Removing Black Stains From Exterior Copper
- Procedure code:
- Ohj - Staff
- Last Modified:
REMOVING BLACK STAINS FROM EXTERIOR COPPER
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
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
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.
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;
3. Potential Hazards: CORROSIVE TO FLESH; CORROSIVE
TO CONCRETE, STEEL, WOOD OR GLASS.
4. Available from chemical supply house or hardware
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
2. Potential Hazards: CORROSIVE TO FLESH; CORROSIVE
TO CONCRETE, STEEL, WOOD OR GLASS.
3. Available from chemical supply house or hardware
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
2. Potential Hazards: TOXIC.
3. Available from chemical supply house, drugstore or
pharmaceutical supply distributor.
E. Clean, potable water
A. Cellulose sponge mop or similar applicator
B. Clean soft wiping cloths
C. Heavy gloves and protective gear
NOTE: TEST CLEAN A SMALL INCONSPICUOUS AREA BEFORE ATTEMPTING
TO CLEAN LARGE AREAS.
NOTE: WHEN CLEANING LARGE SURFACE AREAS, EMPLOY THE USE OF
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
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