Even under natural circumstances such as this rainwater drainage pattern on the veneer of a structure, it is clear that simple water can be an effective technique in removing many instances of atmospheric carbon and soiling.
This wall is an example of how destructive pressurized water can be to a masonry surface, even one as durable as granite. Here the surface has been physically etched by immensely pressurized water shot through a highly focused nozzle at the end of a pressure washer wand.
These cast stone figures were in generally good shape structurally, but the old coatings and paint had failed over the years. Being able to avoid the use of chemical strippers to clean the sculpted surfaces was a great advantage in the choice of using a water-only removal method.
Moderately pressurized water (1,200 psi) applied to the surface effectively removed all traces of the old coatings, while the pressure was low enough to avoid damaging the surface.
This same technique allowed us to remove large areas of old coatings from the building's cast-in-place concrete veneer without the use of chemicals and the attendant disposal and environmental problems.
Here, we are cleaning some brick at the Presidio in San Francisco using a garden hose. No chemicals, no pressure. In this case the original mortar between the bricks was poorly washed beach sand and lime mortar and it may have been the lesser of two evils. Any excessive water pressure would have blown away the original mortar. This was probably the optimum cleaning solution unless the owners were willing to remove and replace the outer 3/4" of all the original pointing mortar. From a strict Historic Preservation point of view, this would be less than optimum.
This is a shot of our basic cleaning test kit: an electric pressure washer (desirable because of low water flow [1 gpm] and pressure [approx. 900 psi]), distilled water (to minimize alkaline/acidic reactions in the masonry substrate), and a sump pump to supply the pressure washer. From a restoration point of view, this equipment is desirable becuase it utilizes a less agressive water washing technique. (Painters and other contractors usually rely on gas-powered machines that generate up to 4,000 psi.)
This is the old Koshman Mansion in Pacific Heights in San Francisco. Upon first viewing the structure, I was stumped at the difference between the stone in the foreground and that of the retaining wall of the main house.
Then it occurred to me that since this was a locally well-known building stone (Colusa Sandstone) it should have a naturally greenish cast. Therefore, the material with a pinkish cast had probably been stained at some point in the past to hide old repairs of grey cement patching mortar. Note the old Portland cement-based repairs in the upper left corner of the facade. (Colusa sandstone is notorious for its tendency to dis-corporate in the presence of moisture.)
A simple wash with moderate water pressure (900 psi) and no chemicals was the least intrusive way of removing approximately 90% of the old stain that had been previously applied to the sandstone surface to hide old repairs.
In this case of a decorative cast stone element in the courtyard of a private home in Berkeley, water only was able to remove the most obvious depositions of carbon and atmospheric soiling.
While most of the soiling has been removed and the surface lightened in value significantly, there are still remaining stains that only a more agressive cleaning method could achieve. But this is the essential element of masonry preservation: getting the most amount of cleaning with the least aggressive method possible (or, as the Secretary of the Interior's Standards proclaim, "the gentlest means possible").
From a short distance, the difference on the dried sample is plainly evident
Even from a greater distance the difference in a cleaned concrete surface is evident. Cleaning the surface of the towers at the Golden Gate Bridge allowed us to then specify the colorant mix for the shotcrete covering the lower areas. Matching the color of the cleaned concrete was an important element of the seismic upgrade project, allowing the new concrete and the cleaned lower areas to weather at a uniform rate.