Patinizing Exterior Copper Elements
- CSI Division:
- Division 5 - Metals
- Ornamental Copper
- Last Modified:
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Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
A. This procedure includes guidance on patinizing exterior copper architectural elements when these have been replaced with new ones and need to match the color of the existing surface or other elements.
B. The development of the natural green patina on a copper sheet or architectural element can take from ten to twenty years, depending on the location and the atmosphere. To speed up this process, patina can be produced artificially—by chemical solutions. The patina solution listed in this procedure includes the mixture of copper ammonium chloride, ammonium sulfate and water. Note: There are numerous types of patination formulas recommended for different uses. For a list of some of these formulas, see 05030-02-S. The RHPO should be consulted for guidance in selecting an appropriate formula.
C. See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO).
A. Copper is initially bright reddish-brown in color, but when exposed to the atmosphere, it acquires a protective patina that turns from brown to black to green over an 8 to 10 year period.
B. The patina is a copper carbonate or copper sulfate formed on the surface of the metal when hydrogen sulfide combines with oxygen or sulfur dioxide. The patina is actually a thin, tough layer of natural corrosion that usually prevents deeper and deeper layers of corrosion (such as rust, which can totally consume iron) to form because of further exposure to the atmosphere; therefore, even though copper corrodes, it is corrosion-resistant.
C. The color of antique copper, which is a little more orange than that of new bronze, was much admired in the late 19th century. Victorian cast-iron hardware was sometimes copper plated, although brass-plated hardware was more common. Cast-iron stair railings and newel posts were sometimes copper plated.
1.03 PROJECT/SITE CONDITIONS
A. Environmental Requirements
- Do not attempt patinizing of the copper in raining or foggy weather.
- Ideal relative humidity for patinization is between 85% and 100%.
NOTE: Chemical products are sometimes sold under a common name. This usually means that the substance is not as pure as the same chemical sold under its chemical name. The grade of purity of common name substances, however, is usually adequate for stain removal work, and these products should be purchased when available, as they tend to be less expensive. Common names are indicated below by an asterisk (*).
A. Copper Ammonium Chloride
- Ammonium chloride is a white crystalline volatile salt that is used in dry cells and as an expectorant.
- Other chemical or common names include Ammonium hydrochloride; Chloride of Ammonia*; Hydrochloride of Ammonia*; Muriate of Ammonia*; Sal Ammoniac*.
- Potential hazards: TOXIC: INHALATION, CONTACT, INGESTION; CORROSIVE TO FLESH; CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS.
- Available from chemical supply house, dry cleaning supply distributor, drugstore or pharmaceutical supply distributor, or hardware store.
B. Ammonium Sulfate
- Other chemical or common names include Sulfate of ammonia*.
- Potential hazards: TOXIC; CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS.
- Available from chemical supply house, drugstore or pharmaceutical supply distributor.
C. Sal ammoniac (a paste in cake form): See ammonium chloride above.
D. Trisodium Phosphate (TSP): TSP substitute includes sodium carbonate with zeolites added. Note: This chemical is banned in some states such as California. Regulatory information as well as alternative or equivalent chemicals may be requested from the Environmental Protection Agency (EPA) Regional Office and/or the State Office of Environmental Quality.
- Strong base-type powdered cleaning material sold under brand names.
- Other chemical or common names include Sodium Orthophosphate; Tribasic sodium phosphate; Trisodium orthophosphate; TSP*; Phosphate of soda* (also sold under brand names).
- Potential Hazards: CORROSIVE TO FLESH.
- Available from chemical supply house, grocery store or supermarket or hardware store.
E. Clean, potable water
A. Soft wiping cloths
B. Soft bristle brushes
C. Heavy gloves and protective gear
D. Paint spray gun
A. Before proceeding with steps to artificially patinize copper elements, it is important to first inspect and check for other damages. Clean surface and/or repair these first.
A. Wipe the surface clear of dust and other debris with a clean soft cloth or brush.
B. Remove all greasy patches by swabbing with a mixture of trisodium phosphate in water. Rinse the treated area with clean, clear water and allow to dry.
C. If possible, rather than washing new copper, allow it to weather through two or three heavy rains, but not longer than three months.
D. Allow the copper sheet/architectural element to dry before application of the patina formula.
E. Mix a solution composed of 6 ounces of copper ammonium chloride, 3-1/2 ounces of ammonium sulfate, and 1 quart of water. NOTE: The double salt, copper ammonium chloride produces a pure green patina, and the addition of ammonium sulfate to the solution gives the true-green shade of the naturally formed patina.
F. Apply the solution when outside conditions are dry by using a paint spray gun to produce a fine dispersion of the liquid at a concentration of 1 pint per 5 square yards.
- At this concentration the surface should be covered with a uniform layer of droplets. Avoid undue coalescence of the droplets, which may occur if too much liquid is applied.
- If the humidity is low, dampen the prepared surface with a fine mist or spray of water. Spray the surface so that it will dry within 5 minutes. Repeat this process at intervals of 2 hours or more until the required patina has formed.
G. After waiting, the surface should be free from any appreciable wetting until a certain amount of weathering takes place. Premature wetting makes the patina flake and peel. The best results are obtained with a relative humidity of between 85% and 100%.
H. To age small new pieces in order to match the old, apply a mixture of sal ammoniac (a paste in cake form) and water to the surface. NOTE: Sal ammoniac is used to clean soldering irons and it is not harmful to copper.