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6.1. Appearance

Appearance changes of field trails were documented using photography and colorimetry throughout phase one of the study. Very subtle changes were seen on stones over time from six months to twelve months after cleaning. However these changes were often not noticeable to the viewer. None of the cleaners left obvious changes, such as yellowing, etc., from possible cleaning residues.

Color change trends were examined by determining the frequency of color changes at ΔE greater than 5 and ΔE greater than 10. Trends were evaluated by cemetery, by cleaners, and by sunny or shady locations. In most cases where color change occurred, headstones were darkening.

From frequency trend data associated with cleaners, Kodak Photo-Flo exhibited the greatest number of color changes greater than 5 ΔE and greater than 10 ΔE, and was likely the worst performer of the test cleaners. None of the other cleaners were readily distinguished based on changes in visual appearance.

It is important to note that, while H2Orange cleaner seemed to perform well based on color measurements, significant visual changes were noted over a six month time period. The appearance of biological re-growth or staining was not always captured by color measurements, since changes often occurred at the outer edges of the headstone.

Moreover, after twelve months, the visual changes had disappeared. Despite the fact that this phenomena was observed at only one cemetery, Jefferson Barracks National Cemetery, it was deemed to be an unacceptable short term appearance change. Appearance changes were subtle during the six and twelve month time period. In general, more time is needed to see significant appearance changes to the headstones.

6.2. Biological Re-growth

Determination of biological re-growth in this study has offered some complex problems, from the sheer numbers of samples to be evaluated and enumerated, to how cleaning history of the stones affect the initial biological activity, to the length of time needed for observing visual biological re-growth.

Biological swabs were taken from many headstones and required considerable time and effort to enumerate in the course of this study. Initial estimates of the number of samples to be examined were 7,880 biological counts, taking over 63,000 hours of work to perform! This was an impossible task and in June 2005, we revised the number of samples to 600 swabs. Still the task was daunting and ultimately, fewer samples were evaluated.

All headstones started with a relatively small biofilm of bacteria and fungi at the beginning of the study, with the exception of headstones located in Santa Fe National Cemetery which displayed a larger biofilm. This is likely due to the fact that Santa Fe headstones are not regularly cleaned in the same manner as those located in the other test cemeteries. Importantly, no algaes or photosynthetic bacteria were observed in the samples. According to Dr. Ralph Mitchell,20 it is likely that algaes or photosynthetic bacteria are the greatest source of visual appearance change found on headstones and thus are the most important to enumerate. Fungi are also sources of visual discoloration, but to a lesser extent.

As of November and December 2006, no algae were detected in samples from any of the five cemeteries sampled. Green coloration in some samples was due to the presence of fungi. Fungi and bacteria were enumerated by plating on solid media and counting colonies after incubation. Numbers of bacteria and fungi in samples were variable. The absence of algae or photosynthetic bacteria is significant. These organisms typically provide the most visual evidence of growth on headstones. Their absence, even from the stones treated with water, suggests it is still too early to determine the effectiveness of the biocides.

NCPTT staff attempted to identify performance trends based on the biological activity documented over the course of twelve months. Performance of each cleaner was ranked based on data from swabs. Rankings from June 2006 results appeared to illuminate differences to a greater extent than rankings from February 2007. This is partly due to the fact that there were six cleaners to rank in June 2006 where as there were four cleaners to rank in February 2007. The latter rankings grouped more closely together thus making it more difficult to see significant differences.

Based on the June 2006 rankings, Kodak Photo-Flo was likely the worst performer of the six cleaners evaluated.

6.3. Physical Changes

Evaluation of physical changes is a significant task in phase 2 of the study. Physical changes will be evaluated for field test stones and for accelerated weathering laboratory samples. To date, NCPTT staff has identified methods to be used in evaluating physical changes to the stones. They include changes in appearance by colorimetry, changes in surface texture to be monitored by laser profilometry, and changes in porosity to be examined by mercury porosimetry and Nitrogen BET absorption porosimetry.

Laboratory samples were examined using colorimetry, laser profilometry, and weight measurements prior to any accelerated weathering studies as the baseline data. Field test stones will be compared to control samples kept in pristine conditions in the laboratory. This work is on-going.

6.4. Chemical Changes

As with the evaluation of physical changes, chemical changes caused by cleaners will be evaluated in the laboratory as part of phase 2 of the study. The possible presence of soluble salts will be evaluated using optical microscopy and analysis of total soluble salts using both gravimetric and conductivity methods. The chemical nature of the efflorescence may be studied using X-ray Diffraction analysis. Detection of possible cleaning residues or minor chemical changes may be studied using Electron Microscopy- EDS, and X-ray Fluorescence Spectroscopy.

NCPTT staff has tested its new portable XRF analyzer for identifying chlorides on field test stones with success. The task of identifying chemical changes to the stones continues.

Mitchell, Ralph. Harvard University, Division of Engineering and Applied Sciences, personal communication, March 2007.

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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