The problem is not that we haven’t been cleaning these buildings. We have. And at shorter intervals.
The problem is HOW we are cleaning.
The reason for the black staining is the way these buildings have been cleaned over just the last 20 years: soaps, pressure washers, and especially a certain class of biocides known by the shorthand “quats” for quaternary ammonium compounds. Quats are common biocides used in many industries and even in our homes (although at much lower concentrations than used on buildings) despite the fact they are persistent environmental toxins especially bad for aquatic life where they disrupt reproduction and encourage antibiotic resistance (and of course most of our cities are built on the coast or rivers).
*** See links to some research below if you want to do more reading about quats *
Before I go on, you might be thinking “Won’t buildings always have biological growths on them?” That’s correct, there is plenty of cyanobacteria and fungus that is airborne and mosses and lichens that will settle in shaded areas. But none of those produce the black biofilm colonies that we have now and which are becoming increasingly difficult to remove.
For context, think about what you’ve been hearing in the last few years about antibiotic-resistant bacteria growth and intractable biofilms in hospital environmental where quats are a common cleaning protocol and cleaning is a non-stop activity so that you wouldn’t think those problems would exist … and yet they are a deadly serious and growing problem.
Before the 1970s-80s many of our buildings were covered in coal soot and other environmental pollutants. They were a different type of cleaning challenge that was addressed as historic preservation became big business after the 1976 historic tax credit programs were initiated. What has changed is how we have cleaned for about the last 20+ years to remove biological growths.
The problems with this approach are:
Pressure washers damage masonry (increase the surface area so there are more places for re-colonization) and drive water and soaps into the stone, but rarely rinse it from the masonry. This is often best demonstrated by the way areas that dry out most quickly (two-sided building parapets and building corners where the wind whips around) also re-colonize the fastest
Soaps that are left behind on buildings are food-sources for bacteria just as much as they are as scum in your shower. (I’ve rinsed buildings without non-ionic cleaners yet had bubbles foaming up and running down the walls with the scent of Ivory Soap or Dawn dishwashing liquid, both common in the 80s, on buildings almost two decades after their last cleaning. It is a misnomer to think that soaps left behind will be quickly rinsed away by the rain, especially on the protected undersides of the cornice, recessed panels, decorative details, or windows.)
While biocides may kill existing bacteria, these are not removed from the building, only driven in deeper. Why do we put a rich compost in our garden? To grow more abundantly. The same is true for the rich compost of dead bacteria we leave on our “cleaned” buildings.
Some buildings, even buildings that are less than 20 years old on the National Mall have been cleaned at least a half dozen times in this manner and the black just keeps coming back within months. Clearly this is not a sustainable solution.
The question is: What can replace it?
Unfortunately laser ablation is becoming the latest fad, with claims that it is non-destructive. Of course the world ablation (“to erode”) says it all. Lasers work by effectively boiling and vaporizing water, whether in the stone or biological materials. While the power can be adjusted, the frequency (intensity) of YAG lasers is not adjustable. As a result, laser ablation means popping the surface. In fact I’ve been on several projects where lasers were tried and immediately rejected because everyone watching received a shrapnel-like spray of minerals off the surface. Because the biological material they are trying to remove has been mining the stone for nutrients for its survival, stone must necessarily be removed in order to clear the biological growths interwoven into the stone. (I’ve been sent pictures where laser ablation was allowed to proceed and even very hard stones — on the Mohs scale — like polished granite end up looking more like pumice after laser ablation.)
This really isn’t any smarter than trying to “clean” buildings by burning the top millimeter of stone off with acids (still happening too often and leading to the activation of iron in the stone which produces the telltale signs of stones turning yellow in part or in whole over the following year) or sandblasting them, both of which “work” by removing the surface of masonry back to pristine surfaces behind, but do so by also increasing the surface area to provide more area for dirt and biological materials to return.
Plus, substituting laser ablation after biocides stop being effective doesn’t remove the soaps that are already in the stone or get to bacteria and fungi deeper than near the surface. Leaving both will lead to quick recolonization.
Where does this leave us?
Needing a better alternative that:
Does not exceed the Mohs hardness of that particular stone so that it does not increase the surface area and with it the area to colonize
Does not poison or disrupt the biological functioning of aquatic life in our waterways (or workers)
Removes residual surfactants (soaps)
Removes the component parts of the biofilm (a complex mixture of bacteria and fungi that feed off one another and the stone itself) rather than leaving dead biological material to act as compost for future growth
Until an alternative that meets those requirements is found, many building owners are choosing to hold off on doing more cleaning rather than waste money on a short-term aesthetic treatment that will actually further damage the building.
Links to some useful reading about the problems with quaternary ammonium (“organic salts”) biocides:
ROLE OF CYANOBACTERIA IN BIODETERIORATION OF HISTORICAL MONUMENTS – A REVIEW. BMR Microbiology, May 2014. http://advancejournals.org/bmr-microbiology/article/role-of-cyanobacteria-in-biodeterioration-of-historical-monuments-a-review/
FATE AND EFFECT OF QUATERNARY AMMONIUM COMPOUNDS IN BIOLOGICAL SYSTEMS Georgia Institute of Technology, School of Civil and Environmental Engineering, May 2009 doctoral thesis. https://smartech.gatech.edu/bitstream/handle/1853/28229/tezel_ulas_200905_phd.pdf
MINERAL ECOLOGY: SURFACE SPECIFIC COLONIZATION AND GEOCHEMICAL DRIVERS OF BIOFILM ACCUMULATION, COMPOSITION, AND PHYLOGENY. Frontiers in Microbiology, March 2017. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5368280/