Historic Preservation - Technical Procedures
Duplicating Plaster Castings
Developed For Hspg (Nps - Sero)
Lath & Plaster
Duplicating Plaster Castings
DUPLICATING PLASTER CASTINGS
A. This procedure includes guidance on duplicating plaster
ornament that is chipped or has deteriorated. Plaster of
Paris is the most common casting material.
B. See 01100-07-S for general project guidelines to be
reviewed along with this procedure. These guidelines
cover the following sections:
1. Safety Precautions
2. Historic Structures Precautions
4. Quality Assurance
5. Delivery, Storage and Handling
6. Project/Site Conditions
7. Sequencing and Scheduling
8. General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing
this procedure and should be followed, when applicable,
along with recommendations from the Regional Historic
Preservation Officer (RHPO).
A. Rubber Casting Materials:
1. Abatron, Inc.
5501 95th Ave.
Kenosha, WI 53144
800/445-1754 or 414/653-2000
2. Industrial Plastic Supply
309 Canal Street
New York, NY 10013
3. Perma-Flex Mold Company
1919 Livingston Avenue
Columbus, OH 43209
4. Polytek Development Corp.
P.O. Box 384
Lebanon, NJ 08833
5. Smooth-On, Inc.
1000 Valley Road
Gillette, NJ 07933
B. Rubber Latex: #660:
1. Cementex Co.
336 Canal St.
New York, NY 10013
1. Gold Bond Building Products Division
National Gypsum Co.
P.O. Box 25884
Charlotte, NC 28229
2. U.S. Gypsum Association
810 First Street NE, #510
Washington, DC 20002
202/289-5440, FAX 202/289-3707
A. Plaster of Paris: "Kal-Kote Finishing Plaster" (Gold
Bond, National Gypsum Co.), "Red Top Molding Plaster"
(U.S. Gypsum Association), or approved equal.
B. Spackle or patching compound
C. Rubber latex such as #660 (Cementex Co.)
D. Paper towels
F. Clean, potable water
G. Cheese cloth or burlap strips
A. Stiff bristle brushes
B. Heating lamp, electric hair dryer or warm oven
NOTE: VERIFY MIX WITH MANUFACTURER'S INSTRUCTIONS
A. 7 parts plaster
B. 4 parts water
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.
3. Clean plaster surfaces to be restored, to remove
loose and deleterious materials that may affect
adhesion or application of new plaster. Reattach
loose lath or install new lath as required.
4. 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
5. All existing plaster to be repaired shall be
stripped down to a bare finish plaster surface with
a non-water based solvent or paint stripper which
is not water flushed clean.
NOTE: IT IS NOT ALWAYS NECESSARY TO DETACH THE PLASTER
ORNAMENT IN ORDER TO DUPLICATE IT. HOWEVER, IF THE PLASTER
ORNAMENT CAN NOT BE REMOVED FROM THE WALL OR CEILING TO MAKE
THE MOLD, CURING MAY TAKE SEVERAL DAYS BETWEEN LATEX COATS AND
SEVERAL WEEKS FOR THE FINAL CURING.
A. Making the Mold:
1. Detach from the ceiling or wall plaster casting to
2. Patch the fragment or fill in any damaged portions
of the pattern using spackle or other patching
3. Carefully clean the fragment for mold making.
4. Brush the master piece with the rubber latex.
5. Cure the latex at a temperature of about 110oF.;
Use an infrared heating lamp, an electric hair
dryer, or a warm oven to provide the source of
heat; Cure for about 1 hour using heat.
6. When the rubber has cured, apply a second latex
coating (the number of coats required is dictated
by the size of the original and the depth of the
relief); Cure as above.
B. Building the Mother:
1. The purpose of the mother is to support the mold
when it is filled with wet plaster.
2. Fill any undercuts in the latex mold with wet paper
towels; Lay a thin layer of wet paper towels over
the entire surface of the rubber mold.
3. Build a frame around the mold to pour the mother
into; The mother should be at least 1/2 inch thick
at the highest point of the mold.
4. Separate the mother from the rubber mold as soon as
the plaster has set (in less than an hour).
5. Wash the interior of the mold with soap and water
and remove any residual byproducts left from the
C. Mixing the Plaster:
1. Add 7 parts by volume of plaster to 4 parts water;
Use cold water to retard the set-up time.
2. Sprinkle plaster powder in slowly; Stir as little
D. Casting the Plaster:
1. Before pouring plaster into mold, fill the mold
with water and then pour water out (this will
moisten the walls and ensure penetration of the
plaster into all crevices).
2. Add plaster and cheesecloth or burlap strips for
strength; Jog the plaster-filled mold gently to
nudge plaster into all the indentations.
3. Level the plaster; Plaster should be hard enough to
remove from the mold in 30 minutes; Allow 24 hours
to air dry before installing.
E. Install the plaster casting using a thin coat of plaster
of paris as an adhesive; Hold the piece in place for a
few minutes until the plaster sets; A mastic adhesive or
epoxy or gypsum board joint cement are other good
adhesives for light pieces.
END OF SECTION