My historical archaeology class has recorded a couple of historic cemeteries
in the area and found that by dusting the surface of a gravestone with flour
the lettering was much easier to read.  This method had the advantage of
being effective, inexpensive, and environmentally friendly.  The stones were
automatically cleaned the next time it rained.  We also took digital photos
before applying the flour.  Manipulating the images (e.g. adjusting the
brightness and contrast) in Photoshop was helpful in making the inscriptions
legible.

Charles R. Ewen, PhD
Professor, Department of Anthropology
Director, Phelps Archaeology Laboratory
A-215 Brewster Building
East Carolina University
Greenville, NC 27858-4353