Liberty Bell Passes Stress Test
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How do you move a wounded, 2,080-pound patriot? Very carefully. Recently, under the watchful eyes of curators, conservators, surveyors, and engineers, a team of riggers deftly lifted the fragile Liberty Bell off of the supports on which it has been resting for a quarter century and confirmed the Bell can be safely moved into its new home this fall.
Recognizable across the globe with its famous crack, the 250-year-old bell is remarkably frail for its size. The metal is far from pure. Rather, it is a mixture of various metals, voids and contaminants, intermingled "like a piece of fruitcake," said Karie Diethorn, Supervisory Museum Curator at Independence National Historical Park in Philadelphia. The park is operated by the National Park Service.
Of particular concern is a hairline fracture that extends from the main crack to the rear of the Bell--if the hairline fracture splits, so does the icon.
"In terms of assessing its fragility, we really don't know, so our plan is to subject it to the least amount of stress possible," said Diethorn.
The Liberty Bell has traveled several times, even across the country, before settling in the 1976 Bicentennial facility where it now rests. The latest move will bring the Bell closer to Independence Hall, its original home.
Although the Bell will move only 200 yards into the new museum, curators are taking every precaution to assure its safety. Steve Arms, president of MicroStrain, Inc. tracked movements of the metal along the crack using tiny, wireless sensors he developed as part of the NSF Small Business Innovation Research program. The devices are extremely sensitive, able to detect motion as small as 1/100th the width of a human hair.
The researchers used custom attachments to place two metal sensors, originally developed for the semiconductor industry, on the metal around the Bell's main crack. One sensor monitored stresses that could widen the crack, another monitored stresses from shearing motions. Stresses along the main crack will warn of dangers that could spread to the hairline fracture. The team also hung a third sensor inside the Bell to monitor potentially jarring rocking motions.
The Bell surface is extremely sensitive to both scratching and chemical damage, so the team could not use glues or directly touch metal to metal. Ultimately, Andrew Lins, chief conservator of the Philadelphia Museum of Art, worked with MicroStrain's Steven Mundell to carefully clamp the devices to the Bell, inserting a special paper where the sensors would otherwise touch, and potentially mar, the surface.
Fortunately, Arms's sensors detected no motions greater than several millionths of a meter, tiny movements that do not seem to stress the Bell.
"We can use the data that we collected to create upper and lower limits for vibration, and sound an alarm during the move to warn the riggers if the limits are exceeded," said Arms. "That will allow us to move the bell to the new location and basically mimic the conditions that we got here today, which we feel are safe." he added.
Because of the nature of the move, all of the technology had to be small and wireless. Despite these restrictions, the devices are capable of continuously streaming enormous amounts of data to a laptop that Arms monitored at the site. However, the proprietary software is web compatible, so all of the monitoring could theoretically have been done from anywhere in the world.
MicroStrain's work with the Liberty Bell, and all of the stress sensors and equipment, were provided pro bono. "I just thought this would be a fun project to take on, and a challenging one," said Arms, "we wanted to try to protect a national treasure."
The Liberty Bell was forged in 1752 at Whitechapel Bell Foundry in England--the same foundry that forged Big Ben (the 13-ton and, ironically, cracked bell within the Great Clock of Westminster) and the bells of Washington National Cathedral.
According to historians, the Pennsylvania Assembly probably ordered the Bell in 1751 to commemorate the 50th Anniversary of Pennsylvania's Charter of Privileges, religious and political freedoms that the state had enjoyed since its founding. The Assembly asked for the biblical inscription: Proclaim Liberty throughout all the Land Unto all the Inhabitants thereof - Leviticus 25:10.
The Bell was to hang in the clock tower of the Pennsylvania State House. The State House was later renamed Independence Hall, and the Bell--once known as the State House bell--was renamed "Liberty Bell" by abolitionists who adopted it as their symbol in the 1800s.
The Bell cracked soon after its arrival in Philadelphia and was recast (from the original metal) by local craftsmen John Pass and John Stow in 1753. Even that casting had problems, and the Bell that now rests in the display hall is the third casting.
Over the next century of continual use, a crack had begun to form that had to be filed down to prevent a jarring noise when the Bell was struck (the filing marks are still apparent today). In February, 1846, the Bell was repaired and rung in commemoration of George Washington's birthday. The repair is visible today as a wide jagged crack spanned in two places by rivets. While it once rang the pitch of E-flat, the Bell has not pealed since 1846.
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