• Ashley Webb

Agents of Deterioration: Light

In December of 2020, I mentioned the ten agents of deterioration, and how it's important to be aware of these agents when considering your collection or family keepsakes.

Light, or rather the overabundance of light, is one of these ten agents.

Think back to the last time you were in a museum - did you notice that it was often dark, or that there wasn't any natural light or a window in sight? This is because light can quickly damage objects with irreversible effects.

There are three types of light radiation:

  1. Visible light

  2. Ultraviolet radiation (UV)

  3. Infrared radiation (IR)

There's a lot of complex stuff that goes on with the eye and light, but the note to take away is that the naked eye can't necessarily see UV or IR. Advancements in technology have given us the ability to see items with UV - think black light posters, and IR through thermal detection.

Light spectrum showing Ultraviolet radiation, Visible Light, and Infrared Radiation - three components that affect the deterioration of objects

Basically, visible light is the light we can see - involving all the colors of the rainbow. Ultraviolet radiation has tighter wavelengths, creating more power, while infrared radiation has looser wavelengths, creating heat.

Visible light causes fading of dyes, or in some cases darkening of paints or varnishes, while UV radiation causes yellowing, weakening of materials, or breaking down of adhesives or chemical compositions of objects. Infrared doesn't necessarily cause photochemical damage from the light source itself, but does cause damage in other ways. This becomes more of an incorrect temperature problem, and will be discussed at a later date.

Unfortunately, with light it's a bit of a compromise. While optimal conditions = darkness until the end of days to keep an object looking pristine or as if it were just made yesterday, that's also not the point of cultural heritage institutions. People want to see the priceless treasures. So, there has to be a trade-off in what the institution is willing to risk in terms of potential light damage.

With the help of a light meter, you can determine how much visible light an object is getting. 50 lux, or 5 foot candles (in the old imperial measurement) has become the standard for low-level lighting - especially when viewing textiles, works on paper, or older color photographs. This is the minimum level of lighting our eyes can accommodate in any given space - for older individuals, however, this low level can be difficult for their eyes. Despite these recommendations though, even 50 lux has been determined to fade extremely sensitive objects over long periods of time.

The maximum amount of light organic items should receive at any one given time is 150-200 lux, and even then the item needs to be carefully monitored and moved or put away after a shorter period of time. To put this in context - regular daylight ranges from greater than 100,000 lux on sunny summer days to 20,000 lux in the shade. Even on overcast days, daylight can reach 2000 lux - 40 times the minimum amount of light that's considered safe for many items.

As I mentioned earlier, lighting is an agent of compromise - you need to be able to have enough lighting for older visitors to see, but still protect objects from fading or breaking down. The ideal way to achieve this compromise is to have objects rotate out off exhibit after a set duration of time in the desired light levels.

There are four categories of light sensitivity that most objects fall under: no sensitivity, low sensitive, medium sensitivity, and high sensitivity. Sometimes objects with medium sensitivity is lumped with high or low, depending on its fade testing qualities. Fortunately, stone, metals, some ceramics, and glass - all inorganic objects - don't necessarily fall under the light restrictions, and can be on exhibit indefinitely. Items at 150-200 lux are considered medium and low sensitivity, while 50 lux is considered for items that have high sensitivity.

Unfortunately, light damage is cumulative, and its not reversible. In other words, once it happens, there's no fixing it, and it'll just keep fading or darkening (depending on the object's composition) the more light it is exposed to.

Level of fading when items are exposed to certain light levels, as well as the amount of time determined for the fade to occur

To illustrate this, there have been new studies on many of Europe's Impressionist paintings, especially those painted by Van Gogh.

Vincent Van Gogh used certain paints that have been determined to be of high light sensitivity - like chromate yellows, reds, purples, and certain greens. Check out the short Dutch video below from the Van Gogh Museum in the Netherlands on how they've determined how much Field of Irises near Arles has faded over the past 100 years, and what they've determined the painting looked like just after it was finished. There's also an article about it, if you prefer to read.

Van Gogh's sunflowers have also been analyzed in a similar way. Instead of fading though, the yellow pigments used are darkening under light exposure. It almost looks like someone took a bad photo on the left and used photoshop to brighten it on the left, but I assure you, the paint has darkened that much over the last 100 years - and all from light exposure!

One of Van Gogh's five Sunflowers. Courtesy of the Van Gogh Museum, Amsterdam

It seems that Van Gogh knew of the volatility and sensitivity of the chemical make-up of his paints. In a letter to his brother, he wrote "all the colours that Impressionism has made fashionable are unstable, all the more reason boldly to use them too raw, time will only soften them too much." This definitely has been the case, but it hasn't stopped museums and galleries from keeping them on display to curb the desires of the public.

So, now that you have a basic understanding of how light affects objects, how do you mitigate these risks with your own keepsakes?

Visible Light

  • Keep precious items from sitting in front of a window or an area that receives high levels of light at any point in the day - this especially goes for wooden objects, taxidermy, and upholstered/textile objects

  • Use light blocking curtains in rooms that get a lot of sunlight

  • Display more sensitive items in darker areas of your house

  • If you don't want items on display, the best course of action is to pack them in archival boxes to keep out any unwanted light

UV Light

  • Avoid using fluorescent lights in your home. Fluorescents give off a high amount of UV radiation, and even with UV filtering films, 100% of the radiation isn't blocked

  • Purchase special UV filtered Plexiglas for your framed items such as photographs, works on paper, and works on canvas

Infrared Light

  • Avoid placing items in direct sunlight, or in a box that can generate heat (this is called a microclimate) when in the sunlight (think of a car on a sunny day)

  • Avoid the use of incandescent lights. Incandescent lights can still be purchased today (the typical lights used for everything prior to LED), and generate heat when powered on, which equals a higher rate of IR.

  • If you do use incandescent bulbs, say if the bulb was already there and hasn't yet died or been switched out, you can monitor its heat output by checking how much heat you feel when your hand is next to it. The more heat you feel, the more IR the light is giving off

I hope this post helps when determining how to display or keep your most precious objects from fading and help them last for the next 500 years. Check back next month when we talk about the next agent of deterioration: Fire.

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