Weathering Steel, Preweathered

Solanum Steel is a pre-oxidized weathering steel surface developed by Zahner. The surface is produced by accelerating the natural weathering process and stabilizing the result. Solanum Steel provides artists, architects and designers with a range of resonant tones on an enduring surface.

One of the well recognized challenges of using steel in architecture is its propensity for the material to rust or oxidize. Although there are many people who appreciate the visual nature of the warm hues of weathering steel, nobody likes it when the material's color bleeds onto other surfaces. The Solanum Steel process reduces the material's tendency to stain adjacent surfaces.

Solanum Steel: The next stage of Weathering Steel

The innovation of adding copper to the steel alloy was the first step towards resolving this challenge. Copper-bearing steel, also known as weathering steel or CORTEN, has a unique design feature. As the material oxidizes, the oxidized layer becomes a protective layer, meaning that the surface won't corrode further. The problem is that manufacturers sell the sheet as a bare surface, and to get the material to a stable state where its no longer rusting will take many years.

This final challenge is what Solanum Steel has achieved, by bringing the natural weathering steel alloy into an insoluble and stable state. Solanum Steel provides a preweathered surface made by using an industrial process to limit bleeding onto adjacent surfaces. 

Sealing Solanum Steel

In some circumstances, the Solanum Steel material can be sealed. This is often desired for trafficked environments where people will touch the surface. For interior applications of Solanum Steel, the material can be sealed with a clear coating to provide a softer touch. 

However, for exterior applications, it is recommended to leave the material unsealed. Sealing is not recommended for outdoor applications because the coating will require further maintenance over time, and may change appearance as the sealant interacts with the changing environmental conditions. 

Gulf Islands Residence in Vancouver featuring Solanum Steel.
Gulf Islands Residence in Vancouver featuring Solanum Steel.
Interior use of Solanum Steel for the Guggenheim Hermitage Museum.
Interior use of Solanum Steel for the Guggenheim Hermitage Museum.
Interior office partitions made in Solanum Steel.
Interior office partitions made in Solanum Steel.
Solanum Steel column covers at the Omni Hotel in Nashville.
Solanum Steel column covers at the Omni Hotel in Nashville.
Detail of the Solanum Steel column cover at the Omni Hotel in Nashville.
Detail of the Solanum Steel column cover at the Omni Hotel in Nashville.

How Solanum Steel is Manufactured

The typical alloy used in Solanum Steel is ASTM A-588, which was developed as an advancement over the original ASTM A-242 alloy used in COR-TEN, a trademark of U.S. Steel. Since the material was first made public, the alloy has been used by architects on a wide number of project. 

The Solanum Steel process begins at the raw steel ingot. Zahner has close relationship with both the steel mill and the rolling mill, and is able to influence the quality of the surface long before the raw sheet arrives at Zahner. This ensures that the material is derived from specific alloying components and that the base raw material is cast to exacting specifications.

The ingot is then processed by Zahner's rolling mill partner, whose finesse with steel ensures that the surface is properly treated. In some ways, this is the most important step, because if you don’t influence the base metal sources the mill will treat the material like basic steel. If the mill treats the material like basic steel, contaminants will reveal themselves in the final product as streaks, rings, and discolorations in the finished surface.


Solanum Steel seen under a microscope. Note the flaky appearance of the typical weathering steel patina.
Solanum Steel seen under a microscope. Note the flaky appearance of the typical weathering steel patina.

Zahner patina engineers inspect each and every plate surface. Each surface of the bare Solanum Steel is cleaned and prepared to eliminate oxides and contaminants. If this step is not carefully followed the finish will not hold up. Once properly prepared, Zahner 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 as permanent as a dark patina. A patinated steel showing yellow and light orange colors will rub off and continue to bleed and stain adjoining surfaces, unless sealed with a durable coating. Zahner allows the orange-toned oxide to grow to a point, and then remove most of it from the surface. The surface is then exposed to a proprietary bath with specific atmospheric conditions to develop a hydrated oxide form of either ferrous oxide or ferrous hydroxide.

At this stage, the steel surface becomes insoluble.

The hydrated oxide form until it reaches a certain point. The material is then arrested with a final bath that inhibits and slows oxidation to a standstill. The material is in its final stage of development.

Projects Using Solanum Steel:

Works featuring Solanum Steel™