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Historic Preservation - Technical Procedures

Spectitle:

Removing Black Stains From Exterior Copper

Procedure code:

0501501R

Source:

Ohj - Staff

Division:

Metals

Section:

Copper

Last Modified:

02/24/2012

Details:

Removing Black Stains From Exterior Copper



REMOVING BLACK STAINS FROM EXTERIOR COPPER


PART 1---GENERAL

1.01 SUMMARY

    A.   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.

    B.   See 01100-07-S 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).

    C.   For additional information on the characteristics, uses
         and problems associated with copper, see 05015-01-S.


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 (*).

    A.   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;
              Phrophosphoric acid.

         3.   Potential Hazards:  CORROSIVE TO FLESH; CORROSIVE
              TO CONCRETE, STEEL, WOOD OR GLASS.

         4.   Available from chemical supply house or hardware
              store.

    B.   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:  CORROSIVE TO FLESH; CORROSIVE
              TO CONCRETE, STEEL, WOOD OR GLASS.

         3.   Available from chemical supply house or hardware
              store.

    C.   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.

    D.   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.

    E.   Clean, potable water

2.02 EQUIPMENT

    A.   Cellulose sponge mop or similar applicator

    B.   Clean soft wiping cloths

    C.   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.

    A.   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.

    B.   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.

    C.   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.

    D.   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.

                         END OF SECTION