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IR Imager Helps Preserve the Alamo

Photonics Spectra
Jul 1998
Ruth A. Mendonsa

The Alamo, a historical landmark in San Antonio, Texas, that many consider the birthplace of the state, is showing signs of aging. This comes as no surprise, since the walls were constructed centuries ago. One culprit that is taking its toll on the walls of the Alamo chapel is moisture. Infrared technology is helping to verify evaporation of moisture and assist in stopping its spread.

The Radiance PM infrared camera from Raytheon provided a thermal map of the moisture buildup in the Alamo chapel wall. Bright yellow and white areas show the greatest concentration.

Ford Engineering is a civil engineering and services business that does a lot of work in preventive maintenance, and about half of this work involves infrared technology. When taking on this unique project, the company chose the Radiance PM infrared imager from Raytheon Systems Co. to detect the moisture. The camera's thermal sensitivity of 0.02 °C made it essential for the thermal detection of moisture. Parts of the wall have temperature variations as low as 0.5 °F.

Damaging salt crystals

The walls of the chapel are made of 4-foot-thick limestone and mortar. Over the years moisture has wicked up from the ground, forming salt crystals that cause the limestone to deteriorate. In the past, moisture readings were manually measured at the wall's surface; although noninvasive, this technique is time-consuming.

The Radiance PM camera provided an additional noninvasive technique that can be accomplished in a few hours. Its images provided a thermal map of all the moisture in the wall. This was possible because the walls absorbed 90 to 100 °F temperatures over several days and the moisture retained the heat. When the rest of the wall cooled down in the evening, the moist areas were still warm and appeared as "hot spots" in the infrared image.

Ford used the camera's 50-mm lens mounted on a tripod 50 feet from the structure. Surveying equipment aligned the camera so the 28 images of small portions of the wall would fit into a full picture. A Windows-based report-generating program called AmberTherm compiled the images into a mosaic.

Although the images could not indicate the amount or depth of the moisture, they clearly showed the location. The largest section was identified in an area 1 to 3 feet above the ground, although more was evident 6 to 8 feet above the ground. These results will help determine the effectiveness of moisture barriers placed in the wall within the last 10 years.

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