Zinc Guide Part 3: Innovative Zinc Patinas and Finishes, And the World’s Most Interesting Architecture
Learn about Zahner patinas and finishes and how they’re being used in projects worldwide to create earth-toned cladding, facades, accent elements, and more.
Zinc has been used as an architectural material from the time of Napoleon, yet many designers still overlook the nuances of this mystifying metal. In our everyday lives we experience zinc at every turn, but we often don’t realize it or recognize it.
Let’s take a closer look at this amazing alloy and the various patinas, colors, textures, and finishes that make it such an important and enduring design element.
The Dark, Variegated Patinas of Zahner Roano™ and Baroque™ Zinc
The ability to quickly and effectively form insoluble compounds is the basis of patinas that can be developed on zinc. The process involves activating a flow of zinc ions into a reactive solution. The solution is an acidic electrolyte containing negatively charged anions that combine with the zinc at the surface to form mineral-like natural compounds. The compounds must join with the surface slowly to arrive at a conversion layer of insoluble, yet adherent compounds.
This patina is a stable, highly variegated surface. As exposure increases the frequency of moisture within mild light industrial conditions, the patina will gradually change. Usually a light white spotting of zinc hydroxide appears as the moisture is held by the semi-porous surface or as moisture collects on drip edges or flat regions. Change in base color is usually minor. The zinc patina surface, particularly around the edges, has some minor zinc hydroxide whitening, otherwise the surface looks very consistent over the course of its life.
For this particular finish, called Roano™ zinc, there are a series of reactions that occur on the surface in a layering effect. As the finish develops, an appearance not unlike the minerals that give some paving stones color forms a tight bond with the zinc.
A variation of the Roano finish is called Baroque™. With this patina the variegated finish darkened, still very stable but with a darkened zinc background. The darker tone adds richness to the design. The chocolatier company, Max Brenner, used this finish to emulate the deep color of the product it produces.
Chocolate company Max Brenner, clad with shingles of Baroque zinc.
The finish has an interesting directionality due to the depth of the crystalline surface that forms. The crystalline surface is rough but continuous with zones of heavy crystal development and areas of openness.
The surface is similar in nature to the mineral franklinite, a zinc iron oxide, and the rare mineral known as stelleite. This amazing micro-topology traps and scatters light similar to the way a natural mineral surface does. The appearance, color, and tone of the reflected light will change slightly as the angle of view changes. This is also why there is an appearance difference from panel to panel. The surface “grows” as the zinc reacts and thickened corrosion products form.
Interior lobby cladded with Roano zinc panels.
Photo by Andrew Merz | ARKO
The process is similar to prepatinated copper or preweathered weathering steel. The surface goes through a chemical reaction as some of the zinc on the surface goes into solution and combines with other elements in the solution. These reactions must occur at the surface and time is needed for zinc ions to diffuse and create bonds with the elements in the patina solution. Like all chemical reactions their efficiency is dependent on time of exposure and temperature of the metal. If the process is rushed you can get separation from the finish. If the temperature is below 20°C (68°F) the chemical reaction is slow to occur as is the case with other patination processes on copper or steel.
There are other variations on this finish produced by layering of the chemistry not unlike layering of patinas on copper alloys. In a similar fashion, each layer must adhere, and this is accomplished when the chemical reaction occurs near or at the surface and has sufficient time for diffusion of zinc out from the base metal into the patina layer.
The parking garage at the Children’s Hospital of Richmond Pavilion on the VCU Campus is an excellent example of zinc variegations. The designer, HKS, wanted to create earth tones using the Roano finish patina process as a base. Several variations of the finish were designed into the parking garage façade to add color to the texture by layering patinas on some of the sheets.
Custom patinated zinc and aluminum panels adorn the parking facade for Children's Hospital Richmond Pavilion.
Others were blackened. Each of these finishes is a patina induced onto the zinc by creating a slightly different compound on the surface. These were not sealed or coated. The patina will age gracefully over time and exposure. The differences in color are mineral in form and will not wear away or fade.
Not unlike the patina on copper or weathering steel, these zinc patinas take time to develop. Each is produced in a controlled environment that enables the zinc ions to diffuse into a solution and react with other ions and form a thin adherent patina on the surface. The patina that develops is insoluble and bonds to the base metal, as on the Taubman Museum of Art in Roanoke, Virginia.
