Removing Black Stains from Exterior Copper

CSI Division
Division 5 - Metals
Section
Copper
Last Modified

Technical Procedures Disclaimer

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.

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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.

PART 1–GENERAL

1.01 SUMMARY

  1. This procedure includes guidance on cleaning blackened exterior copper. When copper weathers, it naturally develops a green-grey patina. Blackened encrustations may result from several things, including contact with chemicals, contact with incompatible metals or air pollution. The resulting stains are generally not harmful to the copper, though they may be aesthetically objectionable.
  2. See “General Project Guideline” for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
    1. Safety Precautions
    2. Historic Structures Precautions
    3. Submittals
    4. Quality Assurance
    5. Delivery, Storage and Handling
    6. Project/Site Conditions
    7. Sequencing and Scheduling
    8. 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).
  3. For additional information on the characteristics, uses and problems associated with copper, see “Removing Black Stains From Exterior Copper”.

PART 2–PRODUCTS

2.01 MATERIALS

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 (*).

  1. Phosphoric Acid:
    1. A syrupy or deliquescent tribasic acid used especially in preparing phosphates (as for fertilizers), in rust-proofing metals, and as a flavoring in soft drinks.
    2. Other chemical or common names include Metaphosphoric acid; Orthophosphoric acid; Phosphoric acid.
    3. Potential Hazards: CAUSTIC TO FLESH; CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS.
    4. Available from chemical supply house or hardware store.
  2. Nitric Acid (HNO3):
    1. A corrosive liquid inorganic acid used especially as an oxidizing agent, in nitrations, and in making organic compounds such as fertilizers, explosives and dyes.
    2. Potential Hazards: CAUSTIC TO FLESH; CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS.
    3. Available from chemical supply house or hardware store.
  3. Sodium Bicarbonate:
    1. Other chemical or common names include baking powder*; baking soda*.
    2. Available from grocery store or supermarket, or drugstore or pharmaceutical supply distributor.
  4. Ammonium Oxalate:
    1. Other chemical or common names include Oxalate of ammonia*.
    2. Potential Hazards: TOXIC.
    3. Available from chemical supply house, drugstore or pharmaceutical supply distributor.
  5. Clean, potable water

2.02 EQUIPMENT

  1. Cellulose sponge mop or similar applicator
  2. Clean soft wiping cloths
  3. Heavy gloves and protective gear

PART 3–EXECUTION

3.01 ERECTION/INSTALLATION/APPLICATION

NOTE: TEST CLEAN A SMALL INCONSPICUOUS AREA BEFORE ATTEMPTING TO CLEAN LARGE AREAS.

NOTE: WHEN CLEANING LARGE SURFACE AREAS, EMPLOY THE USE OF PROFESSIONAL HELP.

  1. Mix 6 parts concentrated phosphoric acid with 1 part concentrated nitric acid, diluted at least 50% with clean potable water. The mixture should have a pH between 1 and 1.5.
  2. Apply the solution to the stained copper surface using a cellulose sponge mop or similar hand applicator. Let the solution sit for approximately 1 minute.
    CAUTION: AVOID APPLYING CLEANING SOLUTION TO METAL IN DIRECT SUN, AS IT BECOMES VERY HOT, AND THE DETERGENT OR SOAP SOLUTION WILL DRY IN STREAKS BEFORE IT IS POSSIBLE TO RINSE IT OFF. THESE STREAKS CAN VERY EASILY BECOME PERMANENT AND CANNOT BE REMOVED WITHOUT ABRASION.
  3. Remove the solution by wiping the surface with a sponge soaked in sodium bicarbonate. Follow this by wiping the surface with a sponge soaked in ammonium oxalate.
  4. Thoroughly rinse the surface with clean, clear water and dry with clean, soft cloths. Repeat the process as required to achieve the desired level of cleanliness.

 

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Last updated: Mar 5, 2024
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