Cleaning and Finishing Bronze Architectural Elements

 Disclaimer

Technical procedures are supplied by GSA only as a reference. All procedures have been reviewed for general consistency with federal standards for rehabilitating historic buildings. 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 Description

1. This specification provides guidance for cleaning and applying protective coatings to bronze architectural elements.
   1. This specification has been developed for use on historic properties (defined as any district, site, building, structure, or object that is listed in or eligible for listing in the National Register of Historic Places) and provides an overview of accepted practices. Site-specific specifications, when appropriate, will be provided by the Architect.
   2. All work described herein and related work must conform to the Secretary of the Interior’s Standards for the Treatment of Historic Properties.
   3. The Contractor shall provide all labor, material, equipment, and operations required to complete the rehabilitation work indicated herein.
   4. All work described herein and related work must have the approval of a Cultural Resources Manager, Conservator, Historic Architect, or other professional who meets the standards outlined in the Secretary of the Interior’s Standards – Professional Qualifications Standards pursuant to 36 CFR.

1.02 Section includes

1. General cleaning of bronze
2. Removal of deteriorated lacquer from bronze
3. Application of a new clear coating to bronze
4. Touch up of damaged clear coating
5. Application of wax

1.03 Related sections

1. Section 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections and should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from GSA Historic Preservation:
   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)
   9. Historic Building Treatment Procedures

1.04 Quality assurance

1. The Contractor performing the work described in this Section shall have a minimum of 5 years experience in bronze cleaning and restoration and shall have successfully completed at least 3 projects of similar scope within the previous 5 years. Contractor shall demonstrate a working knowledge of The Secretary of the Interior’s Standards for the Treatment of Historic Properties with Guidelines for Preserving, Rehabilitating, Restoring & Reconstructing Historic Buildings.

1.05 Submittals

1. The Contractor shall submit to the Architect or GSA Approving Official:
   1. A detailed description and schedule of the areas to be treated, including an assessment of the problem surfaces, any necessary testing, and proposed general cleaning techniques, lacquer removal procedures, clear coating application methods, waxing procedures, etc., for approval once cleaning test panels are completed and approved.
   2. The manufacturer’s product literature for all proprietary products. If a custom wax formula is used, all ingredients must be documented and submitted for approval as with proprietary formulas. Product literature shall include specification data, Safety Data Sheets (SDS), and instructions for storage, handling, and use.
   3. A project safety plan to include at minimum: any personal protective equipment to be used by the contractor’s staff such as disposable clothing, gloves and safety glasses, procedures for emergencies and accidents, documentation of staff training in use of equipment and/or chemical products as required by the manufacturers.
   4. Ongoing care and warranty requirements for all restored and newly installed materials.
   5. Provide a final report of completed work, including all approved submittals and photographs of the areas repaired that were taken before, during, and after the work.

1.06 Testing and mockups

1. Contractor, at inconspicuous locations designated by the Project Team and GSA HP, shall perform small test patches (no larger than 6 inches by 6 inches) for all products to be used. Testing will determine the best method(s) to perform the work and ensure that the chosen products do not cause harm.
2. Following testing, mockup panels shall be created (mockup panels may incorporate successful test patches) using the chosen methods. Size and location of mockup panels shall be determined by the project Team and GSA HP.
3. testing and mockups shall include (as applicable), but not be limited to:
   1. General cleaning
   2. Lacquer and/or wax removal
   3. Finish coat methods
   4. Wax application procedures
4. The methods used, their application, etc. shall be in accordance with manufacturer’s instructions and shall duplicate those procedures proposed for the overall bronze cleaning and protective coating process. GSA HP shall conduct a thorough evaluation of each method after work is complete to determine the best method for the overall work.
5. The mockup shall include an evaluation of the materials and techniques proposed for the protection of surrounding areas from the chemicals used to clean and coat the bronze, and must include an evaluation of the method to be used to collect the cleaning effluent.
6. For proprietary cleaning systems, a representative of the cleaning materials manufacturer(s) shall be present during the preparation and application of the test areas.
7. Each worker proposed for work on the project shall prepare a mockup panel of each activity they are to work on. Workers whose mockups are not approved shall not be permitted to work on the project.
8. Approved mockup panels shall become part of the work and shall serve as the quality standard for all similar work.
9. Any testing, demonstration, or mockup that is rejected by the architect shall be removed in its entirety without damage to the surrounding masonry and a new demonstration/mockup shall be performed for the architect’s review.

