Minimizing Light Damage to Artifacts

Courtesy

Wyoming State Museum

What You Will Learn

This short tutorial will discuss different kinds of light damage and how they are created. It will also recommend different types of light fixtures to use in your museum and discuss ways to limit the amount of light damage your artifacts experience.

Light Damage

Light is a common cause of damage to artifacts. Paper, cloth, leather, photographs, and many artworks are easily damaged by light and controlling the amount of light artifacts receive should be a priority for any museum. Both visible light and non-visible ultraviolet light (UV) provide energy for chemical reactions that lead to fading and deterioration.

How does light damage artifacts?

• All molecules vibrate constantly. When light waves (which are a form of energy) hit molecules, the molecules vibrate more quickly and begin to spread out. Ultraviolet light especially increases the vibration of molecules causing them to expand, then split from each other, then bond again, which leads to deterioration.

• On the next page you will see an image of a piece of paper that was

originally red. One half of the paper was covered while the page was exposed to sunlight. Over time, the light striking the uncovered half caused the molecules in the ink and paper to vibrate more quickly, bump into each other, and change their shape. The light hitting and reflecting off these differently shaped molecules changed the speed of the light wavelength we see with our eye making the original red color look faded.

The right half of this once red page was exposed to sunlight for three months. The damage done is easy to see.

Fading Away to Nothing

When the colors of an artifact fade or change, it is a signal that severe chemical changes are starting to destroy the object. The primary cause of this is light. The photograph on the next page is of a kimono worn by a Japanese doll. Once, it was all the same color purple seen in the top half of the photo. Exposure to light has caused the lower half to fade. A few more years of similar light exposure would have caused the object to turn almost white, weakening the threads in the process. There is no way to repair this type of damage.

The upper half of this kimono was covered for many years.

The lower half was not.

Yellowing and Embrittlement

Another chemical reaction related to light is oxidation. Oxidation occurs when a molecule excited by light energy transfers part of that energy to an oxygen molecule, which then attracts other molecules. This attraction causes chemical changes such as embrittlement and yellowing.

The front of this newspaper was exposed to sunlight for just a few weeks. Its molecules have started to alter causing it to become brittle. Notice that the inside pages have had much less damage since they were not exposed to as

much light.

Types of Light

There are two primary sources of light: sunlight and electrically produced light. As an exhibit light source, sunlight should never be used. It is too intense, quickly causes extensive fading, and has a high UV component, which also causes damage to the basic structure of objects. Electric lights should be used in your exhibits, but what type? The answer will depend on your budget and current light fixtures.

Incandescent Lights

Incandescent bulbs produce light when an electric current is passed through a tungsten filament or wire. They convert only a small percentage of the electrical current into light; the rest is given off as heat. Pro: produce little UV light (but can still damage artifacts with other types of light) Pro: relatively cheap Con: being phased out in many countries Con: don’t last long Con: will raise your exhibit temperatures and cooling bill if you use them in large numbers

Fluorescent Lights

Fluorescent lights contain mercury vapor inside a tube of glass whose inside surface is painted with white fluorescent powder. The electric current passing down the tube excites the vapor atoms which then give off energy in the form of light.

Pro: come in various sizes and shapes

Pro: inexpensive to operate and replace

Con: give exhibits a flat, two-dimensional look

Con: give off a large amount of very damaging ultraviolet light

Con: considered a low-level biohazard

Light Emitting Diodes (LEDs)

Light Emitting Diodes create light by using a semiconductor.

Pro: does not emit ultraviolet light

Pro: generates very little heat

Pro: has a much longer life span than other light bulbs

Pro: very inexpensive to run

Con: much more expensive to purchase initially

Measuring Light Levels

Whatever type of lightbulb you use, measuring the amount of UV and visible light to which an artifact is exposed is extremely important to ensure a long life for the object.

Measuring Ultraviolet Light

Ultraviolet light meters, such as this one distributed by General Tools, cost around $180 and can be purchased on the internet.

How much ultraviolet light is too much?

• To accurately measure ultraviolet light, a UV meter, which measures in microwatts per lumen, must be used. The best range of ultraviolet light for artifacts is between 0 and 10 microwatts per lumen. It should never be more than 75 microwatts per lumen.

• If a UV meter is not available, it is best to assume that sunlight and fluorescent lamps will need light filters to avoid damage to artifacts.

Measuring Visible Light

Visible light levels (not Ultra Violet light levels) are best measured by a light meter using what is called either “lux” or “foot-candles.” One lux is the measure of the intensity of light over one square meter. One foot-candle is equal to about 11 lux.

Measuring Visible Light

Visible light meters such as this one manufactured by General Tools cost around $129.00 and are easily found online.

