On July 11 1998, I responded to an inquiry in the Conservation Distribution List from Patricia Favero patricia.favero@ci.seattle.wa.us. She had asked:
"We have an unpainted Cor-ten steel sculpture in our collection, about 20 years old, that has been hit often by graffiti vandals and subsequently by harsh graffiti-removal chemicals. The resulting discoloration could also be due to the trees above the sculpture or our generally wet weather.
So, I am interested to hear about various experiences anybody may have had with Cor-ten: is it better to use an anti-graffiti coating/sealer or leave the metal bare. Are there any obvious weathering patterns of Cor-ten; is the material affected differently by different climates?"
I my response I provided the text of "Weathering Steel Sculpture" from this site as it stood at that time, and a reference to the web site to provide an opportunity to view the illustrations.
After reading my response, John Scott (nyconsnctr@aol.com), a conservator who has obviously spent a lot of time thinking about weathering steel in sculpture, wrote a reply for the Conservation Distribution List,
"I am a sculpture conservator, active in the care of weathering steel. Incidentally, some who have known me for many years will be glad to hear that I have just (finally!) reinstalled the painted zinc 1879 Lions on Princeton's campus :' ) While grinding through the final weeks of that project, I noted but couldn't respond to the Seattle question on weathering steel and graffiti.
"IMHO, after reading a recent DistList contribution on this topic and visiting the associated web site <URL:http://www.best.com/~drrust> , artists, curators, conservators and others interested should consult further authority on the nature and development of weathering steel and its protective patina, on weathering steel's simple but critically important design requirements, and on the practical effect of chloride ions on that (or on any stable) patina. Readers might begin with any of the following offerings of mine:
"Weathering steel sculpture," article in October 1996 issue of Sculpture magazine.
"Weathering Steel," co-authored chapter in the U.S. National Park Service's 1995 book: Twentieth Century Building Materials: History and Conservation.
"Conservation of Weathering Steel Sculpture," chapter in the Canadian Conservation Institute's 1992 book: Saving the 20th Century: The Conservation of Modern Materials."
In the meantime, a few pointers:
The protective patina on weathering steel has been shown to comprise a two- part layered structure, which includes little or none of the red rust (magnetite) found on other ferrous alloys. The inner part, tightly adherent to the underlying metal, is mainly amorphous ferric oxyhydroxide (goethite), and engenders and is covered by outwardly separating layers of the same in mostly crystalline form. Alternating wet and dry conditions promote the persistence of this protective barrier, not by recrystallization of rust but by dissolution at micro-crevices in the inner layer, of alloying elements such as copper from the underlying steel. These dissolved ions inhibit formation of magnetite and catalyze formation of goethite which seals the crevices. Crevices form anew when the patina dries, and when rewetted, the cycle continues and the protective patina develops to a kind of equilibrium with very little surface corrosion going on.
Normal development of the protective rust patina is relatively slow; depending on ambient conditions the patina can take years to fully develop. However this may be speeded by an active program of water spraying and drying. In any case, good planning and patience are necessary.
It is very unfortunate that salt or acid are commonly used to avoid waiting for protective rust to develop normally. I have observed that chlorided weathering steel surfaces develop rust suffused with a most, glistening quality very reminiscent of ferric chloride rust. Industry studies clearly show that weathering steel's performance is much worse near the sea and in marine environments, where its surface is heavily contaminated by chlorides. The provision of chlorides to the weathering steel surface, whether in the form of hydrochloric acid or as salt, is very detrimental to the formation of protective rust on weathering steel. The effect on long term stability cannot be good. Please *do not* use acid or salt to produce rust on *any* steel you want to preserve, including most definitely of course, weathering steel
Due to the rinsing effect of rain as observed on outdoor surfaces generally, and to the exfoliating nature of weathering steel's protective rust, the contribution of ambient deposition to the observed darkening of color is probably minor. More important very likely is a decrease in surface light-reflectivity/dispersivity due to gradual thickening of the light-absorptive inner part and exfoliative thinning of the more dispersive outer crystalline part.
John Scott, MA, MBA, MA-CAS (sorry, no PhD!)
New York Conservation Center, NYC"
Then I responded to the Conservation Distribution List:
"It appears that my recounting of a successful use of aqueous HCl to renew a rust film on weathering steel has disturbed John Scott. I believe that his concern is due to confusion between a one-time exposure to a corrodent and continual exposure to a corrodent.
No one can dispute that under some climatic conditions the continual exposure of weathering steel to chlorides can prevent the formation of a protective rust layer. I have seen many sad cases of this condition on the coasts of the continental US, in Hawaii, in South Africa, and under leaking bridge joints in the road-salt belt of the US, and have provided consultation in regards to marine exposures in Japan, Europe, and South America. (Please note that in certain coastal environments weathering steel in well-designed applications performs satisfactorily.) However a one time exposure to chloride is a very different situation. My experience and that of other investigators is that without a continuous resupply of chloride or other corrodent from the environment, a chloride or other artificially produced rust will over time revert to the rust film characteristic of weathering steel of that location and exposure condition. Consequently I believe that in general a one-time exposure to HCl or other chloride solution will not produce a permanent degradation to the rust layer on weathering steel, and its use may be justified to produce a quick and relatively uniform oxide layer.