1.07 Delivery, storage, and handling (as applied to products and materials)

1. The Contractor shall:
   1. Deliver restoration cleaning and testing materials and proprietary products to the project site in manufacturer’s or distributor’s packaging, undamaged, complete with application instructions and Safety Data Sheets (SDS)
   2. Transport and store cleaning agents, chemicals, and solvents within the temperature range recommended by the manufacturer and away from direct sunlight. Handle all materials according to manufacturer’s instructions.
   3. Collect and dispose of waste material, packaging, debris, and effluent associated with the work in accordance with local, state, and federal environmental regulations.

1.08 Safety

1. The contractor shall only use chemical products whose safety data sheets (SDS) have been reviewed and considered acceptable by the Government. Any project surfaces containing lead-based paint, as disclosed by the Government, shall be handled by the contractor in accordance with OSHA (29 CFR 1926.62), EPA (40 CFR 261) and State regulations. Any lead-based paint or coating that is removed in a project resulting from this scope shall undergo Toxicity Characteristic Leaching Procedure (TCLP) testing by the contractor. The contractor shall dispose of any lead-paint waste as hazardous if it fails the TCLP.
2. Any paint or coating required to be removed which has not been disclosed by the Government to contain lead, shall be tested for lead by the contractor. All paints or coatings testing positive shall be handled and disposed of as lead-based.
3. The contractor shall isolate the work area with cones, tape or similar means as possible for the duration of the project, to prevent unauthorized individuals from entering the immediate work area.
4. For any work required on elevated surfaces (4 feet or more above the ground) the contractor shall install and use any ladders, scaffolding or platforms in accordance with OSHA regulations (29 CFR Subpart M).

1.09 Project/site conditions

1. The work of this Section shall be executed when all areas of the bronze are at a temperature within the product manufacturer’s recommended temperature range for application. Exterior work shall not commence when rain, snow, or below-freezing temperatures are expected within the next 24 hours. All surfaces shall be free of standing water, frost, and ice.
2. The Contractor is responsible for protecting existing adjacent areas and materials during the execution of the work and shall provide all necessary protection and follow all necessary work procedures to avoid damage to existing material assemblies and vegetation not a part of the work of this Section.
3. At a minimum, the Contractor shall:
   1. Protect woodwork, glass, metal, and any material adjacent to bronze areas to be cleaned from overspray and possible chemical or water damage from cleaning operations. Cover all window openings with waterproof plastic to prevent leakage to the building interior.
   2. Protect surrounding vegetation from runoff during cleaning operations.
   3. Exterior work areas should be accessed from the exterior only. Under no circumstances are hoses to be run or equipment transported through the building during exterior cleaning operations.
   4. If necessary, the Contractor shall erect waterproof enclosures around areas where treatment operations are in progress to protect nearby property and passers-by from overspray of chemicals or rinse water.
4. If applicable, the Contractor shall coordinate treatment operations with the other trades involved in exterior and interior restoration work, including but not limited to masonry restoration, sealing, and painting. Masonry cleaning is to be completed prior to restoration of windows, doors, and metalwork, and prior to any exterior painting in the affected areas.
5. All Contractor personnel performing treatment operations shall be provided by the Contractor with gloves, respirators, protective clothing and any other personal protective equipment (PPE) as recommended by the manufacturer of the products used during the work and required by local, state, and federal regulations.
6. The Contractor shall complete installation of temporary seals and protection at window and door perimeters prior to starting cleaning operations where required to prevent leakage to the interior.

