Replicating Ornamental Plaster Trim
REPLICATING ORNAMENTAL PLASTER TRIM
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
1. Historic Structures Precautions
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
A. American Society for Testing and Materials (ASTM)
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
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
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
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
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
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:
1. Base coat plasters: ASTM C28, types as indicated
a. Gypsum neat plaster such as "Red Top Gypsum
Plaster" (USG.com), "Two-Way Hardwall Gypsum
Plaster" (National Gypsum Co.), or approved
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" (USG), or approved equal.
2. Finish coat plasters: Types as indicated below:
a. Gypsum gauging plaster, ASTM C28; for flatwork
such as, "Red Top Gauging Plaster" (USG), "Gold Bond Gauging
Plaster" (National Gypsum Co.), or approved
b. Gypsum molding plaster, ASTM C59; for crown
moldings such as "USG No. 1 Moulding Plaster" (USG)
"Gold Bond Moulding Plaster" (National Gypsum Co.), or
c. Gypsum casting plaster, ASTM C59; for
ornamental plaster such as "USG No. 1 Casting
Plaster" (USG), or approved equal.
3. Finishing Hydrated Limes: ASTM C206; Type S, or approved
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
F. Anchors for Reattachment of Existing Plaster: Zinc
plated washers with appropriate mechanical fasteners for
existing substrates, (Charles Street Supply Co.), or
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.
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
1. Promptly notify the contracting officer of
conditions that need to be corrected or
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
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
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 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.
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
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
1. For Hand Mixing:
a. Weigh the plaster, and measure or weigh the
b. Sift plaster into water evenly. Do not drop
handsful of plaster into the water at one
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
d. Continue mixing until a smooth, even mass is
2. For Mechanical Mixing: Mechanically mix
cementitious and aggregate materials for plasters
to comply with applicable referenced application
standards and with recommendations of plasters
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
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
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
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.
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
END OF SECTION