An overview of architectural metals and advanced surface treatments

After an earthquake in 1989 wrecked the structural integrity of the M.H. de Young Memorial Museum in San Francisco, architects Jacques Herzog and Pierre de Meuron were charged with redesigning the building. One of the ideas was the development of a variably perforated screen exterior that would mirror the green foilage of the tree canopy in nearby Golden Gate Park. A. Zahner's engineers and software specialists developed a system to create the unique perforation and dimple patterns to create the screening, which can be seen in this canopy covering the museum's outdoor café. This was the first iteration of what A. Zahner called the Zahner Interpretive Relational Algorithmic Process, or the ZIRA Process.

The beauty of metal is often hidden in commercial construction. In most instances, no one really notices the structural steel that provides the skeletal system for some of the most impressive architectural works on record.

That's not always the case, however. Many architects and artists are realizing the impact that metal can have as not just a functional feature in building design, but also as a decorative one. No one enjoys bland and boring, and architectural metal is one way to add visual excitement to a construction project.

Bill Zahner, president and CEO, A. Zahner Co., literally wrote the books on metal surfaces (Steel Surfaces; Copper, Brass, and Bronze Surfaces; Aluminum Surfaces; Stainless Steel Surfaces; and Zinc Surfaces). Founded by Bill's great-grandfather in 1897, A. Zahner Co. got its start with architectural metals by making cornices for new commercial buildings going up all over small towns west of the Mississippi. The company started in Joplin, Mo., but later moved to Kansas City, Mo., where it's located today. A. Zahner has two plants, one in Kansas City and the other in Grand Prairie, Texas, and has about 60 engineers and 100 fabricating employees working to create metal products that are sent all over the world.

"We work all over the place," Zahner said. "We have work going on in Miami, Phoenix, and in New York. Right now, one of our largest projects is doing the restoration on the Air Force Academy's chapel in Colorado Springs."

The U.S. Air Force Academy Cadet Chapel is well-known even to those outside of architect circles. The chapel, built in 1962, has 17 spires reaching heights of 150 ft. In 2004, it was given U.S. National Historic Landmark status.

During the restoration project, a temporary enclosed structure has been built around the chapel to protect the building's interior as aluminum sections used as part of the original construction are removed and replaced. (The original building design called for an intricate network of rain gutters, originally planned to be included beneath the building's exterior aluminum panels. The gutters, however, were scrapped because of budget constraints. As a result, the chapel had only caulk between the aluminum panels to project it from the elements, and that hasn't been the best line of defense over the years. The existing aluminum panels are being removed to allow for the installation of a new panel system that matches the original aesthetic, but with significantly improved performance features than the older panels being replaced.)

Zahner said that as a subcontractor on the project, the company had to identify the specific alloy used in the chapel's construction in the 1960s and have it specially cast so that it could be extruded and rolled out to match the panels coming off the chapel. The goal is to restore the panels, but without the leaks.

"Our reputation is that we can take on challenges on very complex projects and help the designer achieve them for the client," Zahner said.

A. Zahner's work is a very good illustration of some of the metal surface trends being used in architectural metal.

"As you travel around the world, I’m sure you see these old buildings in places like Copenhagen, London, and Frankfort, and the old oxidized copper surfaces are typically the most identifiable and most interesting," Zahner said. "Well, you can do something similar with stainless steel."

FIGURE 1. A thickened chromium oxide layer on stainless steel helps to produce the color visible to the human eye. This can be seen in the panels used in the exterior of the Museum of Science and Industry in Tampa, Fla. A. Zahner has a team dedicated to developing custom palettes of new colors and finishes for various metals, which the company calls its Zahner Surfaces product line.

One way to introduce coloring to stainless steel is through a process developed in the 1970s by Inco Ltd., a Canadian mining company and the world's leading producer of nickel for much of the 20th century. Chemical acid baths develop a thickened chromium oxide layer on the stainless steel surface, and this layer helps to produce the color visible to the human eye.

The phenomenon behind the color creation is called thin-film interference, which has evolved into interference coloring. The thickened layer of chromium oxide helps to influence the color as the light passes through it, hits the surface metal, and reflects back to the viewer. Think of sunlight shining on a puddle of motor oil and water in the driveway and the rainbow effect that is sometimes produced. In that situation, some of the light is reflected off the top layer of oil, some is refracted by that layer and then reflected by the layer of water underneath the oil, and the light is then refracted again as it passes up through the layer of oil. The reflected light in the puddle shows a rainbow because the light contains all components of all wavelengths and the condition is not controlled. When the electrochemical process to introduce the chromium oxide later is controlled, specific colors can be introduced. No pigments, inks, or dyes are used to create the color.

One of the first major projects using this type of interference coloring was the Team Disney building in Anaheim, Calif., designed by Frank Gehry and opened in 1995. That was soon followed by the Museum of Science and Industry (MOSI) in Tampa, Fla., completed in the same year.

Those blue panels are triangular, which adds to visual interest (see Figure 1). A. Zahner and Antoine Predock, architect on the MOSI project, worked together to develop the triangulated panel system in a bid to counteract the metal's reluctance to curve in two directions at once, which was needed to cover the spherical shape of the building's dome.

"Today, you go out there, and it looks exactly like it did when it was first done in the early 1990s. It's this beautiful jewel," Zahner said.

