The Semantic Metal Surface is an article which outlines the sustainable aspects of various metals. Written by L. William Zahner, the piece was originally published in the architectural quarterly, “Forward, The Architecture & Design Journal of the National Associates Committee.”
When defining a logic to use a particular surface material, various aesthetic qualities such as color, texture, patterns and boundaries are often considered. In our pursuit to arrive at materials that perform over a lifetime and do not possess hidden cost to our children's future, considerations of manufacture and eventual recovery and recycling of the material must also play a part. Architectural metals achieve these design requirements. They are durable and lightweight. They can be formed, shaped, pierced, cut and machined in ways only plastics can attempt to copy.
Architectural metals are the family of materials that encompass aluminum alloys, copper and copper alloys, brasses and bronzes, iron and steel alloys including stainless steels, lead, tin, titanium, and zinc.
Each of these metals has a vast array of finishes and textures that add color and interface with light like no other substances on earth. Many of these metals can be coated with other metals to enhance their performance or aesthetic appeal. For example, zinc in the process of galvanizing provides tremendous benefit via galvanic protection to steel. Aluminum and steel are often painted to provide a particular color while adding a barrier to prevent the ambient conditions from affecting the base materials’ performance. In these cases, metals act simply as an affordable ductile form.
Stainless steels are naturally reflective. Oxidation does not develop very rapidly and the surface remains very smooth. Having a tight, smooth surface enhances the corrosion resistance of stainless steel. The more the surface is polished, the better the long-term performance.During the production of stainless steels, the mill surface of the thick plate is cleaned of scale using strong acids. This process, known as pickling, dissolves the heavy oxides and free iron carbides that rise to the surface of the hot plate.Further reduction of the stainless-steel thickness is performed on cold semi-polished rolls. These rolls impart a smooth, reflective sheen on the surface of the metal. This initial sheen is the base for all subsequent finishes. The surface is designated as a No. 2B finish. Dull sheens can also be developed using rolls that have a dull surface. These initial finishes are known as No. 2DThe more specular No. 2B finish can be further enhanced by special annealing processes. Annealing the stainless steel in a controlled atmosphere will create a mirrorlike surface known as Bright Annealed, designated as No. 2BA. The No. 2BA can be the base surface for glass bead, No. 8 and No. 9 mirror surfaces, as well as fine satin finishes. The No. 2BA will provide a consistent color and surface for the more refined surfaces.The reflective character of the various stainless-steel finishes can be divided into three categories as shown in Figure 2.4. The Reflective Finishes can be described as those that reflect light similar to a mirror. A bright light will reflect as a "hot spot." The angle of incidence equals the angle of reflection. Very little scattering of light occurs. The No. 9 finish is the equivalent to a mirror on one scale while the No. 2B finish is somewhat smoky.Disturbing the even reflective surfaces with minute surface fractures or indentations, which scatter the reflected light slightly, produces the Diffused Reflective Finishes. Because these finishes are typically applied over the Reflective Finishes, they possess a brightness, an almost glowing behavior when in strong light.The Low Reflective Finishes possess a dull reflection. Light is effectively scattered by the rough surface. and these surfaces appear flat in most light.Because of its chrome content stainless steel reflects 49 percent of the visible wavelength of light. It is much more heavily weighted toward the blue wavelength and captures well the tone of the sky. On cloudy days, stainless steel will appear with very little luster. This is due in part to the scattering effect of the clouds, which reduces the blue segment of the wavelength of light reaching the stainless-steel surface.Because of the specular nature of stainless steel surfaces, slight variations in plane can affect the relative color. Moving around the surface changes the angle of view from one surface of a plate or panel relative to another. The more direct the reflection, the lighter the color. The panel that is slightly askew will appear darker in strong light. A stainless-steel surface can look dark from one angle of view, then light in color from a different angle of view. The difference can be only a few degrees out of plane. This faceted reflection is common in stainless steel thin-plate surfaces. This is also why "oil-canning" tendencies are greater in reflective metals such as stainless steel. The relative high and low points in a stainless-steel surface reflect light back to the viewer at varying angles, which create apparent visual distortions in the surface.When fabricated and installed correctly, the reflection is not distorted by the undulating surface. Light washes over the diffused reflective surface in straight lines. The mirror reflective surfaces show straight lines as straight images and not curved images.Stainless-steel surfaces, particularly the diffused finishes such as No. 4 satin, angel hair, and glass-bead-blast surfaces, reflect the colors and shadows of the surrounding environment, but in a more scattered, subdued fashion. Visual defects such as chatter are more visible in reflective stainless-steel surfaces than in most other materials. Chatter defects are caused when the polishing belts slip during the application of the finish. Whether the finish is a linear satin finish, such as a No. 4 or No. 3, or a mirror finish, slight reflective differences caused by the slipping of the polishing belts will create a series of visual lines of distorted reflection. The viewer cannot feel the distortion, but it is apparent when viewing the surface at an acute angle. These distortions are not repairable.
Stainless steel, titanium and to a lesser degree aluminum, are known for their unchanging surface chemistry. They react with the surrounding environment, for the most part, at a very slow rate. Their oxides develop rapidly and resist additional surface attack. Other metals, such as copper and copper alloys, zinc, and the weathering steel alloys, are left exposed to react with the surrounding environment. These metals combine with substances in the air and develop very tenacious surface oxides. These oxides enhance the appearance of the metal and provide extremely impervious barriers. The barriers are very close to inert mineral forms that are found in nature.
These inorganic surface coatings, commonly known as patinas, develop as the metal is exposed to external pollutants such as carbon dioxide, chlorine and sulfur. When you think about it, a copper roof is removing carbon dioxide and sulfur from the atmosphere and trapping it in inert mineral compounds formed on the metal surface.
