, and and effect similar to sunburn, where the image darkens over time.
Why does light make colors fade?
Paint, dye, or pigment that fades is called 'fugitive', meaning it goes away, becomes less brilliant, and can shift to another part of the color spectrum. It is important to understand that light affects pigments by both intensity (how bright the light is measured in footcandles fc, or lux) and duration (how long it is exposed to the light.)
Intensity and Duration
Light has the ability to fade paint colors, pigments, and dyes. Fading is increased as the intensity of the light, Ultra Violet component (UV the wavelengths that cause sunburn), and by duration or the number of hours per day a work of art is exposed to light.
Blue pigments have traditionally been the most vulnerable to fading from light exposure, described as 'fugitive.' With the developments of synthetic dyes, and 'fade resistant' pigments there is now a huge range in fading. Care needs to be taken in conserving all art, since to measure the fading of colored surfaces requires light exposure to prove fading, and then it is too late, since the fading has occurred and the damage has already been done and cannot be reversed.
Paper, textiles, and other materials are also damaged by exposure to light, heat, and moisture. 'Yellowing' of paper over time has been witnessed by most of us. Not only has the original color shifted from white to yellow, the fibers themselves have been weakened. Imagine a flag blowing in the sun and wind day after day, eventually it weakens and shreds. We do not want that to happen to our art.
Testing for Color Fading
Blue dyed wool has been traditionally used to calibrate the fading strength of various light sources. Developed for testing under sunlight, today's museums and artists need new methods for testing fading.
To understand fading, consider how a similar effect is achieved with our blue jeans when they are faded on purpose to make them look old. Our blue jeans fade because of the abrasion, wear and tear, of usage and washing and drying. 'Stone-washed denim' has been marketed to give a pre-faded appearance. Occasionally you will find a small piece of pumice stone or lava rocks, in the pocket of your new jeans, evidence they have been stone washed.
Light test strips.
Traditionally blue wool test strips are used to test for light fading materials. a series of blue dyed strips are placed inside a museum exhibit case with a light blocking strip covering a part of the strip. On a pre-determined schedule, new portions of the strips are exposed to light. This results in a graduated color faded sample, allowing museum conservators to evaluate both the damage done by light sources and the stability of the dyes or pigments.
Test strip from the Northeast Document Conservation Center.
Museum Lighting Research - The Getty