Weathering Steel Guide Part 3: Preweathered Solanum Steel—The Next Generation of Weathering Steel

This is part three of our three part series on weathering steels. Read about the history of weathering steels in part 1 and the science of preweathering in part 2.

For nearly 60 years, weathering steel has been a favorite material for interior and exterior surfaces among designers and architects worldwide. With its rugged textures, earth tones, and shear strength, it’s often selected as a stylish choice to complement walls of glass and stone.

However, for all of its enduring beauty, weathering steel has presented a number of challenges in manufacturing, fabrication and ongoing maintenance. Its signature characteristic of iron oxide — rust — can be difficult to create, control and preserve.

At Zahner, we felt the industry could do better. Through testing and trials we came up with a reliable process for manufacturing an improved, preweathered weathering steel that we call Solanum Steel™.

Solanum, the Latin word for eggplant was chosen because the deep purple color of an eggplant is similar to what can be achieved when weathering steel reaches its point of surface equilibrium. Solanum also sounded fitting for a metal, in line with the great Sir Humphrey Davy who named such metals as potassium, sodium, and of course, aluminum.

We grow the same orange-colored oxide but then remove most of it from the surface. We then expose the surface to a proprietary solution and specific atmospheric conditions to develop a hydrated oxide form of ferrous oxide or ferrous hydroxide.

When it is desirable to have a very refined surface on the weathering steel, one that almost resembles the texture of velvet or worked leather, the outer veneer of the oxide can be burnished. This added process takes down the initial layers of oxide and creates a rust red slurry over the surface. As this slurry is allowed to dry, then lightly wetted and reburnished, the color begins to take hold.

Loose particles are removed and a richness in color and texture sets in. These artistic surfaces perform best indoors where the environment is more controlled, and the surfaces are not continually wetted. To create these forms of the surface, fine care must be taken.

As the developing oxide takes shape during the preweathering process, the surfaces are hand worked while wetted not to the level of burnishing but to the point of breaking up this initial oxide development.

The application of steam also helps in this process by providing heat as well as moisture. As the oxide forms, use an abrasive pad or bristle broom to sweep the surface. As it dries, a very consistent, even appearance develops.

At this stage, the colors grow richer and darker, and the patina hardens and stabilizes. We cultivate the growth of the hydrated oxide form until it reaches an optimal point. Then we arrest it with another proprietary solution that inhibits and brings oxidation to a standstill. The patina is fully formed and the surface is now Solanum.

Why Preweathered Weathering Steel Is Superior to Corten and Traditional Weathering Steels

Preweathered weathering steels like Solanum address shortcomings of typical weathering steel.

The most common concern is corrosion-induced metal dissolution, which is often seen as staining on surrounding materials such as concrete. This has led scientists to consider the environmental and health impacts of metal loading in urban runoff waters caused by weathering steel such as COR-TEN® A.

The variability of humidity, pollution, and other environmental factors in an uncontrolled outdoor environment can also affect how quickly and how well weathering steel reaches stability. This can introduce the aesthetic challenge of heterogeneous weathering due to differences in panels or the presence of structures that block a surface’s interaction with the environment. If the steel surface fails to reach stability, it also introduces performance concerns.

While rigorous material selection can help to control for these limitations, preweathering provides today’s architects and designers with a weathering steel solution that greatly reduces or eliminates these concerns.

The most exciting aspects of preweathered weathering steel surfaces are the new opportunities for employing the strength and beauty of weathering steels in applications where they were not previously appropriate.

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