Origins
of the Charters of Freedom Project
Since 1951, the
Charters of Freedom have been safely contained in helium-filled, glass
and metal cases built by NIST (then known as the National Bureau of
Standards). For nearly as long, the Declaration of Independence, the
Bill of Rights, and pages one and four of the Constitution have been
on public display, attracting more than a million visitors annually
to the Rotunda of the National Archives in Washington, D.C. From July
4, 2001 to Sept. 17, 2003, the Rotunda underwent extensive renovations,
which now allow all six pages of the Charters to be on display.
An examination
of the Bill of Rights and the Declaration of Independence in 1995
revealed signs of deterioration in the encasements' glass, although
the precious documents were not in danger. Corrective improvements
were deemed impractical because the cases are soldered shut and cannot
be opened without compromising the seal. Subsequently, NARA concluded
that new technology could enhance preservation of the founding documents.
At the same time, NARA officials decided that new encasements should
be designed to enable the flexibility needed to accommodate future
advances in preservation methods.
In addition to
building new encasements, NIST experts designed, built, and operate
the extraction system that was used to remove the gases inside the
former encasements without disturbing the documents—an intricate
procedure that took several days of preparation. With collaborators
from NASA, they analyzed the gases, which provided essential data
on conditions and reactions inside the old encasements. Since parchment
is made from animal skin, a key question is whether so-called out-gassing
by the biological material would alter the chemical environment inside
the encasements.
![pressure expert](picture-9.jpg)
NIST
pressure expert Charles Tilford (above) who designed the system
for extracting gases from the old encasements monitors the removal
of gas from the one that held the George Washington-signed transmittal
letter for the Constitution (right).
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![Constitution](picture-6.jpg)
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![NARA conservator works on document](pro1.jpg)
NARA
conservator applies minute amounts of adhesive to consolidate
potentially loose flakes of ink on one of the Charters of Freedom
documents. (Photo by Earl McDonald, NARA)
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By
Design: Secure, Functional, Attractive . . .
Preserving and
protecting. Though these were the primary goals driving NIST's contributions
to the Charters of Freedom Re-encasement Project, the customized design
and manufacture of the new cases also responded to other key considerations.
A major challenge was to satisfy NARA exhibit designers' aesthetic
and accessibility requirements while meeting or exceeding demanding
technical-performance criteria.
- Aesthetics:
The documents are encased in a gold-plated titanium frame, a visual
complement to the bronze featured in the Rotunda itself. Inside,
the parchment rests on pure cellulose paper made by the University
of Iowa for this project. Parchment and paper are set up on top
of an anodized aluminum support platform that is machined to conform
precisely to the irregular shape of the document. The jet-black
interior creates the impression that the document is floating. Beneath
the platform, also invisible to the viewer, sits a NIST-integrated
instrument system that monitors conditions inside.
- Flexibility,
ease of maintenance: Titanium frame and aluminum-alloy base
are bolted together, permitting conservators to open the leak-free
encasements if a problem arises or if an advance in preservation
technology warrants. The previous 1950s encasements were soldered
shut.
- Ease
of viewing: The
encasements are in a retraction system inside secure display cases.
The documents were positioned at an angle that make it easy for
adults and children of nearly every height and for people with disabilities
to view the documents.
- Reasonable
weight: The base is made out of light-weight aluminum. For
additional weight savings, NIST machined rectangular pockets into
the base, titanium frame, and the aluminum platform that holds the
document. The resulting waffle-like structure maintains the desired
stiffness of the materials, but makes it easier for National Archives
staff to move the cases, if necessary.
Cut-away drawing
showing major structural features of the new encasements
![Drawing showing structural features of new encasements](Doch.jpg)
Illustration
by Jeffrey Aarons
Piece-by-Piece:
Specifications and Components
NIST built a total
of nine encasements.
Dimensions:
Seven encasements (including one spare) were sized for the Constitution.
These hold the four pages of the Constitution, its transmittal page,
and the Declaration of Independence. Outer dimensions of the titanium
frame are 997 millimeters (39 1/4 inches) by 854 millimeters (33 5/8
inches). Of the two wider encasements NIST made, one contains the
Bill of Rights and the other is a back-up. Outer dimensions are 997
millimeters (39 1/4 inches) by 962 millimeters (37 7/8 inches).
Components:
Base:
Cut from a solid aluminum-alloy block that is 75 millimeters thick
(3 inches), the base-in-the-making underwent further machining to
create beveled edges, a series of weight-saving rectangular pockets
with sides that are only 1.5 mm thick (0.060 inch) thick, and threaded
holes for fasteners. Into the top of the base, two channels, or grooves,
were machined along the entire perimeter; these hold high-quality
seals of the type normally used for ultra-high vacuum applications.
