Case Closed

Glass
The 3/8-inch-thick glass window on the surface of each encasement has two jobs: to protect the documents inside and to allow visitors the clearest possible view of them. Each case's glass cover is a two-layer, heat-tempered sheet capable of withstanding variations in barometric pressure and temperature, and has a light-reflective coating that eliminates glare from the lighting in the Rotunda of the National Archives, where the Charters are on permanent display.

Platform
A lightweight aluminum platform supports a layer of celluloid paper and each Charters document. The documents are all slightly different in size and none is perfectly square, so each has its own specially machined aluminum platform. The platform is perforated with about 4,000 holes, which provide moisture transfer between the document and the environment inside the case. The documents are held lightly onto the platform with small plastic clips that viewers can see when looking into the encasements.

Base
The builders of the encasements crafted their bases by machining away most of the material from which they're made, starting with blocks of aluminum about 40 inches square, three inches thick, and weighing more than 500 pounds each. The end result looks like a large cake pan. The base's inside surface is anodized in jet black, which gives viewers the impression that the Charters are floating in midair.

Document
Each of the Charters was handwritten with gall ink on parchment. They are extremely fragile, even within their cases. The documents sit on single sheets of archival paper made of pure cellulose. The paper absorbs and releases moisture as necessary, and it creates an opaque background for the semi-translucent documents, which are otherwise difficult to read. The environment around the document is maintained at around 67°F with a humidity level of about 45 percent to prevent the parchment from becoming brittle. The case is filled with humidified argon, an inert gas that precludes photo oxidation, the chief cause of fading.

Optics
An intricate optical system sits beneath the platform on which the document rests. Its purpose is to facilitate diagnostic tests of conditions inside the encasements. When special light waves penetrate the case from one of two diagnostic windows on its side, five mirrors reflect the beam and pass it out of the second window, where a specially calibrated detector measures its wavelength and intensity. These readings carry precise information about the conditions inside the sealed case. Conservators usually monitor oxygen and water levels, but they can use the optical system to run many other tests as well.

Ball and Socket
A ball-and-socket joint positioned between the platform where the document rests and the bottom base of the encasement serves to locate and secure the document platform in place. This joint ensures that the document is completely immobile even during moving.

Seal
Experts developed a special vacuum seal between the encasement's base and front glass to ensure a nearly impervious enclosure for the Charters. The seal is made of a C-shaped piece of nickel and tin that deforms as the glass is pulled tightly against the encasement's base, creating a leak-proof barrier. Conservators' specifications for the ideal environment inside the closed cases called for no more than 0.5 percent oxygen content—even after 100 years. Laboratory tests indicated that the seal will outperform these specifications.

Diagnostic windows
Two small windows made of synthetic sapphire are set into the wall of the case's base. They allow an absorption spectrometer's signal—a beam of light from a cathode lamp—to pass into and out of the encasement beneath the document. Readings from the signal help conservators evaluate whether the humidity and gas content inside is stable. Scientists at the National Institute of Standards and Technology chose synthetic sapphire for the windows' material because it does not filter the infrared wavelengths needed to conduct sensitive readings of the case's interior.

Frame
The titanium picture frame that surrounds each of the seven encasements on display in the Rotunda of Washington's National Archives is plated with a thin layer of gold. The frame was designed to be as light as possible yet provide the strength necessary to hold the glass in place on the base and form an airtight seal. The frame also provides an aesthetic complement to the grand décor of the Rotunda, an important component of the Charters re-encasement project.

Pockets
To reduce the weight of the encasements and allow for easier moving when necessary, waffle-like spaces were machined out of the metal wherever possible, including on the bottom of the base, seen here, and concealed from view between the bolts beneath the case's outer frame.