The metals used in architecture will not end up in some future waste heap because of the inherent value they possess. The metal recycling business worldwide is a robust industry employing many thousands of people. No other materials used in building construction are so thoroughly recaptured and recycled for use over and over again than metals. Environmental issues surrounding the mining and concentration of metals are valid but often are taken out of context. Efforts are being made within the industry to address long term affects of metal mining and processing. Recycling of metals reduces the need for mining and reprocessing recycled materials uses significantly less energy. Aluminum recycling, for example, has become a substantial secondary business.
Reducing the ravages on the environment caused by mining, recycling bypasses the large ore refining costs. Aluminum refinement requires tremendous amounts of electricity, some 20,000 kilowatt hours per ton of aluminum refined. Most small towns use less electricity per year than aluminum refinement uses per day. The aluminum scrap recycling industry claims that recycled aluminum saves up to 80 million tons of greenhouse gas emissions per year.
The same can be said for other metals. Over 80% of the copper used to create the beautiful façade on the de Young Museum of Art in San Francisco was derived from recycled scrap metal that was recast and turned into sheet copper. Every single perforation and sheared edge left over from the process of creating the elaborate panels was collected and recycled at the fabrication facility.
Copper is a malleable and ductile metal with high thermal conductivity. A freshly exposed surface of pure copper has a reddish-orange color, which will quickly patinate under normal conditions.New copper can look as if it is on fire in low twilight, when the red wavelength of light is more prevalent. The surface of new copper is specular, moderately polished from the cold rolling process, imparting a burnishing effect on the copper. Like gold, copper absorbs most strongly on the red region of the light spectrum, hence reflect more intensely on the red scale. Thus, cloudy days have only a minor effect on the reflected light from gold and copper surfaces.Eventually, the surface of copper will oxidize and decrease in reflectivity, giving way to earthy tones of a dark matte, surface. After several years, the appearance will show signs of green verdigris in its crevices, and may completely turn green over the course of decades.
Copper has an infinite recycled life. It can be used over and over again. In the event the de Young is ever dismantled, one can be certain the surface will be recycled and used on the next great museum façade. Can you say this of other building materials? There are no significant recycling efforts underway for stone, concrete, glass, fiberglass or rubber membranes. Wood and brick have levels of secondary recycling potential but not anything remotely comparable to the infinite recycling ability of aluminum, copper, steel, titanium and zinc.
Metals are available in many forms designed to take advantage of the inherent character only metals possess. Metals can be rolled into extremely thin sheets, even foils, which have directional attributes such as grain, tensile strength and ductility. Even in these thin forms, corrosion resistance is not compromised. When correctly assembled, thin skins of metal can distribute the stresses that develop from changes in the ambient conditions without affecting long-term behavior. Creating thin surfaces of metal allows for optimizing the material usage while achieving very flexible, yet durable, lightweight enclosures. This attribute of metal is the reason why aircraft and automobiles are shrouded in metal skins.
For intricate building surfaces, metals offer similar advantages over other materials. Metal roofing has long been a lightweight surfacing material that provides protection from the environment. At the same time, metal can be a significant design element used to define the building geometry and establish the aesthetic image.
Technological advances in fabrication processes have taken thin flexible sheets of metal and created stunningly intricate wall surfaces for buildings. Perforating, pressing and forming of metal provides the designer a visual and tactile interface to offer his client and the public to experience. Incorporating shape and texture is no longer a significant ‘artistic’ premium.
Thin, inexpensive surfaces of metal can enclose a building geometry and offer a lifetime of performance with little maintenance. A common means of achieving a metal surface is to assemble smaller elements known by various terms as skins, cassettes, or shingles. These thin, flexible elements allow for intricate surfaces to be enclosed without compromising the long- term performance of the metal. Each shingle acts like a scale on a fish, overlapping and engaging into the adjoining shingle. Stresses do not pass over to the next panel but are released at each edge.
To make the thin surfaces work efficiently, close attention to the edges are necessary. The edges are what the eye captures and most inconsistencies will manifest themselves at the boundaries. They can destroy the appearance, allow moisture to enter behind the metal surface, and add unnecessary clutter to the overall appearance. When skillfully executed, the edges define the surface geometry and allow for the control and distribution of stresses and moisture.
The tendency is to apply covers to overlap the edges of large sealant joints. It is cheaper, quicker and for the most part, it will deter moisture, but it will affect the aesthetic. It will require adjustment and reseal at some point and often can be less affective in performing the function of keeping air and water from entering behind the wall surface. It can be like having patches on a fine suit. One spends the money on the cloth but hires a less skillful tailor to assemble the suit.
L. William Zahner, CEO/President of A. Zahner Company, has worked with many of the world’s leading architects, including Frank Gehry, Antoine Predock, Herzog and de Meuron and Tadao Ando. He has contributed to a number of high profi le projects using metal as a major building material, including the Guggenheim museum in Bilbao, Spain, the Experience Music Project in Seattle and the de young Museum in San Francisco.
- 1. Composite materials that combine metal with plastic cores are not currently recycled. Thus, when their useful life expires, they are sent to the landfill — a true waste of metal.
- 2. Recycled aluminum uses less than 4% of the energy needed in the aluminum refining process. It is predicted by the year 2020 over 30 million tons of aluminum will be from recycled scrap. This is equivalent to 18 years of primary production. Source: Recycle Scrap Industry.
- 3. Excluding copper wire which often is made from refined copper ore, over 75% of the copper used in castings, sheet, brass and bronze work is recycled. Source: Copper.org