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Making A Federal Case
Thanks to great sealing technology, the papers that define this country are safe again.
Paul Sharke -- Design News, April 10, 2006
Some of the children visiting the National Archives on a recent fall day received a short lesson in freedom denied. A photographer assigned to shoot the revamped Constitution, Declaration of Independence and Bill of Rights exhibit had to keep the eager sightseers out of the frame for the minutes he needed to make an exposure in the dim light. The results of which are pictured here. Once he captured the image, the children were free to run up and stare at the original inked words and ghostly signatures these old skins still retain. The guards needed to remind them only occasionally not to touch the glass.
Light, like air and bugs, is tough on parchment. Thus the soft illumination of the Archives — about three candles — make shooting them a challenge to personal liberty. Thanks to new encasements, the documents are again on display following several year's absence when they were removed from their 1950s-era cases, lovingly examined, and robed in the cloths of technology that will keep them for the rest of this century. The National Institute of Standards and Technology was hired to design and build the new enclosures.
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The technology sought was one of passive design, says NIST Mechanical Engineer, Richard Rhorer, one that wouldn't require constant or frequent purging, an encasement that would protect its contents from age even if it sat unattended — post-apocalypse, say — for a generation. Milled from 600-lb aluminum alloy blocks down to 40 lbs and screwed closed to one-piece titanium frames, the boxes incorporate seals made of metal rather than Viton or polymer which could harden over time. A strip of tin plated Inconel, C-shaped in cross section, forms the primary seal between metal and glass. A length of Indium wire forms a secondary seal, creating a small chamber in which the integrity of the main seal can be verified.
Archivists had specified that the encasements would contain less than a half a percent of oxygen after 100 years. The final design exceeded that requirement by nearly two orders of magnitude, producing a leak rate of 3 x 10-7 standard cc/sec and denying the survival of any parchment-eating microbe.
The original cases, soldered tight, performed remarkably well at keeping the elements at bay. After 50 years, five of the seven boxes contained their original 100 percent helium atmospheres, Rhorer reports. But concerns that an interior relative humidity of 30 percent might be devitrifying, or fogging, the glass made a strong argument for opening them. There was concern, too, about surface crizzling — whereby glass develops small cracks. And, some experts worried that vibration induced as the cases descended nightly to their vault might be wearing away the document surfaces.
Unlike the originals, the new cases can be opened without damaging them, should an inspection of the Charters of Freedom — as the four pages of the Constitution and single pages of the Declaration of Independence and Bill of Rights are collectively known — become necessary or if a new preservation method develops.
The documents themselves rest on sheets of pure cellulose, clipped, in turn, to aluminum platforms. The platforms were each CNC milled to match the uneven edges of the parchments and pierced with 4,000 tiny holes to permit the argon atmosphere inside to circulate. The cellulose acts as humidity control, absorbing or emitting moisture as temperatures fluctuate.
The back of each encasement holds twin instrument bays, one containing a pressure transducer and a humidity sensor and, the other, two valves for evacuating the chamber and purging it with Argon. Valves are bellows type that seal metal to metal and open and close without needing a stem to pass through any opening, according to Frederick, Md. Swagelok Distributor John Bevacqua.
The argon which fills the encasements contains a 2 percent helium trace. Argon, a comparatively large atom, has greater difficulty escaping a sealed space than the much smaller helium atom. Helium is the standard gas for testing any high vacuum system, Bevacqua explains.
The encasements also include two sapphire windows for making absorption spectroscopy measurements. An optical mount inside each box lets conservators keep tabs on the atmospheres within.
The NIST built nine cases in all (five for the Constitution and its single transmittal page, one for the Bill of Rights, one for the Declaration of Independence, and two prototypes that became spares). The prototype underwent extensive testing, most notably, of the seal, to see how the case would hold up to the ravages of a tornado and the sudden dropping of air pressure. The seals did fine, holding tight in the vacuum chamber until the low-reflective glass itself shattered.
| Web Resources | ||
| //Check out the links below for more info// | ||
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For the Nova website, which accompanies a PBS documentary of the project, go to
http://rbi.ims.ca/4919-556 |
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Making A Federal Case
Thanks to great sealing technology, the papers that define this country are safe again.
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