Suspecting deterioration in the gun loops during a 2011 conditions survey, a restoration consultant suggested using a small diameter drill to probe around the wood to locate soft spots. “My feeling was that might be a little too invasive,” Taylor recalls. “I suggested the X-ray strategy. It cost a little bit more money but it wouldn’t damage the wood or leave unsightly holes all over the perimeter.”
Despite never having used X-ray technology in his 32-year career, Taylor had read about it and understood its validity for unique construction situations, such as locating reinforcing steel in concrete or determining the condition of an old timber structure. Taylor contacted several companies who provide radiographic inspection services. TÜV Rheinland, which has locations worldwide, including near Pittsburgh in Aliquippa, Pa., was chosen for the project.
According to Chris Dugan, radiation safety director for TÜV Rheinland, X-raying a building is very similar to having your bones examined at a hospital. “Instead of using an electromagnetic X-ray machine, like in a hospital, we use actual radioactive materials that pass through the structure and are recorded on a film,” he explains.
The shades of gray that register on the film illustrate whether there are discontinuities in the material. “Anything that is dense and solid will not allow all the radiation to pass; some of the radiation will get absorbed or reflected,” Dugan continues. “The denser the material, the less radiation, which means the film will be a light shade of gray. If there are a bunch of holes or a large hole inside the structure, the radiation basically would pass unimpeded to the film. Any dark shade of gray means the radiation got through a lot faster than it should have.”
Science of Radiography
TÜV Rheinland’s team used Iridium-192 (IR-192), a radioactive isotope, to X-ray the Fort Pitt Block House. IR-192 can produce images for light-density materials, including concrete and up to 3-inch-thick steel. The radioactive material was contained and transported in a camera to the Block House. During an exposure, the camera and an imaging plate are placed on opposite sides of the object to be radiographed. In the Block House’s case, the camera was inside and the imaging plate was placed outside the building. “Once both items are in place, the IR-192 source is remotely positioned to allow for the narrow beam of intense radiation to be directed toward the wood tier and imaging plates,” Dugan explains.
During each exposure, the electromagnetic radiation travels similar to light waves: in straight lines at very high speeds. It passes through the structure and interacts with the imaging plate.
“Each exposure covered approximately 2 feet of tier length with a total of 50-plus exposures required to inspect both wood tiers,” Dugan adds. “Each exposure was probably a couple minutes long, and there’s setup involved, so it was a three-day process onsite.”
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A comprehensive retelling of the history of the Block House has been published by The History Press Inc. Released as part of the upcoming 250th anniversary of the Block House, The Fort Pitt Block House, was written by site curator Emily M. Weaver and is available for purchase at the Block House, at local booksellers and online at historypress.net.
“The story of the Block House is not just a military history, it’s about the growth of a city and the collective power of a determined group of women who went against the standards of their times to stand up and fight for what they believed in – the preservation of the Block House,” said Weaver. “The most fascinating part of writing this book was being able read original documents – like the deed giving the Block House property and Point District land to Major Isaac Craig and his business partner Colonel Stephen Bayard in 1784 and the original letter from Franklin Nicola, offering $25,000 to the Daughters of the American Revolution for the removal of the Block House to Schenley Park (which they obviously did not accept).”
As curator of the Block House since 2011, Weaver has provided tours to tens of thousands of visitors. She is often amazed by the confusion and misunderstandings that continue to exist about the building and its history. “I am glad we now have this resource to help set the record straight,” Weaver said, “about what the Block House is, its importance to the city of Pittsburgh and early American history, and how it has managed to remain standing for the past 250 years.”