Cor-ten steel sculpture

Patricia Favero <patricia.favero [at] ci__seattle__wa__us> writes

>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've prepared the following response.  On my web site are some
illustrations of descriptions in the text.  See
<URL:http://www.best.com/~drrust/wssculpt.htm>

Weathering Steel Sculpture

This is to provide some basis for conservators to use in considering
possible treatments for the care of weathering steel sculpture. I
would appreciate hearing from others of their experiences so I can
use the information to enhance the content and value of this
article.

In the last half of this century weathering steel (COR-TEN(r)) has
come into widespread use in sculpture. COR-TEN(r) is US Steel's
trade name for a corrosion resistant low-alloy steel that forms a
protective coating of rust (hydrated iron oxide) when exposed in
many natural atmospheres. Because the appearance of the steel is due
to natural processes, as an analogy to the weathering of natural
materials such as wood, it is often called weathering steel. I'll
use that term from here on in this note. The "weathering"
characterization is a reminder that the material can change in
appearance over time due to the environment, which should be kept in
mind in appreciating and caring for works executed in the material.

The rust layer on weathering steel becomes protective when the fine
discrete crystallites of early rust recrystallize into a relatively
intact barrier layer of rust. The formation of the protective layer
requires alternating wetting and drying cycles; the wetting to
generate the rust, and the drying to allow it to recrystallize. If
the steel is not allowed sufficient drying time, the resulting
continual rusting will cause the partially crystallized outer layer
of rust to be shed and will prevent a barrier film from forming.

For some interesting reasons I'll not elaborate at this point, the
appearance of weathering steel depends on the extent to which
recrystallization has occurred and thus indicates the extent to
which the barrier layer is formed. The early rust forms in discrete
crystallites that are fine, red and diffusely reflecting, like
hematite. The massive recrystallized layer is a shiny blue,
approaching the blue-black of specular hematite. Thus portions of
weathering steel that have seen different amounts of wetting and
drying will have different degrees of recrystallized oxide and will
have different appearances.

Most weathering steel sculptures in most environments provide
surfaces that see varied amounts of wetting and drying. Consequently
these areas have varied amounts of recrystallized oxide and have
different appearances. In general the skyward surfaces see more
drying and are bluer and glossier while the ground-ward surfaces see
more wetting by condensation, lack of drying, and runoff and so are
redder and flatter. Runoff of water from upper portions of a
sculpture tend to produce long-lasting streaks or other patterns of
redder oxide on lower portions. Similarly, in wetter climates the
overall color of weathering steel sculptures will have generally
have an overall redder cast relative to those exposed in drier
climates.

The appearance of weathering steel can also be affected by other
factors. During recrystallization the rust will trap particulate
matter on the surface. If this material is colored it will
contribute to the appearance of the rust. For example, in dirty
industrial atmospheres the rust on weathering steel can be almost
black due to the incorporation of airborne dirt. Chemical cleaning
treatments such as acids can convert the hydrated iron oxide to
other iron compounds of different color or appearance. In
atmospheres with significant content of sulfur oxides deposits of
white to yellow ferrous sulfate may appear in the rust on weathering
steel. In some climates organic growth such as moss may be present
and affect the appearance of the rust.

Discolored areas on a weathering steel sculpture could be due to any
of the variety of factors described above, or excessive corrosion.
The rust layer on weathering steel in many U.S. climates does not
consume a significant amount of steel in its formation, so removal
in most cases should not affect the strength of the work. However,
in some cases of inappropriate design crevices or pockets will trap
water and the continual presence of water leads to excessive
corrosion evidenced by rust flaking or observable metal loss. These
should be sealed or coated to provide protection, and may need
reinforcement if there has been significant steel loss. In the case
of discoloration of rust due to other causes, if the rust were to be
removed without a change in some factor in the environment the rust
would eventually return to the original discolored appearance. The
use of a clear sealer, say a polyurethane varnish, over new or old
rust will retard the normal weathering process, but it would
certainly also change the appearance of the sculpture, and might
itself require continual restorative treatment. I have had success
in producing a rust layer on cleaned, previously rusted weathering
steel with the use of a 5 to 10 percent solution of hydrochloric
acid in water. Luckily the rust had acceptable appearance since the
substrate was the walls of the entrance to the U.S. Steel Building
in Pittsburgh, and I was working for U.S. Steel at the time.

For weathering steel sculpture not exposed outdoors, the normal
practice is to expose the work to the weather for some period of
time to build up a rust layer, and then to bring it indoors.
Varnishes are sometimes applied to alter the appearance of the rust,
so a conservator should be aware of that possibility.

I hope this is of some help to Patricia Favero and others.

Patrick Gallagher
Materials Preservation

                                  ***
                  Conservation DistList Instance 12:11
                   Distributed: Friday, July 17, 1998
                       Message Id: cdl-12-11-008
                                  ***