Copper and its principal architectural alloys are relatively active metals which, when left unprotected, tend to oxidize. Long term atmospheric exposure in other than arid climates generally results in the formation of the naturally protective gray-green patina.
Because copper and its alloys afford a broad spectrum of both natural and weathered colors, much effort is expended to either hasten the natural weathering by chemical means or to preserve the bright natural colors through the application of clear protective coatings.
This section presents extensive reference data on the subjects of natural weathering, chemical coloring, clear organic coatings and opaque coatings both organic and inorganic. In addition, information is presented on the advantages and potential pitfalls of oiling, waxing and lead coating.Back to Top
The natural weathering of copper to the characteristic blue-green or gray-green patina is a direct consequence of the mild corrosive attack of airborne sulfur compounds. In the atmosphere, these compounds combine with water vapor to form dilute oxidizing acids which react with copper surfaces.
As natural weathering proceeds, the metal exposed to the atmosphere changes in hue from the natural salmon pink color through a series of russet brown shades to light and dark chocolate browns and finally to a dark, dull slate gray or dull black from which the ultimate blue-green or gray-green patinas emerge. The initial transition from the natural salmon pink color to the russet browns stems from the formation of copper oxide conversion films on the exposed metal surface.
During the initial weeks of exposure, particularly in a humid atmosphere or in areas of frequent rainfall, radical color changes often take place with iridescent pinks, oranges and reds interspersed with brassy yellows, blues, greens and purples. These sometimes shocking color variations result from the initial formation of the oxide surface films which are so thin that rainbow-hued interference colors are seen. As the natural oxide film builds in thickness during continued exposure, the interference colors fade and are replaced by relatively uniform russet brown shades.
Due to varying fabricating procedures, some mills may coat coiled or flat sheet stock with a thin coat of anti-stain oil film. This film may give rise to dark purple or black surface colorations soon after installation and exposure. This is a temporary color phase caused by the thin oil film, which is quickly washed off by rain allowing the natural weathering of copper to proceed.
As weathering progresses, cuprous and cupric sulfide conversion films are interspersed with the initial oxide film. These sulfide conversion films range from chocolate brown to black. As they build, the exposed metal surface darkens appreciably. Continued weathering results in the conversion of the sulfide films to the basic copper sulfate patina (see Natural Weathering Color Chart).
In industrial and seacoast atmospheres, the natural patina generally forms in from five to seven years. In rural atmospheres, where the quantity of air-born sulfur dioxide is relatively low, patina formation may not reach a dominant stage for 10 to 14 years. In arid environments, the basic sulfate patina may never form due to the lack of sufficient moisture to carry the chemical conversion process to completion. Similarly, exposed horizontal surfaces develop the patina more rapidly than sloping surfaces which, in turn, patinate more rapidly than vertical surfaces. The critical variable, in all instances, is the dwell time of moisture on the exposed surfaces.
The progressive oxide, sulfide and sulfate films which develop on copper exposed to the atmosphere are quite thin - two to three thousandths of an inch - highly adherent, but with relatively low abrasion resistance. Neither the oxide nor sulfide films are particularly corrosion resistant. The sulfate patina, on the other hand, is highly resistant to all forms of atmospheric corrosion, once it has had an opportunity to form completely. It thus significantly increases the durability and, hence, the service life of copper roofing and flashing. The natural weathering cycle of copper is illustrated by the 12 sequential color plates in Section 7, Finishes. Although the plates represent a typical sequence, the weathering of any installation will depend on local environmental factors, orientation and amount of residual lubricants.
As stated above, the natural weathering of copper to the ultimate sulfate patina yields the most durable installation, however, architects and/or building owners often express the desire either to inhibit natural patina formation, in an effort to preserve the uniform russet brown or chocolate brown shades of the oxide and sulfide conversion films or to hasten the patina formation through chemical means. There are potential drawbacks associated with both of these approaches which the architect or owner should be aware of before opting to proceed.
In architectural parlance, the natural forming or chemically induced oxide and sulfide conversion films on the surface of copper or its alloys are referred to as "statuary" or "oxidized" finishes. In color, they range from light russet brown to ebony. Chemical coloring of exposed flashings, chimney caps and similar small surface areas has been undertaken from time to time with reasonable success.
Because of the nature of the reaction of coloring solutions with the metal surface, application to large surface areas such as roofs, spires and domes is deemed impractical, since color uniformity is difficult to control and the hand application methods employed are expensive. Where large areas are involved, therefore, natural weathering to the desired shade is encouraged, at which point, further color change can be retarded through the application of oil or wax to the weathered copper surface.
Except in arid climates, copper usually weathers to a uniform russet brown within six to 12 months of initial exposure to the atmosphere. In arid climates, due to the absence of moisture, the weathering process is significantly slowed. It may require several years of exposure for the copper to achieve the desired shade. In addition, the multi-hued interference colors, previously mentioned, may persist for months rather than days or weeks.