How much visible light is too much?

• Most artifacts can be on exhibit for three to four months at 50 to 150 lux and show no fading. A level of 50 lux is similar to the lighting in a home living room in the evening. For comparison, standard office lighting is around 400 lux and direct sunlight measures 30,000 lux.

• Lower light levels are necessary for light-sensitive materials such as watercolors, photographs, leather, textiles, and prints. Materials without color can be exhibited at up to 150 lux. No paper, wood, leather, textile, or other organic object should ever be on permanent display.

• If the light levels are to be higher than 50-150 lux, the length of time an object is on exhibit needs to be decreased accordingly. When making the decision about time on exhibit and light levels, be aware that low light levels for extended periods of time cause as much damage as high light levels for shorter periods.

• A good guideline for exhibiting most museum artifacts is to obtain lux readings from 50 to 100.

What if I don’t have light meters?

• If no visible light or UV light meters are available, it is possible to estimate the damage that might result to an artifact from light exposure by using Blue Scale Fade Cards, available from a number of conservation suppliers.

• Blue wool cards can be purchased for $20 each. They can be cut into several strips to use in different locations.

Blue Scale Fade Cards

The Blue Scale cards visibly show the destructive powers of light and can be used to convince skeptics that light really is a problem for artifacts. Each Blue Scale card contains eight samples of blue-dyed wool. Sample 1 is extremely light sensitive, while sample 8 is the most stable dye available. Sample 2 takes twice as long to fade as sample 1, sample 3 takes twice as long as sample 2, and so forth.

How do I use a Blue Scale Fade Card?

Cut the card into strips then place one strip in total darkness in order to know what the original colors were. Write down the date and set out a strip where you plan to place artifacts. Check every couple of weeks to determine how long it takes the various samples to fade. Since the sensitivity of the first few samples on the card corresponds to light sensitive materials such as watercolors and textiles, the results will give you a general idea of the amount of damage you might expect if those materials were exhibited for the same period of time at the current light level in that location.

Blue Wool Fade Cards

The strip of Blue Wool Fade Card on the left was exposed to sunlight for two months. The strip on the right was kept in total darkness. The fading at the bottom of the left strip is easy to see when compared to the strip kept in darkness.

How do I eliminate light damage?

Unless you keep an object in complete darkness, you can’t eliminate light damage. There are many ways to minimize it, however. Since ultraviolet light is the most energetic, and destructive, it is easy to believe that if UV light is eliminated, damage will cease. Unfortunately, this is not the case. Visible light also causes damage. And while exposure to UV light can be nearly eliminated from exhibit and storage areas, reducing visible light requires different strategies.

Filtering Out Ultraviolet Light

• UV-filtering films exist for windows and exhibit cases, are flexible, and attach to glass and plexiglass. UV-filtering film is most effective if it covers the surface it is placed on completely so that all light has to pass through it.

• Plexiglass that filters UV light can also be purchased for windows and exhibit cases.

• LED lights produce no ultraviolet radiation and are a good choice for lighting exhibits. They are also becoming less expensive every year.

Ultraviolet Light Filters

• UV filter sleeves and films are easy to work with and are relatively inexpensive.

• Ultraviolet light filters for fluorescent tubes can be purchased online for roughly $8 each.

• UV filters should be replaced every five to seven years.

Reduce Light Damage in Storage Areas

To control the damage caused by visible and ultraviolet light:

• put light sensitive items into boxes

• keep the lights off when no one is retrieving materials

• put fluorescent lights in UV blocking sleeves

• cover windows with shades

Reduce Light Damage in Staff or Public Spaces

• Cover windows and skylights with UV blocking film and shades which should be closed during the most intense light of the day.

• Filter tubular fluorescent lights with UV blocking sleeves. • Keep collections off top shelves and away from windows. • Protect any materials that may be particularly susceptible

to light damage, such as framed color photographs or watercolor artworks, by displaying them away from any direct light (sunlight or spotlights) and covering them with UV blocking glass or Plexiglass.

• Keep lights off when spaces are not occupied, especially after hours.

Reduce Light Damage in Staff or Public Spaces

• Don’t aim lights directly at artifacts. Use indirect lighting whenever possible.

• Try to increase the distance between your light source and your artifacts.

• Use dimmer switches on lights or light panels if possible.

• If your budget allows it, replace incandescent lights with LEDs.

• Use motion detectors in exhibit spaces to trigger light if possible.

Where to buy supplies.

Supplies may be found at:

Gaylord Brothers (www.gaylord.com)

University Products (www.universityproducts.com)

An internet search for “light meter” and/or “ultraviolet light meter” will provide information on suppliers of these tools.