As with all cleaning methods, the type of stone or masonry and the condition of the substrate must be carefully analyzed before actual cleaning can commence.
Chemical cleaning involves the use of usually one, and sometimes more than one, chemical to break the bond between the surface soiling agent and the substrate. Here an alkaline compound is being applied to an interior limestone handrail to remove accumulated body oils an dirt.. Alkaline products are usually effective in breaking down organic solids.
This Danby marble surface exhibits the soiling that can occur when baked-on carbon and atmospheric soiling builds up on the surface. The stark contrast in the newly cleaned surface is the result of an alkaline-based cleaner designed specifically for this type of soiling and substrate.
Heavy soiling and carbon deposits on calcareous stones (marble and limestone primarily) in urban industrial areas can be tricky. Use of an alkaline-based component is again specified here, but the product used was a caustic soda, from the far end of the pH scale. Rinsing and neutralization of the cleaned surface is an important component of this process.
This marble veneer was cleaned approximately 6 years prior to the taking of this image. Since the building is located in a busy urban area with an adjacent bus line running by the site, it is no surprise that the carbon buildup could occur so quickly. Regular maintenance is an important aspect of preserving historic surfaces such as these.
Acid-based cleaners are another important tool in the box of the restoration contractor. These new terra cotta units were made using a glaze formula color-matched to the cleaned, adjacent terra cotta surfaces. Acid-based cleaning compounds are frequently the most effective cleaner for fired and ceramic surfaces.
This is the Hearst Mausoleum in Colma, that amazing necropolis just south of San Francisco. The entire skin including columns and roof is made of Sierra White granite, a stone still quarried domestically near Sacramento. In this case the buildup of soiling required a two-stage cleaning process in which a caustic soda (nasty stuff, like lye) is applied to the surface, allowed to dwell for 3-6 hours, rinsed with pressurized water, and then a mildly acidic compound of acetic acid is applied and rinsed. to bring the surface back to a neutral range near pH 7.
An after-shot illustrates how effective these materials and methods can be in removing severe soiling without endangering the important historic masonry substrate.
Here an early 50's bank facade was 'improved' in the early 70's by the application of 12 x 12 ceramic tiles. Since the original polished stone surface was not properly prepared by roughening, the bond between tiles and stone soon began to fail in multiple locations. Because we knew that the stone was a highly mineral-laden material with lots of silica components, we knew that we could use an acid-based compound to remove the old thin-set adhesive without etching the polished surface.
Knowledge of materials is crucial to the proper selection of means and methods for bringing back the original look and feel.
Facades having a combination of materials are common, and usually (but not always) we can use a single chemical to clean both substrates. In this case an acid-based cleaner and water rinse is clearly effective in the removal of surface soiling from brick and decorative terra cotta on this Beaux-arts veneer.
We generally find that these industry-standard acid-based restoration cleaners are safe and effective on most historic masonry, except for any marbles, limestone's, or calcium-based materials.
Sometimes the use of a single chemical can have an extraordinary effect on the appearance of historic masonry.
Within the preservation community, it has been occasionally proposed that cleaning of older historic masonry is an inappropriate intrusion into the look and feel of the historic material, provoking the old Ruskinian debate.
It is hard to argue with these results, however, where the articulation of the carved stone surface is dramatically more apparent once the surface carbon has been removed.
Chemical treatments are also important in the removal of old coatings. Here a thick layer of multiple coats of paint is being removed. The stripper is applied, covered with plastic to retard evaporation, and allowed to dwell for approximately 24 hours before rinsing with pressurized water.
When the coatings were removed from the Indiana limestone surface, it was apparent that the original motivation for hiding the stone was the inappropriate look of the previous patches and repairs.
Sometimes budget limitations can impose severe restrictions on the scope of work. Here the bottom entry area was cleaned but the upper portions of the building, made of the same granite, were left untouched.
The last 15 years have seen the introduction of new and safer but still effective chemical cleaners. This operator is not even required to wear the usual rainsuit and face protection for this cleaning process. These products also tend to be easier and cheaper to dispose during cleanup.
Chemical strippers are also a valuable option when old coatings need to removed from interior surfaces, such as this old terra cotta floor.
Nothing is more satisfying than seeing the return of an older masonry surface after 75-100 years of decay and soiling. Here, a parapet composed of glazed terra cotta being rinsed after a mild-acid cleaning solution was applied.
This project was unusual in that it could not be rigged for swing stage access because of the configuration of the roof.
Regardless of the nature of the chemical agent being used, it is crucial that proper collection techniques be employed to prevent cleaning runoff from entering the local storm drainage system.
Chemical cleaning also extends to the use of a poultice, in which a cleaning agent is mixed with a medium and applied to the substrate to begin a slow-acting stain removal process. Here, a marble wall in an entry lobby has been coated with a mild alkaline poultice and covered with plastic to prevent premature drying.