Copper and Its Alloys: Exciting Options for Wall Cladding
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rom ancient times to the mod- annual NACIA awards program. copper weighs 16 oz. per square foot, and ern day, copper and copper Why the trend? One reason is increased 20-oz. copper weighs 20 oz. per square foot alloys have proven to be reli- interest in green building. Not only is copper and is, thus, thicker. able building materials that 100% recyclable, the sheet copper used for These virtually pure copper alloys are can withstand the test of time. wall cladding is often created largely from not the only game in town. Think back to European cathedrals and scrap material, and it can be used over and high school science or history classes and FJapanese temples built hundreds of years over with no loss of its engineering prop- recall the Bronze Age. Early civilizations ago still have their original copper roofs. erties. discovered a small amount of tin mixed Even the nearly 200,000-pound copper- into molten copper creates an exceptionally cladded Statue of Liberty has remained TYPES OF ALLOYS strong, durable metal: bronze. Although a intact despite over 100 years of corrosive The various colors, surface textures, wide variety of these bronze alloys are still pollution, biting sea winds, driving rains, and product forms of copper allow archi- available today, it’s another copper alloy and beating sun. tects and contractors to constantly create family garnering most of the attention for Now, the protective living patina is wind- innovative designs. As with other metals, wall cladding: brass. Brasses are combina- ing its way around the sides of university, there are many intermetallic mixtures of tions of copper and zinc. medical, government, and residential struc- copper with other elements. The most com- Although common brass alloys do not tures and adding to its robust portfolio. mon sheet copper wall claddings use only a have the strength of a bronze, they are Architects and contractors are selecting cop- small number of copper alloys, all of which slightly stronger than the base copper, per as a cladding material for its durability, consist of 99.5% copper (with traces of silver while retaining considerably more mallea- flexibility, and malleability. It is also light- allowed). When considering copper, system bility compared to most bronze alloys. Most weight, fire-resistant, corrosion-resistant, designers should specify sheet copper to recognizable for their more golden color that and available in an array of finishes and col- American Society for Testing and Materials increases with the amount of zinc, the metal ors. These attributes have allowed architec- (ASTM) Standard B370. It defines compo- has primarily been used architecturally for tural creativity to flourish beyond the limits sition, dimensional tolerances, mechanical storefronts, doorways, and interiors. Now, of traditional wall cladding systems. properties, and characteristics to ensure the metal is transitioning into one of the It’s a dramatic movement—one that’s that the material with the correct properties most popular copper alloy cladding mate- clearly evident in the submissions for the is specified. rials. Brasses commonly available in sheet North American Copper in Architecture The most popular temper designations form include an increasing amount of zinc, Award (NACIA) program, hosted annually for sheet copper for wall cladding are “H00” and thus “gold” tone: C22000 “commercial by the Copper Development Association or “H01” (“cold-rolled” or “1/8 to ¼ hard”) bronze” (10% zinc), C23000 “red brass” (CDA) and the Canadian Copper and Brass and “H02” (“½ hard”) if it’s a system where (15% zinc), C26000 “cartridge brass” (30% Development Association (CCBDA). In fact, higher tensile strength but less malleabili- zinc), and C28000 “Muntz metal” (40% adoption of copper as a wall cladding mate- ty is helpful. It is still common to refer to zinc). ASTM Standard B36 is the appropri- rial has become so prevalent that it often sheet copper thickness in traditional ounce ate standard for brass alloys. It is best to outnumbers roofing submissions in the weight thickness. As an example, 16-oz. refer to brass thickness in decimal inches— 1 8 • I N T E R F A C E N O V E M B E R 2 0 1 5 for instance, “.032 in.” or “.040 in.,” select- bare copper or brass is just a teaser. It rate depends on exposure, of course, but ing a thickness depending on project may look great, but it’s going to change. generally, tones are similar to copper while requirements and availability. Chemicals in the atmosphere cause the emerging more slowly. After many years, it As with any metal (copper or other- metallic surface tone to oxidize, transform- is difficult to tell a color difference among wise), it is important to consider the right ing from orange through deepening russet weathered brass, bronze, or copper alloys. brass alloy based on service conditions and tones, rich browns, to black, and possibly— There are few modern brass alloys con- system design. It is especially important if there is enough exposure—to a blue-green taining percentages of aluminum, specially for design professionals and product engi- patina. Hues can vary from panel to panel formulated, which retain a golden tone con- neering staff to review product use with or perhaps even within the bounds of a siderably longer. alloy-producing technical staff when consid- panel. The Irving Convention Center at Las ering brass alloys over 15% zinc, especially In industrial and seacoast atmospheres, Colinas, in Irving, Texas, which won a in a salt-spray zone, in a high-pollutant the natural patina generally forms in five to 2015 NACIA award, is covered with a mill- area, or when the product may experience seven years. In rural atmospheres, where finish copper façade. The raw “red” copper bending or cyclical stresses. There are spe- the quantity of airborne sulfur dioxide is rel- surface is already moving through a long cial processes and alloy formulations, less atively low, patina formation may not reach process of patination. Since wall surfaces seen but more appropriate, for extreme con- a dominant stage for 10 to 14 years. In are vertical and atmospheric conditionals in ditions (such as naval or admiralty brass, arid environments, the basic sulfate patina central Texas are more arid than most east- developed for saline conditions.) may never form due to the lack of sufficient ern areas of North America, the wholesale moisture to carry the chemical conversion weathering past the deep brown/black hues PATINATION process to completion. The protective chem- will be extremely slow. Designing with copper often involves ical reaction occurs when a corrosive attack For centuries, builders have sought to designing for the long term, both in relation of airborne sulfur compounds leads to a hasten the weathering process by chemical to appearance and performance. Although gradual change in the surface color until means. Because of the number of variables it’s possible to paint copper, most prefer the equilibrium is reached and the change is involved, chemically induced patinas are natural weathered appearance. It’s free, gets stabilized. prone to problems because of lack of adhe- better with time, and never needs recoating. When exposed to the elements, brass- sion, excessive staining of adjacent mate- The metallic sheen of recently installed es and bronzes weather, too. 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Local contractors often fabricate with more common metals. Double- sheet products. Their proprietary methods and install these proven systems, saving lock, standing-seam copper panels involve a chemical conversion process to an extra layer of shipping-related costs cover both the third- and fourth- force a chemical patina at the top molecular and adding flexibility for customization and floor vertical walls and mansard surface of the copper sheet. Significantly, adaptation to site-specific conditions. roof. Standing-seam panel vertical prepatinated products are created in a As with roofing, edge condition and joints line up with the windows on controlled factory environment, removing penetration flashing require special design the third floor—a critical require- the guesswork involved with field-applied attention and supervision at the job site. ment of the design. Because it works chemical solutions. Product manufacturers Fenestrations are especially critical, but great for both walls and roofing, a supply specifications and samples.