In addition, two window ports were milled into one side and two "instrument
bays" machined into the bottom. The bays provide external access
to valves (for filling the encasement with humidified argon) and sensors
(pressure, humidity, and temperature gauges) used to monitor conditions
inside the encasement.
Two sapphire windows
are mounted in the side ports. These allow light to be directed onto
special mirrors mounted in an optics bench sealed inside the encasement
below the document platform. This optical system can be used to monitor
the internal environment without opening the encasement.
The outside of
the base and all seal surfaces are plated with a thin layer of nickel
and then anodized, resulting in a jet-black finish.
NIST
staff members clean the solid aluminum base. The two large rectangular
pockets are ports that will hold valves or instruments to check
conditions inside the encasements. The smaller waffle-like pockets
reduce the weight of the base without compromising strength
or stiffness.
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![A technician affixes indium wire seal](P000343.jpg)
At
NARA's College Park, Md., facility, a technician affixes the
secondary indium-wire seal to outer channel on the perimeter
of the base. The interior of the base has been anodized to create
a jet-black surface.
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Tin-plated
Inconel seal: Into the base's inside channel, NIST specialists
inserted this unusually stiff, pre-formed seal that resembles a "C"
in cross section. Inconel is a hard nickel alloy steel, but the thin
tin coating is soft.
The tin yields—or
"gives"—to pressure when the titanium frame is fastened
to the base, enabling it to fill any minute surface imperfections
that might remain. In contrast, the super-hard inconel acts as a spring
that can block distortions that might occur over time. The result
is a "leak proof" barrier that seals in the encasement's
argon-filled interior and prevents entry of air molecules.
The seal is designed
to be nearly impervious. As specified, the environment inside an encasement
should contain no more than 0.5 percent oxygen after 100 years. Tests
at NIST indicate that the seal is better than the requirement; it
will take more than a century before oxygen accumulates to this minute
level, which provides a wide margin of safety. No known microorganism
attacks parchment when the oxygen level is less than 2 percent.
A NIST engineer
inserts the primary seal into the inside channel machined into the
perimeter of the base. Shaped like a "C" in cross-section,
the seal is made of tin-plated inconel. (Photo by Earl McDonald, NARA)
Indium wire
seal: The outside groove is lined with a thin wire of indium,
a pliable metal that serves as a secondary seal. Its primary purpose,
however, is to a create a narrow chamber adjacent to the inconel seal.
This thin space allowed NIST to conduct tests to make certain that
the primary seal is leak free.
Frame:
Cut as a single piece to avoid irregularities or problems that can
occur when joining sections, the frame is made of commercially pure
titanium, which started out as solid square that measures 1,000 mm
(40 inches) on a side and is 50 mm (2 inches) thick. Threaded bolt
holes are tapped. For aesthetics, the edges of the frame are beveled.
The surface is plated with nickel, textured for appearance, and then
coated with a 1-micrometer-thick layer of 24-karat gold.
![Close up of the titanium](P000603.jpg)
Close-up
of the titanium shows bolt holes and pockets cut into the frame
to reduce weight. The frame is cut from a solid block of commercially
pure titanium.
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![Frame awaits placement](P000665.jpg)
Frame
awaits placement of the glass.
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Document
platform: The anodized, high-grade aluminum platform is designed
to support a parchment page inconspicuously and to allow even distribution
of argon and moisture around the document. More than 100 rectangular
pockets—most 50 mm (2 inches) by 65 mm (2.55 inches)—are
milled into the bottom. Each pocket is perforated with very small
holes—about 4,000 in all. Conservators from the National Archives
traced the perimeter of each irregularly shaped parchment page on
a sheet of mylar. NIST used a coordinate measuring machine to map
coordinates along the outline. The data were used to program the milling
machine that cut each platform to conform to the unique shape of the
page that it supports. Held in place by clear polyester clips, parchment
rest directly on pure cellulose paper, made at the University of Iowa.
The paper sits atop the platform and provides an opaque background.
It also serves as a buffer, absorbing or releasing moisture should
temperature or humidity conditions change. The platform itself snaps
into three support posts.
![Underside of the anodized document platform shows holes](P000585.jpg)
The underside
of the anodized document platform shows holes drilled into milled
pockets to promote even distribution of argon and moisture around
the parchment documents.
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![Fit of the document is tested](P000598.jpg)
The quality
of fit of the document platform inside the encasement is tested.
Note the irregular edge of the platform, which has been milled
to conform to the "wavy" perimeter of the parchment.
