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Headstones that were cleaned in October 2005 were swabbed again in April 2006 to determine biological activity. These results are reported in Appendix E, Analysis of Microorganisms on Headstones in VA Cemeteries, Second Report: June 2006 is summarized here. The work here involved looking at many of the samples, but only a select number of samples were enumerated in this round of analyses.

No algae were detected in samples from any of the five cemeteries. Green coloration in some samples was due to the presence of fungi. In general, the numbers of bacteria were greater than the numbers of fungi found on the stones.

The decreasing order of biological activities was:

Bath > Jefferson Barracks > San Francisco > Alexandria > Santa Fe

Bath had five times more enumerated bacteria than bacteria found in Santa Fe, but both counts are in the same order of magnitude. In Santa Fe, the biological growth of both bacteria and fungi is greater in shady areas. Higher fungal counts were found in Bath and Jefferson Barracks. At Jefferson Barracks, fungal growth was higher in the shade than in sunny areas. Lower biological activity in Santa Fe may be expected because of the drier, hotter climate.

The enumerated biological activity does not fully account for visual changes observed on headstones. This is partly due to the fact that only one stone displaying discoloration, from Jefferson Barracks, was enumerated in the biological activity study. Also, the biological analyses did not attempt to identify specific fungi or bacteria genus present. One hypothesis is that some cleaners are not full spectrum and thus donít fully kill all fungi or bacteria. If most micro-organisms are eradicated, but a few are left behind, then those left behind may grow freely. Resistant fungi that continue to grow may cause discoloration but be limited to a select species.

Further analysis of the biological activity regarding biocidal effectiveness of each cleaner becomes more complex. NCPTT researchers chose to rank the biological activity for each cleaner to determine performance.

3.5.6. Ranking Biocidal Performance of Cleaners

Dr. Tye Botting prepared a ranking of the performance of the cleaners to inhibit biological growth based on the enumerated microbial activities determined from the June report. Botting ranked activities from 1 to 6 with lower numbers indicating higher biological activity found in the count. He ranked the activity by cemetery site, sunny or shady location, bacterial count, fungal count, and cleaner. Once her ranked each cleaner, he averaged the results for overall activity in sunny locations, overall activity in shady locations, and by total growth. He also looked only at bacterial activity and fungal activity. Results are found in Appendix G. Biological Performance Based on June 2006 Report. In this appendix, Botting highlighted those cleaners with average rankings below 3.0. The following observations can be made based on the rankings:

  • Photo-flo was a poor performer at controlling bacteria in both sunny and shady locations at all cemetery sites. It also ranked lowest of all cleaners in limiting biological activity overall.
  • D2 performed well in controlling bacteria and fungi in sunny locations and in controlling bacteria in the shade. Greater activity of fungi was seen on test patches cleaned with D2 in shady locations.
  • D2 and Daybreak performed similarly in controlling overall biological activity.
  • WEG Marble cleaner was a consistently high performer based on this analysis.

3.5.7. Follow-up Biological Activity, January 2007

A second analysis of biological activity focused on headstones that had been previously swabbed in October 2005 and April 2006. Swabs were collected at the designated cemeteries in October/November 2006. Swabs of patches cleaned with D2, Daybreak, WEG Marble Cleaner, and water were analyzed for biological activity, and they are reported in Appendix F.

Trends in biological re-growth were not evident in the evaluation of this data. No consistent differences were found for bacteria or fungi for the remaining cleaners. In general, numbers of bacteria were greater than fungi at all cemeteries. The greatest difference was seen between sunny and shady locations. Greater numbers of bacteria and fungi were found in shady areas. This is most likely due to drier conditions and more intense UV irradiation in sunny locations.

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.

Further ranking of this data, similar to the performance rankings discussed in Section 3.5.6, is shown in Appendix H. In this ranking, the performance of D2, Daybreak, WEG Marble Cleaner, was evaluated based on the numbers of bacteria, fungi, or total re-growth enumerated approximately twelve months after cleaning. Rankings were from one to four, with higher numbers indicating better performance. Ties were allowed. Few trends in biological activity were identified based on this analysis. Cleaners performed about equally in the shady areas. Water was the worst performer in slowing bacterial re-growth in shady areas. WEG Marble Cleaner was a poor performer for slowing bacterial regrowth in sunny locations.

Thus, all indications from the biological activity analysis is that more time is needed to allow significant re-growth to better distinguish between cleaners.

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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