In regards to Mr. Scott's detailed explanation of the weathering steel rust layer formation and behavior over time, I am skeptical of it or any other detailed description of weathering steel rusting based on the crystallographic identity of rust components. I have seen too many explanations of rust formation based on one analysis technique (e.g. X-ray diffraction, Mossbauer spectroscopy, Raman spectroscopy, electron diffraction, optical microscopy) that contradict explanations based on other techniques. The problem is that, as I understand John Scott to agree, the rust on weathering steel or other steels is stratified into layers of different morphology and crystallography and that much of the rust is so microcrystalline it appears amorphous to many techniques. Also, it is the case that different exposure climates and exposure conditions produce weathering steel rusts that show different crystallographic identity at any one time of exposure, and over time.
In any case, I agree that conservators should consult John Scott's publications, and others on the subject. In my experience the more information or opinions that are available to anyone making a materials preservation decision, the more likely a satisfactory outcome.
And John Scott quickly replied:
"Dr. Rust, Thanks for responding to my comments. I don't quite agree to what you suggest I do, but my sense of the Cons DistList audience is that those actually interested can find out what they need to know with or without our exchange.
In many types of chloride corrosion, a small amount of chloride repeatedly cycles back to the metal interface to cause more corrosion; the main factors affecting the rate of such corrosion, besides the presence of chloride, are ambient conditions such as humidity, surface moisture, and presence of other ionic species. In conservation we're interested not only in the gross visible effects of applying salt or acid to promote rusting, but also in effects on underlying structure and long term stability which may be undetectable at first. Conservators prefer to err on the side of caution, although I definitely notice that in the care and restoration of outdoor sculpture, even otherwise thoughtful conservators often give short term results priority over craftsmanship and over long term planning.
If there is more than anecdotal evidence to support your comments, I will be very glad to see it. A very interesting study I'm in no position to conduct, might test your hypothesis about chloride contamination's impact or lack thereof on the development and protectiveness of weathering steel's patina. I guess the study would have to employ some of the instrumental methods you find untrustworthy, since they are among the tools available to investigate the nature of materials at the scale of interest. They are some of the methods normally used in studying surfaces, corrosion products and patinas. It would be instructive to see an explanation of how the chloride is transported away from the steel or is otherwise rendered harmless. Although it would simplify matters tremendously, it would also be very remarkable indeed if weathering steel proved one of the few instances of metal surface to which a small amount of chloride contamination such as supplied by soaking with salt or acid solution was not detrimental. Until this can be proven, I'm afraid that even from an expert witness it just isn't convincing, and I cannot use it in the conservation of art."
And then he wrote, 24 Jul 98, for the Conservation Distribution List:
"I appreciate the opportunity for exchange of views with Patrick Gallagher; we need more such personable interfaces with the industries whose technologies we adapt for the production and care of art and cultural heritage. This particular thread has shown some of the different challenges involved in configuring technical expertise for application to conservation (Dr. Rust) and broadening of conservation expertise through science and engineering (many conservators do try!).
These exchanges beg a larger question about what we as planners, curators and conservators are doing with public art such as most large outdoor sculptures and murals in various media. Is our work "for the ages," or is it only necessary to preserve these artworks for a generation or two (or less), until they and/or their landscapes or architectural settings are redesigned or replaced? Very pragmatic factors will have to be considered along with conservation ideals in addressing this question."
Thinking that we had reached an understanding of each other's positions, even though we had not reached agreement, I did not respond further to Mr. Scott. I thought the question Mr. Scott raised "Is our work 'for the ages,' or is it only necessary to preserve these artworks for a generation or two (or less), until they and/or their landscapes or architectural settings are redesigned or replaced?" was very much a non sequitur, although obviously an interesting issue. The personal question I am left with after our interchange is "How can I convey the conclusions of a long career in preservation, in which most work was unpublished outside the companies for which I worked, to the art conservation community and others who might use the information?". I hope that providing this web site is one part of the answer.
Postlude:
In most fields of endeavor the theoretical and the
empirical vie for the practitioners' allegiance. In the case of
the original grounds of the controversy between John Scott and
myself I've found empirical grounds supporting my use of
hydrochloric acid for the renewal of the patina on weathering
steel. On an inspection in August 1998 I found the facia of
weathering steel that I had treated with hydrochloric acid over
twenty-five years ago in excellent condition.
See photo.
Thus the use of hydrochloric acid on
weathering steel does not inevitably lead to catastrophic
deterioration of the material, as I take Mr. Scott to believe.
Now the theory, or rather its applicability, must catch up.