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Subject: Cor-ten steel sculpture

Cor-ten steel sculpture

From: John Scott <nyconsnctr>
Date: Tuesday, July 21, 1998
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

                                  ***
                  Conservation DistList Instance 12:12
                   Distributed: Friday, July 24, 1998
                       Message Id: cdl-12-12-004
                                  ***
Received on Tuesday, 21 July, 1998

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