Sometimes, despite our best efforts, we are forced to use an aggressive cleaning solution when gentler means prove ineffective. Here a sand (silica)-injection unit has been added to a standard 2,000 psi pressure washer to remove old Portland-cement mortar from a limestone artifact prior to re-installation.
This carved marble lion's head has been smeared with a Portland cement-based mortar in a misguided attempt to make it more resistant to moisture penetration. Silica injection into the water stream of a standard pressure washer is the most effective solution to this problem.
The following three images illustrate why sandblasting has acquired such a heinous reputation. Here you can see a marble veneer stone where the sandblaster removed the original tooling while leaving the returns around a window.
Here the capital of a pilaster has been subjected to sandblasting. Curiously, the right-hand portion of the capital is relatively intact, but the left-hand dutchman (inserted as a repair after the quake and fire of 1906) was cut from a softer piece of stone and thus more susceptible to the abrasive quality of the 'cleaning' process.
Even harder surfaces like this formerly glazed brick and cast stone display the harshness of a misapplied cleaning process. The brick lost its fire-skin during the sandblasting and was then a prime candidate for moisture penetration, so it is now painted on a regular, recurring basis.
Abrasive cleaning is yet another variant in the ongoing conversation of which method to use for the cleaning of historic masonry surfaces. A much-abused method for the last hundred years, it has been unfairly tarred with the brush of "inappropriate" and "harsh" when in truth it was simply misapplied.
Micro-abrasive systems have taken the concept of abrasive cleaning to a new level of sophistication by operating on the theory that the hardness of the abrasive cleaning medium should be matched to the hardness of the historic masonry substrate.
In operation, these systems have proven extremely valuable in cleaning operations where the use of chemicals is not appropriate. However, they tend to be slightly more expensive than traditional chemical and water-rinse systems on a unit-cost basis.
Here the microabrasive system was just effective enough to remove deep-seated graffitti staining in a glazed brick veneer without damaging the delicate surface. Such a solution would not be necessary if normal chemical-based poultice compounds were effective in removing the 'ghosting'.
A section of the parapet wall in the process of being cleaned of graffiti and staining.
This is the State Supreme Court Building in the Civic Center of San Francisco. This is another Sierra White granite facade that displayed the usual signs of urban pollution, much of which was the result of the deposition of carbon on the stone surface over time.
The beginning of a cleaning project is especially important as the intended cleaning agent or method may not be as effective or benign as the specifier or architect intended. Test samples of several cleaning options are always performed to determine if the "gentlest means possible" is necessarily the best or most effective.
In the case of these lead-coated copper decorative elements from the dome of San Francisco City Hall, the micro-abrasive cleaning system allowed us to remove the lead coating without destroying the copper substrate, as a sandblasting technique would have done.
The cleaned and abated decorative units could then be gold-plated prior to re-installation on the newly-resplendent dome.
We have also used the micro-abrasive process in conjunction with an enclosed recycling system where circumstances required complete containment of free media.
With this system we were able to clean the walls in the main entry lobby even after the delicate gold leafing on the ceiling panels had already been applied.
