Timeless Messages of Liberty and Freedom

For almost fifty years, the Declaration, the Bill of Rights, and pages 1 and 4 of the Constitution had been on continuous display, sealed in glass and metal encasements filled with the inert gas helium, with an additional loose sheet of glass resting directly on the parchment. The encasement glass was beginning to show evidence of deterioration, which would eventually affect the visibility of the documents. This glass deterioration was a serious concern because it also indicated that the environment within the encasement was more humid than intended.

The design of the encasements did not permit easy access to the documents, since the encasements could not be opened and resealed. Advances in techniques for mounting parchment for exhibition and a greater understanding of materials also argued for removing the Charters of Freedom from the encasements that had protected them for fifty years. All of these conservation and scientific factors led to the decision to re-encase the Charters.

A multiyear collaborative project resulted, involving National Archives staff as well as scientists and technical experts from the National Institute of Standards and Technology (NIST) and other agencies and organizations. Now at the completion of the project, the Charters of Freedom are protected in new state-of-the-art encasements. But this story begins with the first critical moment of opening an encasement to reveal a historic sheet of parchment that had been sealed in inert gas for fifty years.

Science in Support of History

Opening an encasement was a step in a process that began more than twenty years ago with questions raised about the safety and stability of the Charters' encasements.

In 1982 the National Archives invited a panel of respected scientists and preservation professionals to assess the preservation needs of the Charters of Freedom. They advised comparing images of the Charters made at intervals over time to look for changes that might raise concerns.

The National Archives turned to the Imaging Processing Lab at the Jet Propulsion Laboratory (JPL) to assist in this effort. Borrowing technology from America's space program, the JPL developed an imaging system like that used in space exploration. The resulting Charters Monitoring System (CMS) created digital image files by scanning one-inch squares on each document.

During imaging, the encased document lay on a tabletop with legs that floated on nitrogen in cylinders, which acted as shock absorbers to eliminate vibration. An overhead charged-couple device "camera" captured the relative brightness of 1,024 lines of 1,024 pixels in each patch through glass layers, using precise positioning to allow return to the exact spot in future scans.

The National Archives received the Charters Monitoring System in 1987. Conservation staff made baseline measurements for patches on the pages of the Charters. In following years, patches were re-scanned and compared pixel by pixel to the baseline image, looking for physical changes.

In 1996, after more than 125 scans, staff reported the findings. The CMS did not reveal feared changes in ink intensity or loss of ink. In all the scans on the seven encased documents, just one insecure flake of ink was noted on a raised ridge of parchment on the Transmittal Page of the Constitution.

But if the ink of 1787 was holding its own, the encasements of 1951 were not.

The CMS space-age technology ultimately confirmed findings made in 1987 with the microscope: minute crystals and microdroplets of liquid were found on surfaces of the two glass sheets over each document. The scans confirmed that these changes in the glass progressed between 1987 and 1995. Conservators using a binocular microscope could see crystals and liquid droplets on the glass surfaces. These signs of glass deterioration were a clue to the relative humidity inside the encasements. Glass deteriorates at a relative humidity greater than 40 percent. But the encasement helium had been carefully humidified to 30 percent. This low humidity was intended to minimize parchment hydrolysis, a chemical term that means "water cutting." The CMS scans confirmed evidence of progressive glass deterioration, which was a major impetus in deciding to re-encase the Charters of Freedom.