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.
The deep red, brown, and purple hues of Solanum as seen on column covers at the Omni Nashville Hotel in Nashville, Tennessee.
The Process for Preweathering Solanum Weathering Steel
Solanum’s primary function is to outperform traditional weathering steel by using a multi-stage preweathering process that accelerates the base steel alloy to that of a weathering steel that has been exposed to the elements for years. But with richer colors yet none of the rust runoff and staining that comes from natural oxidation.
Our weathering process results in a stabilized patina, meaning that water and other elements won’t further corrode the surface. This also reduces any bleeding or rust spots on adjacent surfaces. The colors may be magnificent, but they are the result of putting the material’s function and utility first. The deep garnet reds and burnt umber tones are simply an added aesthetic benefit, also serving as a natural indicator that the material is in its most durable end state.
Before preweathering is initiated, the base metal must be treated with care by suppliers, otherwise it’s treated like basic steel: heavily oiled, scratched, and scarred. This is because these contaminants show themselves in the final product as streaks, rings, and discolorations in the finished surface.
Streaking and heterogeneous weathering are common challenges with traditional Corten weathering steels.
Solanum's preweathered finish offers a significant improvement in surface consistency.
To prevent this, we work with mills and suppliers to ensure that the raw steel ingot is derived from specific alloying components, and that the base material is cast to exacting specifications. Next, we select a mill that understands the finesse required when working with bare weathering steel. Special care is taken to prevent scratching and keep it clean so obvious imperfections are avoided.
Before the preweathering process begins, we inspect each and every steel plate. Each one is cleaned and prepared to eliminate oxides, greases, and oils.
Once properly prepared, we force the surface to an initial ferric oxide. This finish is slightly orange in color and is characterized by a crusty texture. An orange patina is not a permanent patina. A finish with yellow and orange will rub off, it will continue to darken, bleed, and stain adjoining surfaces. The orange is unstable, soluble, and not the mark of a premium patina.
Solanum Steel seen under a microscope. Note the flaky appearance of the typical weathering steel patina.
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.
Steel Surfaces, fourth in Zahner’s Architectural Metals Series, provides a comprehensive and authoritative treatment of steel applications in architecture and art. It covers everything from the history of the metal and choosing the right alloy, to detailed information on a variety of surface and chemical finishes and corrosion resistance. The book also features case studies that offer strategies for designing and executing successful projects using steel.
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.
Facade penetrations and other elements can negatively impact weathering patterns on Corten steel.
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.
Even if direct contact is not a concern, preweathered steel surfaces can be installed along side — and look as good as — other finish surfaces when installation is complete.
The Australian Embassy complex in Jakarta, designed by Denton Corker Marshall, features buildings clad in Solanum Steel, anodized clear and gold aluminum, and Zahner's Star Blue Copper and Hunter Zinc.
Take Advantage of Preweathered Steel
If you are interested in preweathered steel and want to learn more about how it can be used for your next building, sculpture, or design feature, we’re here to help: