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.
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.
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.