At times, there is concern with regard to color variations during or immediately following installation of large expanses of copper roofing or flashing. Such color variations are common and usually transient, tending finally to disappear as natural weathering proceeds. There are two principal sources for such color variations. One involves the storage conditions from the time of manufacture until the time of installation, while the second involves exposure conditions following installation. For both conditions, time is an important variable. Moisture is also a key factor.
Once a copper sheet is sheared to size in the mill, it is usually packed on pallets with a lining of moisture resistant kraft paper. For coiled stock, the copper is palletized and protected with "shrink-wrapped" polyethylene sheets and desiccants. When stored in a heated mill or warehouse, oxidation (the principal cause of color change) proceeds at a slow rate, since the environment is relatively dry and the copper surface is usually still coated with a thin film of rolling oil.
The first significant oxidation of the metal may occur when it is shipped or unpacked in a warehouse. In transfer, the cases may be exposed to rain, snow or high humidity conditions on open loading docks. Temperature changes can cause condensation within the paper wrappings inside the case. With moisture present on the metal surfaces, oxidation accelerates. Lengthy storage in unheated premises can, and often does, produce similar results.
Once received by the installing contractor, the metal may be subject to storage conditions which promote continuing slow oxidation. Fabricated goods may be further exposed to the weather during shipment to and storage at the construction site prior to installation. When installed, oxidation is continuous, but the rate fluctuates depending on the amount and duration of moisture present, temperature changes, contaminants in the air, the sunshine and even wind velocity.
Since installation of large expanses of copper roofing may take weeks or even months depending upon weather conditions and working conditions, color variations can and frequently do result from differing lengths of exposure and climatic changes. Natural weathering eventually produces a uniform appearance in every case.Back to Top
Because of the time required for copper to weather to the ultimate blue-green or gray-green patina, men have sought for centuries to hasten the process by chemical means.
Coloring however is an art, mainly a matter of craftsmanship and experience. Chemical coloring techniques depend upon time, temperature, surface preparation, humidity and other variables which influence the ultimate result. A wide range of colored finishes may be produced on architectural copper-base alloys by conversion coatings that are chemical in nature. The metal at the surface is converted into a protective film, usually an oxide or sulfide of the metal involved, or a compound is precipitated which forms a surface film. The purpose is to hasten the natural weathered effect that generally results from exposure to the elements.
Several conversion treatments are in general use which produce the patinas (verde antiques) and statuary (oxidized) finishes.
Patinas are primarily developed using acid chloride treatments or acid sulfate treatments. Because of the number of variables involved, chemically induced patinas are prone to such problems as lack of adhesion, excessive staining of adjacent materials and inability to achieve reasonable color uniformity over large surface areas. These potential shortcomings should be considered when specifying such treatments. The following treatments have exhibited some degree of success:
Ammonium Chloride (sal ammoniac): A saturated solution of commercial sal ammoniac is brush-or spray-applied. Several applications may be required to obtain the desired result.
Cuprous Chloride/Hydrochloric Acid: Apply by spray, brush, or stippling. Store and use in nonmetallic containers. Wear suitable protective clothing and equipment. Consists of the following formulation:
- 164 g. cuprous chloride
- 117 ml. hydrochloric acid
- 69 ml. glacial acetic acid
- 80 g. ammonium chloride
- 11 g. arsenic trioxide
- Water to make one (1) liter
Ammonium Sulfate: Spray apply, six to eight applications. Consists of the following formulation:
- 111 g. ammonium sulfate
- 3.5 g. copper sulfate
- 1.6 ml. ammonia
- 1 liter water
Because production of an artificial patina on copper is dependent upon a number of variables, including temperature, humidity, wind velocity, surface condition of the copper and method of application, wide variations in the result achieved have been experienced. Reliability of all present methods can, at best, be considered only fair to poor.
Factory applied pre-patination systems are continuously being investigated for architectural applications. Contact CDA for updated information on such systems.
Statuary (oxidized) finishes are produced in light, medium and dark colors depending upon both the concentration and the number of applications of the chemical coloring solutions.
Two to ten percent aqueous solutions of ammonium sulfide, potassium sulfide (liver of sulfur), or sodium sulfide (liquid sulfur) are swabbed or brushed on the surfaces to be treated producing the statuary (oxidized) finishes from light to dark as desired. Oxide pretreatment may be employed to enhance adherence. Final hand toning or blending may be required to achieve acceptable color match and color uniformity.Back to Top
Coatings over architectural copper alloys can be described as one of two types of coatings: (1) transparent coatings to preserve the natural color, warmth and metallic tones inherent in these alloys, or (2) opaque coatings which utilize the basic properties of the alloy as a substrate to achieve corrosion resistance, longevity and forming capabilities.
