Terra-cotta patching and glaze repair

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 architectural terra-cotta patching and glaze repair.
   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 Project Team.
   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 Project Team, 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. Terra-cotta patching
2. Terra-cotta glaze repair

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 five years experience in Project Teamural terra-cotta patching and glaze repair and restoration and shall have successfully completed at least three projects of similar scope within the previous five years. He/she 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.
2. The contractor shall obtain terra-cotta coatings and patch materials from an established manufacturer. Plant to be that of a producer recognized by the industry as a manufacturer who can demonstrate successful completion of work of comparable quality and scope.
3. Warranty: The Contractor shall provide a warranty in writing against defects in material and installation for a period of five (5) years.

1.05 Submittals

1. The Contractor shall submit to the Project Team or GSA Approving Official:
   1. A detailed schedule of the areas for repair, including an assessment of the problem areas, material analysis, and detailed procedures for terra-cotta repair and replacement for approval.
   2. The manufacturer’s product literature for all proprietary products. 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.

1.06 Material samples

1. Patching Mortars: The Contractor shall provide three cured samples of each proposed patching mortar to be used for the project. The number of colors for which samples will be required will be determined by the range of colors found on the project. Mortar samples shall present a minimum surface of 3x3 inches or three inches in diameter for evaluation of color and texture.
2. Coatings: Proposed glaze and coating samples for terra-cotta repair shall be submitted to the project team for approval. Samples should cover the expected range of color and texture of surface finish as required to match original finish. Samples selected and approved by the project team will be the standard for work covered in this section.
3. Anchors: The Contractor shall submit two of each type of the dowel, anchor, bolt, and fastener to be used in the work under this section. Samples will be approved by the project team.

1.07 Testing

1. The Contractor shall arrange for sampling and testing of terra-cotta prior to selection of appropriate mortars. Allow at least thirty (30) days from time of sampling for completion of tests.
2. All testing shall be performed by an independent testing facility meeting “Recommended Requirements for Independent Laboratory Qualifications” published by the American Council of Independent Laboratories and ASTM E 699.

1.08 Testing and mockups

1. Contractor, at inconspicuous locations designated by the Project Team and GSA Historic Preservation, 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 Historic Preservation.
3. Testing and mockup panels for terra-cotta repairs at a location selected by the Project Team or on pieces of appropriate masonry, using the approved repair mortars as directed by the Project Team. The Contractor shall demonstrate all methods, materials, and workmanship required for the project, and prepare test panels for each color, texture, and type of material to be used. Coatings and finished mortar patch mortar samples will be reviewed based on color, texture, finish, and techniques employed. Retain mockups during construction as a standard for judging completed work.
4. The Contractor shall prepare up to three additional mockups of each type if required at no additional cost to the Owner.
5. Approved mockups shall become part of the work and shall serve as the quality standard for all similar work.
6. Any testing, demonstrations, or mockup that is rejected by the project team shall be removed in its entirety without damage to the surrounding masonry and a new demonstration/mockup shall be performed for the project team’s review.

1.09 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. Store cementitious materials off the ground in a clean, dry location. Cementitious materials shall be protected from contamination by foreign matter and deterioration by moisture or temperature.
   3. 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.
   4. Collect and dispose of waste material, packaging, debris, and effluent associated with the work in accordance with local, state, and federal environmental regulations.
   5. Proposed storage locations shall be approved by the Owner prior to the delivery of materials.

1.10 Safety

1. The contractor shall only use chemical products whose safety data sheets (SDS) have been reviewed and considered acceptable by the Government.
2. 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.
3. 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).

1.11 Project/site conditions

1. The work of this Section shall be executed when the air and surface temperatures are 40 degrees F and rising or less than 90 degrees F and falling. When temperatures are expected to fall outside this range, the Contractor shall employ hot and cold weather procedures as published by the manufacturer of the patching materials and/or the Masonry Institute of America.
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 sills, ledges, and projections from droppings. Immediately remove all patching materials that come in contact with such surfaces. Prevent masonry patching materials from staining the face of masonry or other surfaces to be left exposed.
   2. Minimize levels of dust during mortar removal and masonry repointing operations.
   3. Protect open copings, reglets, and other vulnerable areas from water penetration to prevent leakage during the course of the work. Open areas shall not be left exposed overnight or when inclement weather is predicted.
   4. Temporarily remove and store metal downspouts and downspout boot covers to remain during terra-cotta/cast stone work. Install temporary drainage leaders if required. Reinstall immediately upon completion of work in the immediate area.
   5. Protect existing scuppers and downspout outlets from damage and accumulation of mortar debris during restoration of terra-cotta/cast stone parapets.
   6. Exterior work areas should be accessed from the exterior only. Under no circumstances is equipment to be transported through the building during exterior repair operations.
   7. Where existing masonry components are to be removed, protect adjacent masonry units and surfaces from chipping and cracking during the removal process. Where components are to be reinstalled, the Contractor shall store salvaged components in a safe location, and protect them from theft and damage until reassembly.
   8. Cover partially completed work when work is not in progress.
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 roofing, masonry restoration, sealing, and painting.
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 provide visible barriers and/or warning tape around the perimeter of the work area for visitor protection, and shall protect nearby vehicles and adjacent structures from damage during the course of the work.