Interference coloring can only deliver primary colors. Physical vapor deposition (PVD) coloring, the second major method of producing color in stainless steel, can produce a more varied palette.

The PVD process involves the application of a coating that is vaporized and applied to a heated metal surface in a vacuum chamber. The ionized metal vapor, when bonded with the surface metal, creates a coating only a few molecules thick. The result is a surface that differs from the original metal properties and that is more protective than powder-coating, electroplating, or anodization.

The addition of a titanium nitrate to the PVD process results in a gold color. Titanium carbide helps to produce a deep black. In both examples, the finish does possess color, unlike in interference coloring. (It should be noted that the thinness of the PVD coating allows for light to pass through it and reflect off the surface metal, creating some light interference.) Other colors that can be produced by the PVD process include bronze, blue, and red/purple.

Everyone in the metals industry knows what COR-TEN is. It's pretty much what you think of when someone mentions "weathering steel."

First developed by U.S. Steel Corp. in the 1930s for the construction of railcars used to transport iron ore and coal, COR-TEN since has been marketed as a corrosion-resistant steel alloy for architecture and art applications, but not limited to those areas. COR-TEN and other weathering steels have some elements, most importantly copper, that interact and create a distinctive oxide barrier over the surface. As this ferric oxyhydroxide grows, the base metal is afforded corrosion resistance. Once the oxide layer develops, further changes in the surface material occur slowly in most environmental settings. The weathering steel produces an earth-tone finish that make it a popular choice for buildings looking to blend into a natural environment.

FIGURE 2. The exterior of the Science Center at Amherst College, Amherst, Mass., is an example of the preweathered look that can be attained without waiting around for nature to do its think.

The issue that A. Zahner recognized is that architects and designers can get really frustrated waiting for the corrosion to occur and produce the expected surface finish. That's where preweathering can help.

"That's one of the things that we’re doing with our plant in Texas," Zahner said. "We take the COR-TEN alloy and weather it. What we are trying to do is get to the point where the ferric oxyhydroxide bleeds off and turns into a much more stable ferric oxyhydroxide."

At A. Zahner, preweathering means taking COR-TEN and other high-strength, low-alloy weathering steels and exposing the metal surfaces to an oxidizing agent, then following that up with a series of wetting and drying cycles. The company believes its preweathering processes can help the surface of these materials reach the dark, rich oxyhydroxide it would acquire when aging naturally in the elements.

An example of this type of preweathering is the Science Center at Amherst College (see Figure 2). The welded and formed steel sections were blasted to thoroughly clean and prepare the surface for the initial oxidation treatment. Once dry, the surfaces were treated with a strong electrolyte that is a reducing agent, which starts the oxidation process. When evidence of corrosion was present, the surface was rinsed with deionized water and allowed to dry. Follow-up treatments of wetting the metal and allowing it to dry result in a thick, dark oxide on the metal surface.

Anyone can do perforations. A. Zahner was the first to use that punching machine to punch out an image (see Figure 3).

"We were the first to approach taking an image and converting it to machine language," Zahner said. "So we started messing around with controlling perforations on the surface using the punch, but then we started thinking, ‘How can we trick the machine to not perforate, but rather bump the material to create different ways of creating textures?’"

What started with the precise punching of different-sized holes to replicate an image on several metal panels soon developed into something much more intricate. Zahner said that M.H. de Young Memorial Museum in San Francisco is a good example of this (see Figure 4).

The museum, first opened in 1895, was heavily damaged following the 1989 Loma Prieta earthquake. Swiss architects Jacques Herzog and Pierre de Meuron were selected in the late 1990s to redesign the building.

Herzog and de Meuron came up with the idea of a perforated screen exterior to mimic the tree canopy of nearby Golden Gate Park. The duo worked with A. Zahner's engineers and software specialists to develop a way to create unique perforation and dimple patterns that would replicate the patterns of light as seen through trees.

The job was immense. Just over 1.1 million lbs. of copper were used for the job. That translates to about one sheet of copper that would be 1 m wide by 21.6 miles long.

FIGURE 3. A. Zahner's ImageWall perforated metal panels give architects the ability to create elaborate wall designs for places like lobbies, parking garage facades, or partitioned outdoor spaces. In this particular installation in an office building in Ottawa, Canada, 1,563 sq. ft. of custom perforated aluminum panels and associated sub-framing was used to create wall art that showcases the journey along the Ottawa River adjacent to nearby Parliament Hill.

In terms of fabrications, the building and its tower accounted for 1.7 million perforations and 1.5 million bumps on the surface, including four levels of embossments and four levels of outward forms.

The next evolution of textured panels is likely to lean more heavily on robotics, according to Zahner.

"If you look at the Bloomberg Center in New York, for example, we partially cut the metal and then used a robot to bend it selectively," he said. (Figure 5 shows an outer wall of the Bloomberg Center on Cornell Tech's Roosevelt Campus in New York City.)

That's only a portion of what's going on at A. Zahner for the architectural metal world, but it does reinforce the fact that metal is easy to overlook when it comes to making a statement. For too long, people were OK with simply using paint to provide surface protection or create visual interest on the metal surface. That doesn't have to be the case.

Zahner said that A. Zahner will continue to look at robotics to deliver textured shapes to metal that would be almost impossible to accomplish manually. He also mentioned that nano-coatings will emerge that will provide material protection and give the metal a more predictable finish over time.