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.
ORNAMENTAL PLASTER WORK IS A SKILLED CRAFT REQUIRING THE WORK OF A QUALIFIED AND EXPERIENCED CRAFTSMAN.
A. This procedure includes guidance on replicating ornamental plaster trimwork to replace that which is deteriorated or missing.
B. See 01100-07-S for general project guidelines to be used along with this procedure. These guidelines cover the following sections:
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing this procedure and should be followed along with recommendations from the Regional Historic Preservation Officer (RHPO).
A. American Society for Testing and Materials (ASTM) www.astm.org
A. Product Data: Submit manufacturer's product data for plaster materials, lath and accessories.
B. Design Data/Test Reports/Certificates:
- Material Certificates: Submit producer's certificates for plaster aggregates to show that materials comply with requirements.
- Schedule of Work: Submit a schedule of spaces that require ornamental plaster, and a list of ornamental plaster items or areas that require repair or replacement in each of the affected spaces. Indicate procedures to be used, and time requirements.
1.04 QUALITY ASSURANCE
A. Regulatory Requirements: Reference Standards: Comply with applicable requirements of Chapter 9 - Ornamental Plaster, in "Plastering Skills", published by American Technical Publishers, Inc. Comply with other specified standards.
1.05 PROJECT/SITE CONDITIONS
A. Environmental Requirements:
- General: Comply with requirements of referenced plaster application standards and recommendations of plaster manufacturer for environmental conditions before, during and after application of plaster.
- Cold Weather Protection: When outdoor temperature is below 55 degrees F., maintain continuous uniform temperature of not less than 55 degrees F., nor more than 70 degrees F., for not less than 1 week prior to plaster application, during its application, and until plaster is dry but for not less than one week after application is complete. Distribute heat evenly; prevent concentrated or uneven heat from contacting plaster near heat source.
- Ventilation: Ventilate building spaces as required to remove water in excess of that required for hydration of plaster. Begin ventilation immediately after plaster is applied and continue until it sets.
- Protection: Protect adjacent work from soiling, splattering, moisture and other harmful effects which might result from plastering. Obtain approval of the protection from the contracting officer.
A. National Gypsum Co.
Gold Bond Building Products Division
B. USG Corporation
C. Perma-Flex Mold Company
D. Charles Street Supply Co.
A. Gypsum Plaster Materials:
- Base coat plasters: ASTM C28, types as indicated below.
- Gypsum neat plaster such as "Red Top Gypsum Plaster" (USG.com), "Two-Way Hardwall Gypsum Plaster" (National Gypsum Co.), or approved equal
- High strength gypsum neat plaster with a minimum average dry compressive strength of 2,800 psi per ASTM C472 for a mix of 100 lbs. plaster and 2 cu. ft. of sand such as "Structo-Base" (USG), or approved equal.
- Finish coat plasters: Types as indicated below:
- Gypsum gauging plaster, ASTM C28; for flatwork such as, "Red Top Gauging Plaster" (USG), "Gold Bond Gauging Plaster" (National Gypsum Co.), or approved equal.
- Gypsum molding plaster, ASTM C59; for crown moldings such as "USG No. 1 Moulding Plaster" (USG) "Gold BondMoulding Plaster" (National Gypsum Co.), or approved equal.
- Gypsum casting plaster, ASTM C59; for ornamental plaster such as "USG No. 1 Casting Plaster" (USG), or approved equal.
- Finishing Hydrated Limes: ASTM C206; Type S, or approved equal.
- Aggregates for Base Coat Plasters: ASTM C35, sand.
B. Clean, potable water
C. Bonding Compound for Gypsum Plaster: ASTM C631.
D. Plaster Molds: Urethane rubber liquid mold material that can produce detailed impressions (Perma Flex Mold Co.), or approved equal.
E. Reinforcing Fibers: Sisal hemp fibers, or polypropylene fibers, not more than 2" long (Plastic Tooling Co.), or approved equal.
F. Anchors for Reattachment of Existing Plaster: Zinc plated washers with appropriate mechanical fasteners for existing substrates, (Charles Street Supply Co.), or approved equal.
A. Plaster Base Coat Compositions: Comply with ASTM C842 and manufacturer's directions for gypsum plaster base coat proportions for three-coat work:
- Scratch Coat: High strength gypsum gauging plaster with job-mixed sand.
- Brown Coat: High strength gypsum gauging plaster with job-mixed sand.
B. For Finish Coat: Proportion casting plaster and molding plaster to comply with the manufacturer's instructions. Proportion gypsum gauging plaster to comply with ASTM C842; 1 part plaster and 2 parts lime.
A. Examine surfaces to receive ornamental plaster and conditions under which the ornamental plaster will be installed. Proceed with the ornamental plaster work only when substrate surfaces and conditions comply with referenced standards, to ensure satisfactory installations.
- Promptly notify the contracting officer of conditions that need to be corrected or coordinated.
A. Surface Preparation:
- Remove ornamental plaster that is damaged or loose and requires replacement, or that has to be reset.