PART 2 – Products

2.01 Products

1. Lacquer: UV Resistant, long lasting lacquer coating
2. Wax: conservation-grade outdoor sculpture wax and Clear Paste Wax
3. 3-5% solution of non-ionic detergent
4. Conservation grade metal de-corroder
5. Pure acetone
6. Patina (mineral patinas, Dye‐Oxide Patinas…)

2.02 Materials

1. Soft, lint-free polyester-blended cotton cloth
2. Soft natural bristle buffing brush
3. Duct-tape
4. Bamboo skewers

2.03 Cleaning of bronze

1. Investigation and methods selection
   1. Identification of substrate conditions, previous treatments, and any existing coatings. Testing may require additional technical expertise from a materials scientist, architectural conservator, microbiologist, and/or other technical expert. The Contractor shall choose the gentlest method possible to clean the bronze and remove any existing coatings without damaging the substrate.
   2. The Contractor shall conduct cleaning, coating removal, and protective coating application test patches, in unobtrusive locations on the surface to be cleaned. The purpose of the test patch is to determine the gentlest, most effective method to clean, strip and apply protective coatings to  the bronze. Several cleaning, stripping and coating methods are preferably tested side by side in small patches, size and location to be determined by GSA HP and the project team.
   3. Damage to the bronze, including its patina, cannot be addressed by cleaning, and is therefore not covered under this section.
   4. Existing conditions and processes must be documented before, during, and after work.
2. Cleaning methods:
   1. Brush away all dirt and debris with dry, clean, soft bristle brushes. Wrap metal ferrules of brushes with duct tape to avoid scratching the bronze. Use small natural bristle brushes and bamboo skewers to dislodge debris from crevices. Metal tools shall not be used.
   2. If the bronze is adjacent to surfaces of a different material, flood that material with clean water or protect it carefully if water is not appropriate, to prevent absorption of cleaning effluent.
   3. Rinse sculpture with potable water delivered from a garden hose or pump sprayer.
   4. After rinsing, clean the bronze with a 3-5% solution of non-ionic detergent in water. Apply with soft natural bristle brushes and rinse repeatedly with water until all cleaning residue is removed from the surface. Surface corrosion may be reduced by lightly buffing with a pre-wetted soft scrubbing pad for sensitive surfaces and cotton cloths, then rinsed. Areas of stubborn corrosion may be treated with a conservation grade metal de-corroder. During corrosion removal, the bronze patina shall not be disturbed. Clean, absorbent cotton cloths will be used to thoroughly dry the bronze.
   5. Blasting, such as pulverized walnut shell and CO2, may be used to clean the bronze without removing patina. When needed, a corrosion inhibitor may be applied following the blasting, prior to the application of a wax coating.
   6. To remove extensive mineral scale: Medium to high (1000 psi to 2800 psi) pressure washing methods may be utilized during washing to aid in the removal of mineral scale deposits, reduce corrosion, and remove any found surface accretions. Always start at the lowest psi and check for damage. To safeguard against over cleaning, the washer wand will be equipped with a minimum 25‐degree fan tip nozzle and positioned with a minimum 12‐inch standoff distance from the bronze object. Where required, mineral scale may be softened prior to washing with a mild acetic acid poultice, or the like, to ease mineral scale removal during washing efforts.
   7. Remove existing lacquer only if necessary, using pure acetone, applied with a cotton cloth wiped along the grain of the metal. Allow the acetone to stand on the metal for several minutes and then wipe it away with a cotton cloth followed by a water rinse. Several applications of the solvent may be necessary in order to remove all traces of the lacquer from the metal.
   8. Depending on testing results of existing finishes, cleaning may also include partial or whole stripping of existing finishes with an environmentally friendly stripper. This step will ensure proper surface preparation for the new coatings.
   9. Provide an additional cleaning with a 5% solution of non-ionic detergent in water as noted above to remove residues from corrosion removal and stripper application.
3. Patina Touch-up (only perform if deemed necessary)
   1. Patina may be touched up in areas of bare metal to match the characteristics of the existing patina. The patina formulation will be customized for this purpose, but should include traditional mineral patinas, Dye‐Oxide Patinas, and patinas as found in existing documentation of the piece(s) or determined through testing. 