Transparent Coatings: These coatings are designed to preserve the distinctive colors of the copper alloys. While copper alloys are extremely resistant to corrosion, a superficial discoloring tarnish results from weathering and handling. The natural appearance can be preserved by the application of thin, clear protective coatings. The coatings are essentially organic chemicals which are dry at ambient temperatures or require heat for curing or to hasten solvent evaporation. No coating, however good, can perform satisfactorily unless the surface is properly prepared to receive it and the application is performed immediately.
Numerous transparent coatings have undergone extensive testing to determine performance characteristics and compatibility with copper and copper alloys surfaces for interior and exterior applications. Test results and data sheets on the various tested coatings are available by request to CDA.
Oiling and Waxing: The primary purpose in oiling or waxing is to provide a barrier layer which excludes moisture from the copper surface and, hence, prevents the chemical conversion reactions from proceeding. Since oils and waxes tend to degrade and dissipate when exposed to the weather, reapplication at periodic intervals is required in order to successfully inhibit the natural weathering process. For roofing and flashing work, oiling predominates. Waxing is generally reserved for architectural components subject to close inspection and/or traffic. The oils are usually applied to the metal surface with the aid of a clean cloth, swabs or mops. The oil must be applied sparingly, otherwise the resulting tacky surface tends to attract and hold accumulateddust and dirt which detracts significantly from the desired appearance of the roofing or flashing.
The choice of oils is critical. Linseed oil, both boiled and raw has been widely specified for such purposes in the past. The use of this oil is still frequently referenced in the literature. Despite past acceptance, the suitability of the oil is open to question. Boiled linseed oil contains a small amount of varnish. When applied to copper surfaces exposed to the atmosphere, the varnish dries or sets producing a thin, hard, impermeable film. Since the film excludes moisture, it is desirable in this respect. Unfortunately, upon prolonged exposure to the elements the film degrades. As it breaks down, it becomes translucent, altering the appearance of the copper surface. More importantly, it peels off the copper surface unevenly. As a result, some areas of the copper are exposed and begin again to weather, while other portions continue to be protected by the varnish film. This ultimately results in a mottled or variegated surface coloration.
Raw linseed oil contains no varnish or driers. It remains tacky for an extended period of time following application. Failure to remove excess oil results in the accumulation of dust, dirt and debris over the copper surface. In addition, excess oil remaining in contact with the copper for any length of time reacts with the copper to produce insoluble organic copper salts which are characteristically dark green in color. The formation of these salts defeats the basic purpose of oiling; that is, to retard natural patina formation.
Crude oil has been used in the past with some degree of success. When applied with a heavy hand, it tends to be a dirt catcher. In recent years, evaluation of various oils has led to the tentative conclusion that, as a group, high grade paraffin oils, which are light-bodied petroleum distillates, are best. They are readily available, easy to apply, provide a reasonable degree of protection, and, when applied sparingly, are less likely to accumulate dust and dirt. One potential drawback associated with the paraffin oils is the fact that, since they do contain paraffin wax, the wax is deposited on the metal surface. Frequent and/or heavy applications tend to produce a build-up of wax on the metal which alters the color of the surface and produces streaking.
Where oiling is employed on copper roofing or flashing installations, reapplication as infrequently as once every three years can effectively retard patina formation. In arid climates, the maximum time span between oilings may be extended to from three to five years.
Opaque Coatings: These coatings are used primarily for work applied over copper when substrate integrity and longevity are desired but a specific color other than the naturally occurring copper hues is required.
Paints: Copper is an excellent substrate for a painted finish. Prior to painting, the surface must be free of grease, oil, dirt, fingerprints, drawing compounds and surface passivation treatment chemicals.
A first coat of an industrial wash primer should be applied according to the specific manufacturer's recommendation. The finish coat should be two coats of an oil alkyd enamel designed for exterior (or interior) use. Appropriate finish coatings are industrial enamels and silicone alkyd enamels of the specified color.Back to Top
The natural weathering of copper from its bright pink to the characteristic blue-green color (patina) is a direct consequence of its reaction to the corrosive action of the atmosphere. In industrial and seacoast atmospheres, the natural patina generally forms in 5 to 7 years. In rural atmospheres, with relatively clean air, patina formation may occur in 10 to 14 years. In arid desert environments, the patina formation may never occur and the copper surface may remain a dark brown or bronze color.
The large number of requests by the architectural community for a man-made patina has prompted US copper mills to research and develop pre-patinated copper sheet products. The patina is a chemical conversion process whereby the top molecular surface of the copper sheet is enhanced and forced to produce a natural patina.
Currently there are three brass mills who produce a factory applied patina. Each product is produced using a different method and the resulting finishes differ in their hues and colors. It is therefore recommended that the architect review each product prior to final selection. Complete specifications and samples can be supplied by contacting each of the manufacturers.Back to Top