| Recommendations for Controlling Light
This study reveals that certain wood finishes
and historic materials may be damaged by visible light as well as by
ultraviolet radiation.
As the study demonstrates, oil stains hold up better than some other
finishes traditionally used on historic material, but none is immune
to the destructive effects of light. It is imperative, therefore, to
control light --all forms of it-- as much as possible.
Ultraviolet Radiation - The destructive nature of ultraviolet light
can be controlled to a large extent through the use of special filters.
With many collections housed in historic buildings, it is fortunate
that UV filters have only a minor effect on light which is seen and
thus alter little of the coloration and reflective qualities of the
window glass.
There are different options for installing ultraviolet filters. A common
technique is the application of filtering films to the existing glass.
This approach has been used for many years, and indications are that
such filters can have a service life of 10 or more years, depending
upon the materials and conditions involved. Most of the problems with
recent film applications have occurred where the film has not been
correctly installed, resulting in "bubbling" or other forms
of adhesion failure. Experienced personnel must be used for installations.
Removal of the film is very time-consuming and costly, since experienced
people must be used to avoid scratching to the glass and gouging to
the wood. Once installed, special care must be taken in the routine
cleaning of the glass.
When investigating the options for controlling ultraviolet light, particularly
when dealing with historic window sash and old glass, two important
factors should be considered. When in the future the film requires
replacement, there is the distinct possibility of damage to the historic
glass and even the sash. And second, highly trained personnel must
be employed in the preparations, application and future removal of
the film, whether or not historic sash and glass is involved.
There are alternatives to the applications of filtering films. One
is the addition of specially treated glass or plastic glazing panels,
preferably to the room side, which can also serve as a storm panel,
help lower dirt infiltration and reduce some visible sunlight. With
this approach, a careful assessment would need to be made beforehand
of the following: (1) the visual impact on the historic character of
the building; (2) the best way to install the panel with minimal damage
to the historic woodwork; and (3) the potential for moisture entrapment
between the sash and panel. Installation of a test panel over the full
seasonal cycle is recommended.
Even in rooms with no windows, there can be ultraviolet light from
artificial sources, particularly fluorescent lights. Filtering sleeves
can be installed over fluorescent tubes and lights are available with
a built-in filter. Try to lower ultraviolet light levels to no more
than 75 microwatts per lumen (see figure 3).
Visible Light - Besides the damage caused by ultraviolet light, this
study suggests that visible light through windows can damage certain
historic furnishings as well (see figure 3). With non-historic buildings,
the use of specially tinted window film or glass effectively reduces
light infiltration and therefore is often recommended as a way to help
protect the historic furnishings.
When dealing with historic buildings, a number of additional factors
must be considered in any plan to reduce light damage to historic materials.
The use of visible light filters at windows in historic buildings can
change the historic character of both individual windows and the overall
building. Specially treated glass, films, and acrylic or polycarbonate
sheets for control of visible light all have a distinct color because
of the limited ways that visible light can be filtered.
Green, gray and bronze are the most common colors of the various filters.
There are many factors which help determine the visual impact of visible
light filters on the historic appearance of the buildings, such as
the color of the building, the depth of the window reveal, the artificial
lighting within the building, the color of window blinds or draperies,
and the visibility of the windows. In some cases, the use of a light
non-reflective gray tint on secondary elevations may be acceptable,
although not acceptable on the primary facades.
National Park Service Guidelines for Preservation and Protection
of Museum Objects
Light (visible): The maximum acceptable illuminance level for light-sensitive
materials is as follows:
50 lux (5 footcandles) for especially light-sensitive materials,
e.g., dyed and treated organic material, textiles, watercolors, tapestries,
prints and drawings, manuscripts, leather, wallpapers, natural history
specimens including botanical specimens, fur and feathers.
200 lux (20 footcandles) for undyed and untreated organic materials,
oil and tempera painting, and finished wooden surfaces.
Generally, other materials are less light-sensitive and may be exposed
to higher levels up to a maximum of 300 lux. However, when these
materials are exhibited with light-sensitive materials, light levels
must be controlled at the levels acceptable for the most sensitive
materials.
Except for short durations required for access or housekeeping, no
light is acceptable for museum objects in storage.
Light (UV radiation): All forms of lighting (e.g., daylight, fluorescent
lamps, tungsten [incandescent] and tungsten-halogen lamps) used in
museums emit varying levels of UV radiation. Monitor all light sources
of UV radiation and record levels. If the UV radiation level exceeds
75 microwatts/lumen, it is mandatory to control it by installing
filtering material (e.g., plastic solar control film for windows,
UV filtering film or sheet for windows or picture frames, and filter
sleeves for fluorescent tubes) between the light source and museum
objects. Periodically monitor UV radiation to ensure that filtering
material is effective.
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| Figure 3.
National Park Service "Guidelines
for Preservation and Protection of Museum Objects from Visible
and Ultraviolet
Light." |
As with the installation of films
that filter only ultraviolet light, films that filter both ultraviolet
light and visible light can have
potential long-term
damaging effects on the historic glass and even the sash when it
has to be removed in the future. Unlike ultraviolet light filters,
however,
films that filter visible light may pose an additional hazard to
the historic glass these films cause the glass to absorb additional
radiant
heat from the sun, causing higher levels of expansion and contraction
in the glass. This action can lead to the cracking of some historic
glass, particularly where old hard glazing compound exists and with
typical old glass that had more edge imperfections than found today.
Obviously if the historic glass or sash has been replaced, any potential
physical damage is of much less concern.
The best way to assess the visual impact of this type of filter is
to install a field mock-up and observe it through seasonal changes.
A light non-reflective gray-color film or glazing panel probably will
have the least visible impact, although in many cases it still may
be inappropriate for historic buildings.
A number of other steps can be taken with both historic and non-historic buildings
and furnishings to control visible and ultraviolet light (see figures 4 and
5). Where the application of filtering films or specially treated glass is
determined
not in keeping with the historic character of a building, the following treatments
are particularly important to consider:
1. Install window roller shades and use them at times of direct sunlight penetration
and whenever a room is not in use, including at closing time.
2. Utilize existing shutters and Venetian blinds to control light into a room
throughout the day.
3. Close draperies and curtains during times of direct sunlight and whenever
a room is not in use. Where historic draperies exist, have reproductions made
and use them in place of the historic draperies or curtains.
 |
| Figure 4. A number of steps can be taken to control or reduce the damage
to historic furnishings caused by light entering through windows. In this
house museum, a reproduction table cloth is appropriate for use in protecting
the mid-nineteenth century dining room table. The chair would be better
protected by moving it away from the window. The dark window roller shade
should be drawn in this house museum during periods of direct sunlight
and when the building is closed to the public. Photo: Mike Wiltshire, National
Park Service. |
4. Locate particularly light-sensitive furnishings away
from direct window light. With some historic museum settings, this may mean
that a particular
furnishing
cannot be placed in its exact historic location (see figure 6).
5. Cover historic furnishings with muslin or other material when a room is
not in regular use. Store light-sensitive furnishings darkened facilities.
6. Where awnings are historically appropriate, utilize them to reduce sunlight
from entering the interior.
7. Carefully study the existing landscape and, when appropriate, plant additional
trees and vegetation that may help reduce sunlight.
8. In repairing and restoring historic woodwork, use stains and finishes
that exhibit high color stability and that are compatible
with the historic stain and finish. This test suggests avoiding aniline dyes.
9. Take light readings both for ultraviolet light (microwatts per lumen)
and visible light (lux or foot candle) on a seasonal basis to make sure that
the
levels are within the recommended range for the room or building.
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