Removing Copper Sulfate from Bronze Features

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.


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 removing copper sulfate deposits from bronze features. Two methods are described under Part 3.

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. Cleaning and refinishing bronze is highly specialized and should be conducted under the supervision of a metals conservator. All chemicals and cleaning methods should be approved prior to use by the metals conservator to ensure protection of historic metals.



A. Industrial Soap Co.

B. 3M Consumer Products Group


A. Non-ionic Detergent: A controlled suds, silicate buffered, non-ionic, rinseless-type synthetic detergent, containing no soap, alkali, solvents, abrasives, acids or caustics such as "Zyfo" cleaner concentrate (Industrial Soap Co.), or approved equal.

B. Solvent: 75% toluene + 24% acetone + 19% butal acetate.

C. Abrasive: Abrasive shall be silicon carbide abrasive pads such as "Scotch-Brite" (3M Company), or approved equal.

D. Water shall be clean, potable, non-staining and free of materials detrimental to all surfaces to be cleaned.

E. Cloth: Clean, cotton waste.


A. Stiff bristle brushes

B. Wide tip nozzle

C. Oxidizing agent: Aluminum chloride or liquid sulphur



A. Protection: All exterior stone that may come in contact with the cleaner shall be rinsed before, during and after cleaning in order to avoid staining the stone.


A. If the copper sulfate formation has not advanced too far (light streaking), remove the sulfate layer and expose the original patina. Select gentlest method - "A" or "B" which satisfactorily removes sulfate as determined by sample panels.

1. Method "A":

a. Wash with non-ionic detergent scrubbing the stained areas with a stiff natural bristle brush.

b. Rinse with cold water to remove all traces of detergent and residue. Rinse water pressure shall not exceed 100 psi and shall be sprayed through nozzles fitted with 15-20 degree wide tips. Nozzles shall be held perpendicular to the surface at a working distance of 1.5 to 2.5 feet.

c. Apply clear protective coating. See 05010-08-P for guidance.

2. Method "B":

a. Wash with solvent and abrasive pad. Avoid abrading existing patina finish.

b. Rinse with cold water as specified in 1b above.

c. Dry with clean, soft cloth.

d. Apply clear protective coating. See 05010-08- P for guidance.

B. When the finish cannot be restored by removal of sulfate formation, chemically strip to bare metal and repatinate. See 05010-24-R for guidance.