A team from the NASA Langley Research Center in Hampton, Va., has put atmospheric savvy to work protecting a trio of documents known in the United States as the Charters of Freedom—the Declaration of Independence, the Constitution, and the Bill of Rights. The documents are handwritten with iron-gall ink on animal skin parchment and, since 1951, have been housed in airtight enclosures to preserve them from natural deterioration.

The National Bureau of Standards, predecessor of the National Institute of Standards and Technology, had responsibility for the project, and two stipulations. The enclosures had to contain an inert gas, because atmospheric gases, including oxygen, could prove destructive. Moisture had to be controlled because too much of it could support microorganisms that live in anaerobic, high-moisture environments, and too little could leave the documents brittle and in danger of cracking.

The solution was a mixture of helium and water vapor at a relative humidity of 25 to 35 percent at room temperature. The documents remained in their protective cocoons for nearly 50 years.

In 1998, a NASA Langley senior research scientist, Joel S. Levine, received a call from Margaret Kelly, a chemist at the National Archives and Records Administration, which is entrusted with the documents. One of Levine's interests is the evolution of the atmosphere over time. Levine said archivists had noticed that tiny white specks had begun to appear on the inside surface of the glass enclosing the documents.

Experts from the original glass manufacturer, Libby-Owens-Ford, and from museums were consulted.

In July 1998, Levine assembled three teams at NASA Langley to determine the chemical composition of the atmosphere inside the cases. Two teams used non-invasive techniques to study the atmosphere through the glass. A third team analyzed the atmospheric samples extracted from the cases.

The researchers used non-invasive laser spectroscopy to determine the helium content. Readings from the cases were compared to sample mixtures of helium and water vapor produced at a Langley lab. Analysis on two of the documents indicated a match with the laboratory measurements for 100 percent helium—with a 2 percent margin of error. The finding disproved one theory—that helium had leaked through the glass.

National Archives contacted Levine again, asking him to test the level of humidity in the enclosures.

NASA Langley scientists use a non-invasive spectroscopy technique to determine the relative humidity in the case holding the U.S. Constitution.

The Langley researchers suggested that the cases be chilled until droplets formed on the inside surface of the glass. The temperature of condensation would indicate the level of humidity. But National Archives was concerned that condensation could smear the ink.

Two Langley technicians, Cecil G. Burkett and James W. West, came up with an acceptable alternative. They would chill only a portion of the encasement, away from its document. They placed thermal coolers on the glass, and shut them off when droplets began to form.

They found relative humidity levels of 55 to 65 percent—roughly twice as high as originally thought. The findings were verified by a dew-point hygrometer analysis performed on samples extracted from the cases. Experts from the glass manufacturer concluded that the tiny white flakes were alkaline compounds that had leached from the glass in the humid and oxygen-deficient atmosphere.

A paper chemist at the University of Iowa, Tim Barrett, fingered another suspect: the backing paper in each of the cases. Washington, D.C., is a humid place. Although the parchment was enclosed in a controlled environment, the backing paper enclosed with the parchment had absorbed humidity from the air, Levine said. Over the years, the outgassing water vapor raised the enclosed humidity and led to the formation of the alkaline crystals.

National Archives is currently transferring the Charters of Freedom to new, state-of-the-art enclosures made of aluminum, titanium, and glass. They will be filled with argon, a bigger molecule than helium, which will be less likely to leak out. The University of Iowa's Center for the Book developed special backing paper that will help stabilize the humidity in the cases. Libby-Owens-Ford, now part of Pilkington plc, is providing the glazing.

The Charters of Freedom should be on display in the new cases at the National Archives Rotunda in September.

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