3.5.3. Cleaning Headstones

Jason Church began cleaning headstones after (1) documenting their visual appearance using digital photography and colorimetry and (2) sampling their surfaces for biological activity.

A grid was taped onto each stone using one inch wide 3M blue tape. Cleaners were applied to the surface of the headstone following manufacturersí recommendations. An 8.5 x 11 inch acetate sheet was used to insure that cleaner did not run to a second test grid. If the stone had raised or engraved lettering, the acetate was taped to the irregular surface. Cleaners were applied in the same test grid on each stone, as shown in Illustration 1 After each test patch was cleaned, the area was thoroughly rinsed with tap water.

Based on visual observations, all cleaners effectively removed soiling and biological growth from the stone. Water removed soiling and to a much lesser extent staining from micro-organisms. Changes in appearance were recorded by photographs and color measurements. In general, there was natural variability in the results. NCPTT staff evaluated color change and visual appearance between April 2006 and November 2006, representing changes from six months to twelve months after cleaning. Most changes in appearance during this time were subtle.

Changes in color measurements were calculated from CIElab coordinates as !L* (changes in lightness or darkness), Δa^* (changes towards red or green), Δb^* (changes towards blue or yellow), and ΔE (total Color change). These results are reported in Appendix B, Color Measurements on Field Trials.

Further evaluation of the data focused on establishing color change trends by counting the frequency of color changes at ΔE greater than 5, and ΔE greater than 10. Again, any changes of color observed were subtle to the human eye. Researchers looked at frequency trends compared by cemetery, by cleaners, and by sunny or shady location. The results are reported in Appendix C, Color Analyses by Cemetery, Test Patch, and Location. Once the frequency tables were created, photographs of each headstone were carefully examined to determine if measured color changes could be observed in the photographs. Two important points should be noted about the data. First, additional data regarding color changes will be measured in forthcoming field trips, thus data for some headstones continues to be collected. Second, some data from Bath National Cemetery is missing. These analyses exclude data from Bath National Cemetery at this time. By looking at the frequency of ΔE color changes, researchers were able to initially identify headstones that displayed some subtle appearance changes. For example, data from headstones in Jefferson Barracks is shown in Table 6. Table 6. Frequency of color change greater than 5 (in yellow) and color change greater than 10 (in gold) for headstones at Jefferson Barracks National Cemetery, St. Louis, Mo.

By looking at this data, headstones 72 1273 and 72 1370 were identified as displaying color changes from April 2006 to November 2006. The same analyses were undertaken for data found in Appendix C from each cemetery. Based on color measurement changes headstones at Jefferson Barracks displayed the most occurrences of color change in patch 6 (Kodak Photo-Flo). Next, researchers closely inspected photographs of headstones that displayed color changes to see if further visual changes could be noted. For example, upon closer inspection of headstone 72-1273, biological re-growth was noted in patch 4 (H₂Orange Cleaner) along the inset center cross, see Figure 6. Further review of the color measurement data indicated that the stone had darkened slightly (based on negative ΔL^* values for all patches).

On closer inspection of headstone 72-1370 in Jefferson Barracks, scientists noted a mottling appearance that could be seen in most patches, see Figure 7. The headstone is darkening. There may be biological re-growth associated with the veining seen in the stone.

Similar observations can be made at other cemeteries. For example, color change was noted in several headstones in San Francisco, including WS 1032 B shown in Figure 8. Based on color measurements, five of the six test patches have changed color by !E greater that 5. Most of this color change comes from the darkening of the headstone (negative ΔL^* values). Visual observation supports these measurements. Biological growth can be seen on patch 2 (D2 cleaner) patch 4 (H₂Orange cleaner) and patch 5 (WEG marble cleaner). Also, the brown staining seen on the lower portion of this stone is frequently found in the cemetery. Researchers hypothesize that the staining is due to the use of iron fortified fertilizer used by contractors in San Francisco National Cemetery.

Next, frequency data of ΔE color change was analyzed by cleaner, see Appendix C. The frequency of color change is given in Table 7.

From this data, Kodak Photo-flo exhibited the greatest number of color changes both greater than 5 and 10. Based on this frequency analysis, the worst performer was likely Kodak Photo-Flo. Although Photo-flo test patches, location #6 (Illustration 2), were lower on the headstones it is unlikely that these changes were a result of rain water backsplash since water, in adjacent location #3, did not show the same frequency trend.

Finally, frequency data of ΔE color change was analyzed based on the location of the headstone in sunny or shady locations within the cemetery. The frequency analysis of this data is given in Table 8. The frequency of color changes greater than 5 is equal in sunny and shady locations (n = 22), indicating that there is a equal chance of seeing a color change in a sunny location or a shady location on the headstones. However, there is a greater chance of seeing a color change greater than 10 in a shady location than in a sunny location. Fungi and algae tend to grow in shady locations and may lead to greater visual appearance changes.

Some unusual visual observations were made at Jefferson Barracks National Cemetery. It was at this cemetery that Church inadvertently cleaned more stones than were needed for the study. In follow-up visits, biological activity was observed on these headstones as well as headstones included in the study. Eight headstones were cleaned at Jefferson Barracks in October 2005 ñ six headstones were in the shade and two headstones were in the sun. On all shady headstones, the reoccurrence of biological growth was seen predominantly on patches cleaned with H₂Orange Cleaner (#4). The re-growth was sometimes seen on other patches #5 and #6 below the H₂Orange Cleaner. Figure 9 shows examples of biological re-growth on four of the stones. Sample and swabs and biological analysis did not indicate any significant differences between patch 4, cleaned with H₂Orange cleaner, and other test patches. Interestingly, six months after these photographs were taken the dark biological growth had disappeared on all samples! It may be possible that H₂Orange cleaner did not kill all microbes initially, and it took some time for all growth to die. Alternately, the growth seen on these headstones was seasonal and may return again in the future. In any case, the reoccurrence of microbial activity left an undesirable surface appearance for a period of time, thus NCPTT staff recommended the exclusion of H₂Orange cleaner from Phase Two of the study.