The Roano patina was originally developed for this project, designed by the late Randall Stout. He wanted a surface that related to the natural fall colors of the Blue Ridge mountains that surround the city.
It is critical to understand the patina that grows on zinc like that on copper or weathering steel is a chemical reaction. The ability to precisely match one sheet to another is equivalent to searching for pebbles in a stream that look exactly alike. You will get some close but never an exact match. However, the end result should be within a set family of color tones.
There have been instances when a range is required to be established. Producing range samples are an attempt to objectify whether something is acceptable or not. Such attempts to express a condition that is more abstract into concrete terms can lead to frustration. It is best to arrive at agreeable and controllable parameters that can actually be achieved.
Detail view of Zahner's preweathered Hunter Zinc surface.
Another patina, developed in a similar fashion, but somewhat simpler in make-up, uses the oxides of zinc to create an adherent marbled surface. This finish goes by the trade name Hunter™.
The Hunter Museum was also designed by Randall Stout. He was struggling with an economical surface material that would fit well with the dolomite cliffs of the Tennessee River. The museum was to be constructed on top of the cliffs overlooking the river. Stout wanted a surface that looked as if it was formed out of the rock cliffs.
Winding path to the entrance of the Hunter Museum.
The finish had been under development for several years. It all began with a restoration project of a Donald Judd art piece that had been damaged by exposure to moisture while in storage in London. The art piece was a series of large spatial forms in the shape of a cube. They were made from thin hot-dipped galvanized steel.
The galvanized surface had developed a thick, zinc hydroxide and zinc oxide crust over sections of the surface. Those of us that work with sheet metal inevitably experience the effects of moisture entering between coils or sheets of galvanized steel. The resulting stain, a whitish crust is tenacious and near impossible to remove without damaging the underlying zinc.
The Hunter Museum features Zahner's preweathered Hunter Zinc™.
Several months were spent working with methods to remove the stain from the artwork without harming what remained of the galvanized surface. The stain is superficial and does no harm. It actually adds a level of corrosion resistance to the surface of the work on the sculpture, an example of the tough, inert nature of zinc compounds and how once they are present, removal by any normal means is futile.
The zinc hydroxide was resistant to all mild solvents, acids, and bases. One could conjecture if this could be reproduced it would resist the milder effects presented by the natural environment and last indefinitely.
View of the Hunter Museum of American Art at sunrise.
The base surface of the Hunter zinc is a preweathered surface. The zinc hydroxide is “grown” onto and out of this base surface. The effect is a natural-looking, mottled color not unlike the dolomite cliffs the museum is built over. Dolomite, the mineral, has no zinc in its makeup; however, the mineral has a similar, hexagonal crystal structure like zinc. Dolomite is made of calcium magnesium carbonate and has a white marbling over a gray base, much like the marbling created on the preweathered zinc to produce Hunter.
The Hunter oxide surface has variations induced onto the surface. The zinc hydroxide must have time to develop on the surface as zinc ions diffuse from the surface and react with oxygen in the electrolyte to form into the oxide and hydroxide. The result is a white, thickened outgrowth from the zinc surface. This roughened white substance adheres to the underlying zinc. As it develops, differing thicknesses and intensities create variations on the surface of the darker zinc. In some cases, other compounds in the electrolyte form and the streaks can go dark in contrast to the lighter oxide.
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Processes of application can alter the exposure of the zinc surface to create the mottling in a selective manner. Once the oxide forms, it slows down the further development of the patina. This selective corrosion of the surface is neutralized to stabilize and limit further oxidation.
On interior applications, the surface is very stable and changes in appearance over time will be negligible. This finish can develop on formed panels or on sheets. The finish is not flexible but it is thin and very adherent. Post bending, the surface may create microfractures, but these are hardly visible.
The zinc hydroxide will not flake or rub off the surface. For the Art Gallery of Alberta, the panels were made into trapezoids, keeping a “water-line” horizontal, while breaking the vertical line. Each panel was patinated separately to further enhance the texture of the overall zinc surface.
