NIST built a total of nine encasements.
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).
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.
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.
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.
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.
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.
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.
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.
|Notice of Online Archive: This page is no longer being updated and remains online for informational and historical purposes only. The information is accurate as of 2001. For questions about page contents, please inquiries [at] nist.gov (contact us).|