Created by Image Permanence Institute with support from Tru Vue
FORCES OF PHOTO DECAY
Decay on Display
Most people think that light causes most of the damage to photos on
display, but there are other forces that contribute to the decay of
photographs, including heat, pollution, moisture, and even the framing
materials themselves. Below are descriptions of each of these, but we
will start by discussing exactly how light harms photographs.
Light Fading
The rate at which photographs are damaged on display and the type of
damage that appears are determined by two properties of light: quality
and quantity. Light quality doesn't refer to how well the bulb
was manufactured or how long it will last before burning out, but to
the type of light: tungsten, fluorescent, or sunlight. Quantity of light refers to its intensity,
that is, how bright the bulb is. The light from a child's nightlight
is similar in quality to that from a hundred-watt lamp bulb, but the
quantity of light emitted by the nightlight is far less.
The quality of light is often correlated with the color of light.
Most people know that tungsten light tends to be yellow and that
fluorescent light is fairly white (although older bulbs often made
things look green). Sunlight varies in color throughout the day and
with different types of weather. Sunsets are marked by their glowing
orange, while an overcast day can look dark grey-blue. The color of
light affects both the way in which photos on display change and the
speed with which they change. Blue light is higher in energy than
green, yellow, or red light, and the higher the light energy, the more
likely it is to damage photos. Right next to blue light in the spectrum
(see Fig. 7) is ultraviolet (UV), which can be the most damaging of
all. The term UV light is really a misnomer. The word light
applies only to the energy that we can actually see, and UV is
invisible to humans (although some animal species can see in this
range).
It is more appropriate to refer to UV energy. Next to red in the spectrum is infrared energy.
Damage by UV Energy
Each type of light (tungsten, fluorescent, or sunlight) contains
different amounts of UV energy. Sunlight contains the highest UV levels
and is the most damaging, followed by fluorescent. Tungsten carries
very little UV energy.
UV energy can damage photos by fading the image as well as yellowing
and/or embrittling the paper. One of the ways it fades photos is by
breaking the internal chemical bonds of the color molecules that form
the image. This causes the molecules to become colorless and invisible
to our eyes. The more this happens, the more the image faces. This
process is called photolysis.
Another way that UV energy fades photos is by photo-oxidation.
UV energy excites the color molecules, making them more sensitive to
oxidation (the same process that causes iron to rust). This is a
two-step process (excitation-oxidation) that won't take place without
both UV energy and an oxidizing agent. Unfortunately, our air is full
of oxidizing agents, most notably ozone. Both photolysis and
photo-oxidation can cause photographic prints to yellow or become
brittle over time (see Fig. 8).
Tungsten lighting emits fairly low levels of UV. Because of this,
many people mistakenly believe that, since they light their homes with
tungsten bulbs, they don't have to buy UV-blocking glazing for their
frames. However, research by a major manufacturer of photographic
prints has shown that the dominant type of light in homes is still
sunlight through window glass.
(1)
This means that the photos we display are actually being subjected to
the harshest of type of light and therefore really do need protection.
Damage by Visible Light
Visible light (non-UV) can also be damaging to photos. Visible
light usually isn't strong enough to cause photolysis, but it can still
cause some photo-oxidation. So, while it is very important to reduce a
photo's exposure to UV energy, that won't prevent all fading. We can
reduce damage by visible light by reducing the quantity of light, and we
can do this by placing our framed photos in low-light areas or by
moving photos and light sources further away from each other. Because
our framed photos must be exposed to some visible light if we
want to see them, it is all the more important to reduce any damage from
UV energy or poor-quality framing materials.
Damage by Infrared Energy
Like UV energy, infrared energy exists in varying amounts in each
type of light. Tungsten and sunlight have more infrared than
fluorescent. As mentioned, infrared energy can heat framed photos.
This can dry out the photograph, causing it to warp or shrink and pull
at its mounting points. Heat also accelerates other types of decay. It
sometimes takes longer to notice damage caused by heat than to see
damage caused by UV or visible light, but the damage is always
occurring, even when UV-protective glass is used. Again, keeping framed
photos in naturally low-light-level areas in the home will reduce the
amount of damage over time. If tungsten light is aimed directly at a
photo, feel the glazing for warmth. If it feels hotter than other areas
of the room then lower the light levels or move the photo and the light
source further apart.
