I cleaned the stones with D/2 Biological Solution, which is a monument cleaner. The stones were clearly much cleaner immediately after the initial cleaning. However, after almost three months since I restored the site, the stones are noticeably brighter and cleaner. I am extremely satisfied with the results of this product. A link to D/2 is http://www.gravestonecleaner.com/welcome-to-cemetery-preservation-supply-llc/
Anyone wanting to clean gravestones should check out this product. It costs $40 per gallon plus shipping but in my opinion it is worth every penny. Below are final pictures of what this product can do.
Above: This photo was taken today, 29 September 2013
Above: This photo was taken in May 2013
Additional photos from another ancestor's grave site clearly show what D/2 Biological Solution can do for cleaning old marble tombstones.
This grave stone is for John Albert, a German immigrant from Baden. He is one of my great-great-great grandfathers. He is also buried in Our Lady Help of Christian's cemetery. Note the black coloring on the stone. This is a combination of lichens and the effects of acid rain on the stone. Around 1980 I cleaned this stone with a wire brush. I do not remember how "dirty" the stone was but I know now that scraping/rubbing the stone with a wire brush is an incorrect method for cleaning it. This stone was photographed in February 2013.
Note how bright and clear this stone is.I cleaned this stone in June 2013. This photo was taken today, 29 September 2013. The cleaning method was to scrub the stone with a soft nylon bristle brush and clean water. This lifted and removed a lot of the loose lichens on the stone. Then I heavily sprayed the stone with D/2 and after 15 minutes I used the same brush and then re-scrubbed the stone. After another 10 minutes I scrubbed the stone with clean water and the cleaning process was complete. The stone was clean but over a period of a few weeks the stone brightened and became cleaner. The chemical continues to work after your work is done.
To clean this stone I spent about 10 minutes of active work to get the job done. Most of the time spent was on waiting for D/2 to do its work. This was a very easy process and very rewarding.