Copper and its alloys can be finished to embark a particular look, feel, and/or color. Finishes include mechanical surface treatments, chemical coloring, and coatings. These are described here.
Mechanical surface treatments. Several types of mechanical surface treatments exist. Mill finishes are derived by normal production processes, such as rolling, extrusion, or casting. "Buffeted" finishes impart bright mirror-like appearances after grinding, polishing, and buffing. "Directional textured" finishes provide a smooth, velvety satin sheen in a continuous pattern of fine nearly parallel scratches. "Non-directional textured matte" finishes achieve a rough texture, primarily on castings, as spray sand or metal shot are applied under high pressure. And "patterned" finishes, made by pressing a copper alloy sheet between two rolls, produce a textured and embossed look.
Chemically induced patina. Architects sometimes request a particular patina color at installation. Factory-applied chemically induced pre-patination systems can produce a wide range of colored finishes similar to natural patination. Pre-patinated copper is particularly useful in repairs when there is need to provide close color matches to old copper roofs. Pre-patination is also considered in some modern building materials, such as vertical cladding, soffits, and gutters where patination is desired but would not ordinarily occur.
Chemical coloring of metals is an art involving craftsmanship and experience. Coloring techniques depend upon time, temperature, surface preparation, humidity and other variables. Pre-patinated copper sheets are produced by fabricators under controlled environments using patented chemical processes. Green patina finishes are primarily developed using acid chloride or acid sulfate. Treatments with ammonium chloride (sal ammoniac), cuprous chloride/hydrochloric acid, and ammonium sulfate are somewhat successful. Statuary finishes can be produced in light, medium, and dark brown, depending on the concentration and number of coloring applications. One advantage is that the treatment disguises surface markings on bright mill finish copper and can advance the natural patination process.
Because of the number of variables involved, chemically induced patinas are prone to problems such as a lack of adhesion, excessive staining of adjacent materials, and inability to achieve reasonable color uniformity over large surface areas. Chemical patination applied in the field is not recommended due to variations in temperature, humidity, and chemical requirements. Warranties are prudent when purchasing pre-patinated copper for architectural projects.
Useful techniques and recipes for coloring copper, brass, yellow-brass, bronze, cast bronze, gilding metal, along with various physical and chemical textural finishes are available.
Coatings. Clear coatings preserve the natural color, warmth and metallic tone of copper alloys. However, particularly on exterior applications, they introduce maintenance into what is naturally a maintenance-free material. They are organic chemicals that are dry at ambient temperatures or require heat for curing or solvent evaporation. Examples of clear organic coatings include alkyd, acrylic, cellulose acetate butyrate, epoxy, nitrocellulose, silicone, and urethane. Further details are available.
Oils and waxes exclude moisture from copper surfaces and simultaneously enhance their appearance by bringing out a rich luster and depth of color. Oiling is typically used to prolong the time exposed copper remains in a brown to black tone. It will not keep copper shiny on an exterior installation. Oils and waxes offer short-term protection for exterior applications and longer-term protection for interior applications.
Oiling predominates for roofing and flashing work. The most popular oils are Lemon Oil, U.S.P., Lemon Grass Oil, Native E.I., paraffin oils, linseed oil, and castor oil. On copper roofing or flashing, reapplication as infrequently as once every three years can effectively retard patina formation. In arid climates, the maximum span between oilings may be extended to from three to five years.
Waxing is generally reserved for architectural components subject to close inspection and/or traffic. Mixes considered satisfactory include Carnauba wax and wood turpentine, or beeswax and wood turpentine, or paste waxes.
Opaque paint 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.
Zinc-tin coatings are an alternative to lead coatings since they have approximately the same appearance and workability.
Vitreous enamel coatings are used primarily for art work over copper.
More details on copper finishes are available.