Dark Fading
While light fading is easy to describe, dark fading is more
complicated. Dark fading is the damage that occurs when photos are
stored in the dark, as in boxes or photo albums. Where it gets
confusing is that dark fading also occurs in the light, simultaneous to
light fading, so it's always happening to our framed photos too. Dark
fading is not caused by the dark; it is the sum of the damage caused by
forces that do not need light, and those forces continue whether the
lights are on or off. The forces behind dark fading are heat,
pollution, and humidity.
The heat that damages photos is not the kind of heat we associate
with an oven or even a very hot day. Room temperature for us is hot for
a photo. Most photos last longer when stored in cool or even cold
storage spaces. Since we prefer our living conditions to be around 70
to 75 degrees, the lives of our photos are forcibly shortened. There's
really not much we can do to prevent this for our photos on display.
The one thing we can do is make sure that the lighting we use to
illuminate our pictures does not also heat them up.
Pollution in the air can come from outside the home in the forms of
ozone, oxides of nitrogen, and other gases that are the products of our
industrial society, but pollution can also come from sources inside the
home, such as household cleaners, electronic equipment, and curing paint
and adhesives. Don't clean your frames with chemicals, and always let a
freshly painted room cure for at least two weeks before rehanging your
pictures. Pollution was the cause of the image fading shown in Fig. 9.
Damage by moisture comes in many forms. In the case of framed images, one of the most common and the most destructive is blocking,
which is the adhesion of a smooth surface - in this case the glazing -
to the surface of a photo. In high humidity the top layer of many
photos can soften and become like glue, bonding the photograph and the
glazing together. It is often impossible to remove a blocked photo
without destroying it (see Fig. 10). This is why it is so important to
use a window mat or frame spacers to prevent direct contact between
photograph and glazing.
High humidity can also result in mold growth that damages the image
(Fig. 11). Mold is also a health hazard. If mold growth is not too
severe, a professional conservator can remove it. The dyes used in some
modern digital photos have a tendency to bleed at high humidity.
Details in the image will blur, making the photo look out of focus (Fig.
12); the colors can shift as well.
Damage from Poor-Quality Framing Materials
The materials used to frame a photograph can also cause damage, if
they are of poor quality. Usually, the worst culprits inside the frame
are the paper materials that make up the mat board, the window mat, the
filler board, or the backing paper. These can fade, mar, or yellow the
photo.
(2) Wood frames and some adhesives can also cause damage.
The reactions caused by these materials, like those we see from air
pollution, are often oxidation reactions that result in image facing.
In a black-and-white image the faded silver can migrate to the surface
of the print and be converted back into metallic silver by other
pollutants (either from the framing materials or the air), forming a
mirror-like sheen on the print's surface. In the case of the photo on
the left (Fig. 13), a poor-quality mat covered the outside edges of the
print and caused the silver mirroring. The oval shape in the center of
the print is where the mat did not cover the print. Often this form of
damage is caused by papers that contain lignin. Lignin is a natural
substance in wood that, if not removed during pulping, can be very
hazardous to photos. Not only can it cause fading or mirroring like the
example shown here, but it can also cause severe yellowing of photos.
It is not always the image that is damaged; sometimes it is the paper
support. Acidic mat boards, filler boards, and frames can "burn"
paper, causing it to turn brown and become brittle (Fig. 14). A
photograph with this type of damage must be handled very carefully to
void breakage. All of these problems can be prevented by selecting
good-quality framing materials to begin with.
Self-Destructive Photographs
Some photographic materials themselves can give off harmful chemical
gases that turn around and cause further damage to the photo. This is
called autocatalysis, because the photo catalyzes its own
decay. Any photo printed on a poor-quality paper or unstable plastic
can deteriorate and release chemicals that then accelerate decay.
Confounding Combinations
Not only to the different forces of decay described so far occur
independently, they also exacerbate each other and make the damage even
worse. The framed photograph and detail in Fig. 15 show what has often
been called the "picture-frame effect." This black-and-white photo was
framed and displayed on a wall in an office. UV energy entered the
frame and reacted with one of the layers of the photo creating a
pollutant that then faded the image. The faded silver was then
converted into an orange substance in the dark areas of the print by yet
another pollutant that came from either the photo itself or the framing
materials. In order to occur, the effect needed UV energy, an unstable
print, the pollutants, and the closed environment of the frame.
Removal of any one of those factors could have prevented the damage we
see.
Thus an important point to remember regarding the ways in which
photos are harmed when framed and on display is that the damage to the
photograph isn't always caused by light or UV energy. Damage can also
be due to the framing materials, moisture in the air, pollutants from a
variety of sources, and sometimes all of the above.