Part 2 – Products

2.01 Glaze repair

1. Materials
   1. Glaze Repair Material: One-part, opaque, water-base exterior coating, containing silicone resin binders with excellent UV stability and allowing at least 90 percent moisture vapor transmission in accordance with ASTM E 96, or other industry standard materials as approved by the Project Team. All colors to be tested and approved by the Project Team.
   2. Water: Clean, potable water, free of oils, acids, alkalis, and organic matter.
2. Equipment
   1. Brushes
   2. Clean sponges
3. Factory mixed terra-cotta patching materials
   1. The Contractor shall obtain materials from an established manufacturer of industry-recognized products appropriate for the work in this section. Selected materials and manufacturers shall be approved by the Project Team.
   2. Factory-prepared mortar mix for small patches shall be vapor permeable, frost and salt resistant, shrink resistant, and be physically compatible with the substrate, including but not limited to porosity, tensile, and compressive strength.
      1. Mortar mix shall be formulated to match color and texture of existing terra-cotta bisque. Only approved samples may be used.
      2. Mortar mix should not contain any synthetic additives.
4. Job-mixed terra-cotta patching materials
   1. Job-Mixed Setting and Patching Mortar shall meet standard compressive strength and permeability requirements for terra-cotta in accordance with ASTM C 270-08a. The mortar must always be lower in strength and higher in permeability than the existing terra-cotta.
      1. Portland Cement: ASTM C 150/150M-09 Type I or II. Portland Cement should only be used under very careful consideration of the age and properties of the existing terra-cotta and Project Team approval.
      2. Hydrated Lime: ASTM C 207-06, Type S or equivalent lime putty.
      3. Masons Sand: ASTM C 144-04, clean sand, free of contaminants. Match samples provided for proper sand color.
      4. Mineral Colorants: Only when approved and necessary to match terra-cotta matrix or surface color.
      5. Other Admixtures: No accelerators (Calcium Chloride) or additives shall be used.

2.02 Accessory materials

1. Stainless-steel wire armature, or threaded rods and bolts. All materials are to be reviewed by the Project Team.
   1. Wire shall be 12 gauge or heavier.
   2. Threaded rods shall be sized according to the need and loading requirements.
   3. Stainless steel shall be American Iron and Steel Institute Type 316.
2. Water: Clean, potable water, free of oils, acids, alkalis, and organic matter.
3. Glaze replacement coating: See paragraph 2.01 above.
4. Biocidal solution for cleaning exposed surfaces before glaze repair. Selected product must be approved by the Project Team.

2.03 Equipment

1. Masonry drill.
2. Hacksaw blade.
3. Power drill with mixing paddle.
4. Clean bucket.
5. Stiff, natural bristle brushes.
6. Hammer and wooden mallet.
7. Chisels.
8. Trowel.
9. Putty knife.
10. Natural or synthetic bristle paint brushes.
11. Other tools as needed or specified by the manufacturer.

Part 3 - Execution

3.01 General

1. Project-specific specifications are required for all terra-cotta work contained herein. As material and methods of construction vary greatly, each building must be evaluated and work scheduled accordingly. The Contractor shall submit schedules including methods and materials to be used.
2. Terra-cotta repairs and restoration shall use techniques, methods, and materials as similar as possible to those of the original. Work should be sequenced not only to assure the stability of the structure and protection of personnel during the work, but also to address a sensible order of construction to integrate new work with existing work.
3. The Contractor shall complete a survey to determine the extent and severity of the deterioration and whether the deterioration involves visual aspects of the surface terra-cotta, the failure of joints between units, the structural failure of the units themselves, the failure of support systems, or the overall structural stability of the building façade.
   1. Existing general failures that contribute to terra-cotta deterioration shall be noted and should be scheduled for repair prior to completion of the planned restoration work or in conjunction with terra-cotta replacements.
   2. Complete testing shall be conducted. The extent and type of analysis is to be determined by the Project Team. Work shall not proceed until the test results have been submitted and reviewed.
4. The Contractor shall protect adjacent materials, installed non-masonry materials, and openings.
5. Follow manufacturer’s instructions for mixing and installation of proprietary materials and equipment.
6. Complete façade cleaning prior to beginning terra-cotta repair or restoration work.
7. Stabilize deteriorated and loose units that are not scheduled to be removed or that cannot be easily removed. Areas where units are missing or have been removed should be protected against moisture penetration until replacement units can be installed.