There are other patinas that can develop on zinc, each with various levels of durability. As discussed previously, zinc is never found in nature in the native, pure state. Zinc is so intertwined into various minerals because of its drive to combine and form compounds. Patination can take advantage of this by introducing to the surface compounds that once form and stabilize into a barrier of enhanced color.
Patina technicians in the Zahner Dallas facility create the custom patina on zinc.
Producing these patinas requires a clean, oil-free surface. Some of the zinc sheet manufacturers treat the product with a light protective oil or thin organic film. This must be removed in its entirety for patination to work.
As with copper, developing the patina requires time for the chemical reaction to occur. The zinc surface is homogeneous, and once degreased and clean, the reaction forms over all the exposed surface of the sheet. It should be understood when developing patinas on metal, any initial oxidation treatment establishes a barrier to further oxidation. This can thwart subsequent patination processes and result in weak or spotty patina on the surface.
For any patina on zinc, it must adhere to the surface. Some are thick and will fracture on bending, others are thin and will allow shaping. The initial patina must etch into the surface, essentially becoming part of the surface of zinc. This is done by an activation process on the surface you need to “wake it up” in order to get the reaction. After the surface is active it will seek out other compounds. The delicate nature of this period of time is short and you must neutralize the reaction once it begins to stabilize the patina and to relax the zinc.
For zinc, the patinas typically develop dark earth tones or white oxides. The darker tones tend to lighten on exposure as zinc hydroxide or zinc chloride forms on the surface. For the most part, these patinas are produced cold rather than with the addition of heat like the copper patinas.
Some of these take several days to fully develop. Like copper and preweathered steel you want time for the zinc to diffuse out into the patina surface to make a metallic bond with the patina. Heating zinc to produce the patina needs to be kept at levels below 100°C. You do not want to dry out the surface nor damage the zinc.
Zinc, similar to other metals, can develop clear interference coatings of purples and golden bronzes with shades of browns and blacks. They are somewhat uneven and are produced by the reactions of the zinc surface to create a film of molybdenum oxide and zinc oxide. The color produced and its coating are durable and adherent.
The coating can be further burnished and protected with wax. The zinc surface needs to be clean and free of oxides, then immersed in a molybdenum solution for a few seconds. The result is an iridescent coating on the zinc. The color produced is highly variable. Purple and red hues, coupled with bronze to blacks, can be obtained.
Zinc B001, a concept surface from Zahner, is a preweathered zinc patina that rivals the appearance of blackened steel.
Blackened Zinc: The Classic Finish That Goes with Anything
Zinc can be blackened in several ways and is available in a matte, charcoal black color. The black produced is very even and consistent across the surface of each sheet of zinc. Black can also be applied to cast parts and even assembled shapes.
In sheet forms, blackening is performed during the preweathering process.
The blackened surface ages and lightens slightly as the surface is exposed to the atmosphere. Moisture that collects on the surface from condensation or water vapor in the ambient air will diffuse zinc ions through the darkened layer. These zinc ions will combine to form zinc oxide and zinc hydroxide on the surface. Zinc hydroxide is white in color, and these tiny white crystals will form on the dark surface.
The more challenging aspect with the blackened zinc surface is to be certain the white zinc hydroxide stain does not develop. This can happen when shavings of zinc are not collected after drilling holes. There have been occurrences where the drilled shavings were allowed to remain or fall behind the panel being attached. The shavings oxidized and streaked over the blackened surface creating unsightly white corrosion streaks over the black zinc.
Custom zinc ceiling soffits and staircase at the McMurtry Building in Stanford, California.
The Enduring Beauty and Performance of Zinc
No matter which zinc patina you ultimately choose, you can be assured of a truly unique surface that as time progresses, will gracefully age into an elegant and timeless cladding. With its constant desire to change, it will tarnish richly and rapidly as it absorbs moisture and oxygen, interacting with the environment to reveal a combination of minerals and patterns not easily found or replicated in other materials.
Artifacts and surfaces constructed of zinc are natural, beautiful, and durable, and give the designer and architect another unique surface that can be arresting, yet aesthetically pleasing at the same time. And when allowed to form rich patinas, zinc can and will perform for generations to enjoy.