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NCPTT staff is currently undertaking phase two research activities that focus on understanding physical and chemical changes to the stone. These efforts include laboratory studies involving accelerated weathering and comparison of accelerated results will field experiments that have been undertaken simultaneously.

5.1. Laboratory Studies

Laboratory studies in phase two consist of accelerated weathering studies at NCPTT laboratories and analytical evaluation of the laboratory samples and field test samples placed in the five chosen cemetery sites.

5.1.1. Accelerated Weathering Studies
Figure 10. Georgette Lang cores a sample of Colorado Yule for use in the accelerated weathering study.
Figure 10. Georgette Lang cores a sample of Colorado Yule for use in the accelerated weathering study.

In June of 2006 NCPTT began the first of two accelerated weathering studies. The purpose of these studies was to simulate the long term use and exposure of the five selected cleaners on two types of marble. Newly quarried Colorado Yule marble and Cherokee White Georgia marble were obtained from the NCA contracted quarries. ìNewî marble was selected for these studies so that any residual chemicals found on the stone after the accelerated study could be attributed to the cleaner used and not to any prior treatments on the marble. By doing a laboratory accelerated weathering experiment; factors could be controlled such as humidity and light and dark exposures. Thus the samples and cleaners were compared under the same controlled conditions.

All accelerated weathering studies used a Q- Panel Lab Products model QUV/ Spray Accelerated Weather Tester (weatherometer). This instrument uses panels of UVA-340 lamps to control a programmable cycle of light and dark. The bulbs irradiance level is calibrated to a constant level of 0.77 W/m2.

Figure 11. Marble samples being removed from the QUV weatherometer during a dark cycle to be treated with cleaner.
Figure 11. Marble samples being removed from the QUV weatherometer during a dark cycle to be treated with cleaner.

For both accelerated studies, the Weatherometer was programmed for a continuing cycle of UV exposure for 4 hours at 60 degrees C followed by 4 hours of condensation at 50 degrees C. Note that this step was in the dark (no UV light) to mimic the natural cycle of night and day, and the temperature drop encouraged condensation from the surrounding humid air inside the Weatherometer. The water that condensed inside the Weatherometer initially comes from a lower holding pan that was supplied from a filtered water system that generated 18 megohm-cm purity of water. These cycles repeat for a total of 800 hours.

Marble samples were prepared in the same manner for both accelerated weathering studies. Newly quarried marble was placed on a drill press and cored with a water jacketed diamond coring bit to a diameter of 1 5/8 inches. Then cores were sliced with a water cooled MK tile saw to a uniformed thickness of 1/2 inch. Once all of the samples were cut to size, they were placed on a Buhler Ecomet 4 fitted with an Automet 3 rotating head and polished to remove any remaining saw marks. The Colorado samples were polished for 5 minutes at 30 rpm with 7 lbs. of force using a 120 grit sanding disk. The Georgia samples were polished for 5 minutes at 30 rpm with 12 lbs. of force using 120 grit. These steps were then repeated using 220 grit paper.

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National Center for Preservation Technology and Training
645 University Parkway
Natchitoches, LA 71457

Email: ncptt[at]nps.gov
Phone: (318) 356-7444
Fax: (318) 356-9119