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Historic Preservation - Technical Procedures

Spectitle:

Replicating Ornamental Plaster Trim

Procedure code:

0920005R

Source:

The Old Custom House/Ny, Ny - Gsa/Pbs, 2-27-91

Division:

Finishes

Section:

Lath & Plaster

Last Modified:

02/24/2012

Details:

Replicating Ornamental Plaster Trim



REPLICATING ORNAMENTAL PLASTER TRIM


ORNAMENTAL PLASTER WORK IS A SKILLED CRAFT REQUIRING THE WORK OF A
QUALIFIED AND EXPERIENCED CRAFTSMAN.


PART 1---GENERAL

1.01 SUMMARY

    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:

         1.   Historic Structures Precautions

         2.   Submittals

         3.   Quality Assurance

         4.   Delivery, Storage and Handling

         5.   Project/Site Conditions

         6.   Sequencing and Scheduling

         7.   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).

1.02 REFERENCES

    A.   American Society for Testing and Materials (ASTM), 100
         Barr Drive, West Conshohocken, PA 19428, (610) 832-9585
         or FAX (610) 832-9555.

1.03 SUBMITTALS

    A.   Product Data:  Submit manufacturer's product data for
         plaster materials, lath and accessories.

    B.   Design Data/Test Reports/Certificates:

         1.   Material Certificates:  Submit producer's
              certificates for plaster aggregates to show that
              materials comply with requirements.

         2.   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:

         1.   General:  Comply with requirements of referenced
              plaster application standards and recommendations
              of plaster manufacturer for environmental
              conditions before, during and after application of
              plaster.

         2.   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.

         3.   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.

         4.   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.


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.   National Gypsum Co.
         Gold Bond Building Products Division
         P.O. Box 25884
         Charlotte, NC  28229
         704/536-0023

    B.   United States Gypsum Association
         810 First Street NE, #510
         Washington, DC  20002
         202/289-5440, FAX 202/289-3707

    C.   Perma-Flex Mold Company
         1919 Livingston Avenue
         Columbus, OH  43209
         614/252-8034

    D.   Plastic Tooling Co.
         303 Commerce Dr.
         Exton, PA  19341
         610/363-5440

    E.   Charles Street Supply Co.
         54 Charles St.
         Boston, MA  02114
         617/367-9046

2.02 MATERIALS

    A.   Gypsum Plaster Materials:

         1.   Base coat plasters:  ASTM C28, types as indicated
              below.

              a.   Gypsum neat plaster such as "Red Top Gypsum
                   Plaster", "Red Top Two-Purpose Plaster (United
                   States Gypsum Association), "Two-Way Hardwall
                   Plaster" (National Gypsum Co.), or approved
                   equal.

              b.   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" (United States Gypsum
                   Association), or approved equal.

         2.   Finish coat plasters:  Types as indicated below:

              a.   Gypsum gauging plaster, ASTM C28; for flatwork
                   such as "Champion Gauging Plaster", "Red Top
                   Gypsum Plaster", "Star Gauging Plaster" (U.S.
                   Gypsum Association), "Super-White Gauging
                   Plaster" (National Gypsum Co.), or approved
                   equal.

              b.   Gypsum molding plaster, ASTM C59; for crown
                   moldings such as "USG Molding Plaster" (United
                   States Gypsum Association), "Super-White
                   Molding Plaster" (National Gypsum Co.), or
                   approved equal.

              c.   Gypsum casting plaster, ASTM C59; for
                   ornamental plaster such as "USG No. 1 Casting
                   Plaster" (United States Gypsum Association),
                   or approved equal.

         3.   Finishing Hydrated Limes:  ASTM C206; Type S, such
              as "Ivory Finish Lime", "Snowdrift Finish Lime"
              (United States Gypsum Association", or approved
              equal.

         4.   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.

2.03 MIXES

    A.   Plaster Base Coat Compositions:  Comply with ASTM C842
         and manufacturer's directions for gypsum plaster base
         coat proportions for three-coat work:

         1.   Scratch Coat:  High strength gypsum gauging plaster
              with job-mixed sand.

         2.   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.


PART 3---EXECUTION

3.01 EXAMINATION

    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.

         1.   Promptly notify the contracting officer of
              conditions that need to be corrected or
              coordinated.

3.02 PREPARATION

    A.   Surface Preparation:

         1.   Remove ornamental plaster that is damaged or loose
              and requires replacement, or that has to be reset.

         2.   Reinforce and secure loose ornamental plaster that
              is to remain, using appropriate concealed methods
              (see 09210-04-R for guidance).

         3.   Make molds or screeds as necessary for replacement
              of ornamental plaster.  Match existing plaster
              profiles.

         4.   Make accurate reproductions for missing,
              deteriorated or damaged ornamental plaster.

         5.   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.

         6.   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.

         7.   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.

         8.   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 "Gypsum Construction
    Handbook" published by United States Gypsum Association
    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.

         1.   Coat metal sheet with layout dye - a blue dye that
              makes the scribed pattern easier to follow.

         2.   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.

         3.   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.

              a.   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.

              b.   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.  

         4.   For circular shapes:  

              a.   Follow procedures described in Section 3.03
                   A.1.- A.3. above, except scribe the center
                   line of the circular shape on the template.  

              b.   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.

         1.   For Hand Mixing:

              a.   Weigh the plaster, and measure or weigh the
                   water.

              b.   Sift plaster into water evenly.  Do not drop
                   handsful of plaster into the water at one
                   time.

              c.   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.  

              d.   Continue mixing until a smooth, even mass is
                   produced.

         2.   For Mechanical Mixing:  Mechanically mix
              cementitious and aggregate materials for plasters
              to comply with applicable referenced application
              standards and with recommendations of plasters
              manufacturer.

              a.   Weigh plaster and measure water accurately.  

              b.   Sift or strew plaster slowly and evenly.  

              c.   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:

         1.   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.

              a.   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.  

              b.   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.

              a.   For bench work, place a center post in a hole
                   in the bench.  

              b.   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.

         3.   Duplicate plaster molds by taking impressions from
              existing plaster molds; create new rubber molds
              from these impressions and cast plaster into these
              molds.  

         4.   Remove plaster molds after plaster has cured,
              following plaster manufacturer's instructions.

         5.   Strip and clean existing decoration to produce a
              clean, sharp mold.

3.04 ADJUSTING/CLEANING

    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.

                         END OF SECTION