Our recommendations here are from the Indiana Limestone Institute of America (ILI), which says its preferred method for cleaning natural stone like limestone or granite is pressure washing.
Use water from a pressure rig at no pressure higher than 1,200 p.s.i., from a 45 fan-shaped nozzle held no closer than 6″ to the face of the stone.
ILI recommends that architects and their clients decide in advance how clean the building must be, keeping in mind that the dirt is not harmful, and that its complete eradication almost always removes remaining original surface and increases surface area. If 80 to 90 percent clean is the target, the relatively benign high-pressure method is a good solution. Note, however, that even high-pressure water can cause damage. Pressure, nozzle size and working distance are all controlled by the operator and can be abused.
If you or your client is considering a major cleaning of a natural stone surface, ILI recommends inviting operators to clean a sample area using their preferred method. This procedure will reveal much about the operator, the workers and the equipment, the amount of disruption likely in his or her procedure, all in addition to how well that operator’s procedure works, and how much damage is done. Use the cleaned area as the target sample for bidders and as the comparative area for the punch‑list.