3.02 Terra-cotta examination and condition assessment

1. The Contractor shall inspect surfaces for the following:
   1. Missing units.
   2. Deteriorated or missing mortar.
   3. Large cracks (particularly those running through multiple units).
   4. Material failures such as spalls and parallel crazing.
   5. Presence or absence of water-shedding devices such as flashings, gutters, and downspouts.
   6. Bulges in the terra-cotta, particularly at floor level and at piers.
   7. Rust stains from failing anchors.
   8. Efflorescence from excessive moisture in the wall.
   9. Internal damage: Strike surface with a wooden mallet to determine internal damage. Damaged units produce a flat thud, while undamaged units give off a clear ringing sound. Check for consistent tonal values to aid in the location of damaged units.

3.03 Terra-cotta glaze repair

1. Repair only those terra-cotta surfaces that have lost their surface glaze. Do not apply coating where glaze remains intact.
2. Lightly brush spalled areas to remove loose material and glaze that has failed but has not fallen from the face of the unit.
3. Clean exposed terra-cotta surfaces with a sponge and biocidal solution applied in accordance with manufacturer’s instructions. Allow to dry completely before application of glaze replacement coating.
4. Prime areas to be sealed with specified primer using brush or roller application according to manufacturers instructions. Promptly remove all primer from surrounding surfaces. Allow primer to dry 10 hours before applying masonry coating.
5. Apply breathable masonry paint in two coats, strictly adhering to manufacturer’s instructions and recommendations. Brush apply first coat, wait 24 hours, and brush apply second coat.
6. Coatings shall only be applied to the masonry units. Do not coat or seal masonry joints with glaze or sealants.
7. Verify color match after material has dried.

3.04 Terra-cotta repair patching

1. Remove broken, spalled, and cracked terra-cotta and mortar at areas to be patched by scraping and chipping the damaged areas to expose sound, clean, terra-cotta surfaces.
2. Cut back to sound terra-cotta or a minimum of 1/2 inch with chisel or hammer. Square off edges. Score surface to receive patch with chisel.
3. Where the depth of patch exceeds 1-1/2 inches and the size exceeds six inches square, set anchor pins (four per square foot), and install wire armature to hold patching material. Ensure a minimum of one inch coverage over all reinforcing for the finished patch.
4. Undercut at least two of the edges to provide a good key for the repair.
5. Thoroughly clean all loose terra-cotta particles, mortar, and other dust and debris from surfaces to be repaired by blowing with compressed air and then with a soft brush and water.
6. Pre-moisten terra-cotta with clean water and a stiff natural bristle brush to prevent patching mortar from drying out prematurely. Do not over-saturate or leave standing water.
7. Unless indicated otherwise in the mortar manufacturer’s instructions, apply a thin slurry coat of mortar over the patch area.
8. Mix patching mortar, following manufacturer’s recommendations, into a stiff, workable consistency that holds its shape when applied and will not run, sag, or crumble. Mix for at least five minutes.
9. Apply mortar mix with a trowel in lifts, working material under and around all reinforcing armature. Allow an initial set before proceeding with the next layer. Hand spray with water before each application. The depth of lifts should be adjusted to the type of mortar, wind, humidity, and temperature conditions for adequate curing.
10. Slightly overfill the patch to allow for shrinkage and finishing. Do not feather edge.
11. Finish: Allow patching material to harden for length of time recommended by manufacturer, then carefully cut back and tool to match adjacent surfaces. Maintain mortar joints; do not install patching material across mortar joints.
    1. Finish to match unglazed units. For glazed surfaces, paint to match the glaze color and texture. See 3.03 – Terra-cotta glaze repair above.
    2. For damaged areas that are smaller than 1/2-inch deep or for old patches where the color does not match the terra-cotta, repair by applying an approved material to match terra-cotta color.
    3. Wipe away all excess mortar as the work progresses. After mortar is thoroughly set and cured, clean new masonry surfaces of excess mortar and foreign matter using stiff nylon or bristle brushes and clean water.

Part 4 – Final report

4.01 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.