Weathering Steel

In the last half of this century weathering steel (COR-TEN®) has come into widespread use in sculpture. COR-TEN® 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. The appearance of weathering steel is due to natural processes.The weathering characterization is a reminder that the material can change in appearance over time due to the environment. This should be kept in mind in appreciating and caring for Weathering Steel Sculpture

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.

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.

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 areas should be sealed or coated to provide protection, and may need reinforcement if there has been significant steel loss.

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. Organic coatings are sometimes applied to alter the appearance of the rust, so a conservator should be aware of that possibility. Also, for weathering steel exposed only to indoor atmospheres, artificial patinas may be created by chemical treatment, then coated.