2.04 Application of clear coat

1. Lacquer - new coat
   1. Where lacquer coatings are required, the use of a UV Resistant, long lasting lacquer coating is recommended. Clear coating is to be applied to dry surface within 4 hours of cleaning and according to the manufacturer’s instructions. It is not to be applied in inclement weather or when the relative humidity exceeds 60%. The clear coating should be sprayed on using conventional spray equipment.
   2. Remove any dust that may have deposited on the surface after cleaning with a soft lint-free cloth.
   3. Apply each coat in a thin mist. Allow to dry a minimum of 30 minutes between coats. Number of coats (generally between 5-8)will depend on the environmental conditions to which the object is exposed.
   4. Minimum thickness of full coating shall be 1 millimeter.
   5. If the lacquer is too glossy, a polyethylene dispersion can be added to the final coat of lacquer to bring down the sheen.
   6. Final drying time will be 24-48 hours before the bronze can be touched or used. Dry time will vary depending on humidity levels.
2. Lacquer - Touch up
   1. Spot repair areas of lacquer coatings that have been damaged or are missing.
   2. If a protective wax coating is present, remove with appropriate solvent.
   3. Clean surface to be repaired with a soft cloth, water and appropriate mild detergent. Use cloth dipped in cleaning solution to wipe the area of repair, do not wet the entire surface. Rinse area with water and allow to dry completely.
   4. Clear coating is to be applied to dry surface within 4 hours of cleaning and according to the manufacturer’s instructions. It is not to be applied in inclement weather or when the relative humidity exceeds 60%. Lacquer should be spray applied to achieve uniform coverage. Size of sprayer should be chosen based on size of area to be covered with lacquer.
   5. Apply each coat in a thin mist. Allow to dry a minimum of 30 minutes between coats. Number of coats (generally between 5-8)will depend on the environmental conditions to which the object is exposed.
   6. Minimum thickness of full coating shall be 1 millimeter and shall match the surrounding surfaces.
   7. Final drying time will be 24-48 hours before the bronze can be touched or used. Dry time will vary depending on humidity levels.
3. Wax
   1. Hot Wax (not for use with lacquer coatings).
      1. Bronze shall be cleaned as detailed above.
      2. Protection must be used to protect surrounding materials from wax and from application procedures and equipment.
      3. A thin coat of a conservation-grade outdoor sculpture wax shall be applied to the bronze. The wax chosen shall be formulated with consideration given to the bronze object’s location and environmental conditions and the weather during the period of application. Hot wax shall be applied by mildly heating surfaces with a propane torch while preventing disturbance or discoloration of the underlying bronze and patina. Soft bristle brushes with wrapped ferrules shall be used for application. After the wax has set, it shall be buffed with natural bristle brushes and clean, lint-free polyester-blended cotton cloths. If necessary for added protection, a second coat of hot wax shall be applied and buffed as described above. Two coats of hot wax are recommended for enhanced protection if the bronze frequently gets wet or is subject to de-icing salts.
      4. One coat of approved Clear Paste Wax will be applied at ambient temperature to the bronze, allowed to set, and buffed as described above to a lustrous finish.
   2. Cold Wax
      1. One coat of approved Clear Paste Wax will be applied at ambient temperature to the bronze, allowed to set, and buffed to a lustrous finish with soft natural bristle buffing brushes and soft lint-free cloths. Two coats of cold wax are recommended for enhanced protection if the bronze frequently gets wet or is subject to de-icing salts. 

2.05 Final report

1. The Contractor Shall:
   1. Provide a final report of completed work, including all approved submittals and photographs of the areas cleaned that were taken before, during, and after the work.
   2. Provide a written summary of the project and results upon final inspection and approval. The summary shall include a discussion of steps taken or new findings not specified in the initial documentation. Summary should also include any applicable ongoing care and warranty requirements.

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.

Rewrite
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.