- Reinforce and secure loose ornamental plaster that is to remain, using appropriate concealed methods (see 09210-04-R for guidance).
- Make molds or screeds as necessary for replacement of ornamental plaster. Match existing plaster profiles.
- Make accurate reproductions for missing, deteriorated or damaged ornamental plaster.
- Clean plaster surfaces to be restored, to remove loose and deleterious materials that may affect adhesion or application of new plaster. Re-attach loose lath or install new lath as required.
- Spray existing plaster to be pointed or patched with water, or use bonding agent, to ensure adhesion of new plaster. Install keying systems for larger pieces as necessary. Comply with referenced standards.
- All existing plaster to be painted shall be stripped down to a bare finish plaster surface with a non-water based solvent of paint stripper which cannot be water flushed clean.
- Measure and layout ornamental plaster accurately to maintain pattern and alignment of the work.
3.03 ERECTION, INSTALLATION, APPLICATION
NOTE: Conform to the requirements of the latest edition of "Gypsum Construction Handbook" published by USG Corporation concerning application of ornamental plaster.
A. Make a template from sheets of metal (brass) cut full-scale in shapes to match existing; use to screed or form gypsum plaster to desired shapes and contours.
- Coat metal sheet with layout dye - a blue dye that makes the scribed pattern easier to follow.
- When the dye is dry, scribe, cut and carefully file the template to remove any roughness or other imperfections that might be transferred to the finished work.
- Use a sled to guide the template as it is pushed through the plastic mass. Basic parts of the sled include a "slipper-board" and the board to which the template is nailed.
- For bench work the sled is guided by the table edge. Bench-run molds are installed on the job with mechanical fasteners or adhesively attached with gypsum plaster.
- For run-in-place work, a guide strip is attached to the wall or ceiling and forms one edge of the pattern; after work is completed the guide strip is removed.
- For circular shapes:
- Follow procedures described in Section 3.03 A.1.- A.3. above, except scribe the center line of the circular shape on the template.
- Center a pivot plate to this line and attach to the template board.
C. Mix Plaster: Use special plaster designed and manufactured for ornamental plaster work.
NOTE: Mix the plaster a little below normal consistency for the first mix, then allow the mix to "cream" a bit before placing it on the bench in front of the template. For run-in-place work a slightly stiffer consistency is desirable.
- For Hand Mixing:
- Weigh the plaster, and measure or weigh the water.
- Sift plaster into water evenly. Do not drop handsful of plaster into the water at one time.
- Allow to soak 2 minutes undisturbed, then mix thoroughly. Stir from bottom, forcing material to top. Take care to avoid beating air into mix. Proper mixing forces air out of mix.
- Continue mixing until a smooth, even mass is produced.
- For Mechanical Mixing: Mechanically mix cementitious and aggregate materials for plasters to comply with applicable referenced application standards and with recommendations of plasters manufacturer.
- Weigh plaster and measure water accurately.
- Sift or strew plaster slowly and evenly.
- Allow it to soak 2 to 5 minutes, and mix for 2 to 5 minutes.
NOTE: For batches of 10 to 50 lbs., use a 3" diameter, 3-blade, 25 degree pitch propeller with 1/4 or 1/3 hp. 1,760 rpm direct-drive motor. For batches of 50 to 200 lb., use a 4" diameter, 3-blade, 25 degree pitch propeller with 1/2 hp. 1,760 rpm direct-drive motor.
D. Replicate the Ornamental Trim:
- For Run Work or Straight Mold:
NOTE: This method is used to produce staff work or pattern stock having parallel edges or to form a straight molding.
- For bench work, some plaster should be allowed to lap over the end of the bench to act as an anchor and prevent the mold from loosening from the bench before work is completed.
- Press small lumps of modeling clay onto the bench before the plaster is placed provide a grip for the plaster and help hold it in place.
- 2. For Circular Shapes (Turning): Like run work, turning can be done in the shop or run-in-place.
- For bench work, place a center post in a hole in the bench.
- For run-in-place work, position the center post accurately at the center of the circular shape to be formed and securely attach or brace to the structural supports.
NOTE: THE CENTER POST SHOULD HAVE A RUNNING THREAD, WITH A DOUBLE NUT FOR HEIGHT ADJUSTMENT. THE PIVOT POINT MUST BE ABSOLUTELY RIGID AND SET HIGHER THAN THE TOP OF THE PATTERN TO BE MADE, SO THAT THE TEMPLATE WILL SCREED CLEARLY AS IT ROTATES AROUND THE PIVOT POINT.
- Duplicate plaster molds by taking impressions from existing plaster molds; create new rubber molds from these impressions and cast plaster into these molds.
- Remove plaster molds after plaster has cured, following plaster manufacturer's instructions.
- Strip and clean existing decoration to produce a clean, sharp mold.
A. When replication and plasterwork is completed, remove unused materials, containers and equipment. Clean floors and adjacent surfaces of all plaster debris, including plaster droppings daily.
B. Provide final protection and maintain conditions, in a manner suitable to the RHPO, which ensure ornamental plaster being without damage or deterioration at time of substantial completion.