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Optics bench:
Acting like a sentinel, a NIST-designed optical system was snapped
into the base of the encasement, beneath the platform. There, it stands
ready to detect incursions of oxygen and water. A device called an
absorption spectrometer is positioned at a pair of side-by-side sapphire
windows measuring about 13 mm (0.5 inch) in diameter. Light from a
cathode lamp enters the encasement through one of the circular windows.
It strikes the first of three NIST-made mirrors and then is reflected
by the two others before exiting the second sapphire window, where
a detector is situated to record the wavelength and intensity of the
arriving light. Conservators can tune the spectrometer to detect substances
in addition to oxygen and water. The system is designed for easy upgrades.
![Two sapphire windows](P000278.jpg)
Two sapphire windows
installed in the base will permit an absorption spectrometer to search
the encasement interior for undesirable chemicals--initially oxygen--and
to ascertain the moisture level. Sapphire does not filter the infrared
wavelengths needed to identify the substances.
Glass:
With the Charters of Freedom now back on display in the rotunda of
the National Archive, visitors may view the documents through laminated,
tempered glass with an anti-reflective coating.
![Glass placed on the titanium frame](P000670.jpg)
Glass
placed on the titanium frame will soon be bolted down to seat the
seals. The blue protective film is removed during the later stages
of assembly.
Bolts:
Stainless steel bolts—a total of 70 each for Bill of Rights-sized
encasements and 66 for the seven others—were used to fasten
frame and base. They supply the pressure necessary to "squeeze"
the laminated tempered glass and to seat the tin-plated inconel seal,
creating the impervious barrier.
Encasement
environment: Following assembly and placement of a parchment
page, the encasement was purged of air. Through a valve in the base,
the interior was filled with humidified argon gas containing a trace
of helium (2 percent) and then sealed. Argon, an inert gas, replaced
the helium used in the old encasements. Argon atoms are larger than
helium atoms, making them less likely to diffuse out of the encasement.
(The small amount of helium included with the argon is used in the
final check of the main seal.) The relative humidity inside the encasement
is 40 percent, preventing the parchment from becoming brittle.
Testing
and Assembly
NIST built two prototype encasements, which were tested extensively
to evaluate the design and to analyze the performance and integrity
of each component. The prototypes now serve as the back-ups that the
National Archives hold in reserve, just in case.
Pressure
and leak testing: NIST experts on pressure testing, leak rates,
and chemical analysis went to great lengths to make certain that the
new encasements met NARA's demanding performance requirements. For
example, they calibrated the pressure sensors and specified the components
of the equipment system that purge the encasements of air and fill
them with humidifed argon. NIST researchers also developed an extensive
leak testing protocol that is used to test the integrity of the primary
seal. This same prescribed procedure may be used by NARA staff to
test the seal periodically after installation of the documents.
Before installation
of the charter pages, all encasements underwent full-system tests.
In all tests, seal performance far exceeded the design requirement.
NIST also conducted
studies to determine whether the encasement glass could withstand
abrupt and large changes in atmospheric pressure. The prototype encasements
were subjected to wide swings in pressure until the glass finally
broke. "There are no known ways," researchers concluded,
"that the encasements would see the level of pressure difference
causing glass failure, except due to a gross error. . ."
![close up of glass](P001271.jpg)
Systematically
exposed to wide swings in pressure, the laminated glass finally gives
way. NIST experts concluded that the glass can withstand a large range
of pressure differences, providing protection well beyond the extremes
it might experience.
Assembly:
As the encasements were completed, they were shipped in special
containers to NARA, where final testing and installation of the Charter
pages was done. Several steps in the process are shown here:
![NARA conservators prepare page for humidty chamber](pro5.jpg)
NARA conservators
prepare to put the transmittal page into a humidity chamber.
The treatment raises the moisture content of the parchment,
permitting it be flattened and dried under restraint to reduce
distortions. (Photo by Earl McDonald, NARA)
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![Preparing to place frame on base of encasement](pro11.jpg)
NIST and
NARA staff members prepare to place the titanium frame on the
base of the encasement.
(Photo by Earl McDonald, NARA)
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![Spectrometer being adjusted](pro12.jpg)
NIST
mechanical engineer Chris Evans adjusts the spectrometer situated
outside two sapphire windows milled into the base. (Photo by
Earl McDonald, NARA)
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![Placing transmittal page into cabinet](pro13.jpg)
NIST
engineering technician Mike McGlauflin and Richard Judson, NARA's
manager of the Charters project, place the newly encased transmittal
page into a cabinet for interim storage.
(Photo by Earl McDonald, NARA)
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