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Historic Preservation - Technical Procedures

Spectitle:

Duplicating Plaster Castings

Procedure code:

0920011R

Source:

Developed For Hspg (Nps - Sero)

Division:

Finishes

Section:

Lath & Plaster

Last Modified:

02/24/2012

Details:

Duplicating Plaster Castings



DUPLICATING PLASTER CASTINGS


PART 1---GENERAL

1.01 SUMMARY

    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

         3.   Submittals

         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).


PART 2---PRODUCTS

2.01 MANUFACTURERS

    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
              212/226-2010

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

         4.   Polytek Development Corp.
              P.O. Box 384
              Lebanon, NJ  08833
              908/534-5990

         5.   Smooth-On, Inc.
              1000 Valley Road
              Gillette, NJ  07933
              908/647-5800

    B.   Rubber Latex:  #660:  

         1.   Cementex Co.
              336 Canal St.
              New York, NY  10013
              212/741-1770

    C.   Plaster:

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

         2.   U.S. Gypsum Association
              810 First Street NE, #510
              Washington, DC  20002
              202/289-5440, FAX 202/289-3707

2.02 MATERIALS

    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

    E.   Plywood

    F.   Clean, potable water

    G.   Cheese cloth or burlap strips

2.03 EQUIPMENT

    A.   Stiff bristle brushes

    B.   Heating lamp, electric hair dryer or warm oven

2.04 MIXES

    NOTE:  VERIFY MIX WITH MANUFACTURER'S INSTRUCTIONS

    A.   7 parts plaster

    B.   4 parts water


PART 3---EXECUTION

3.01 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.

         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
              referenced standards.

         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.

3.02 ERECTION/INSTALLATION/APPLICATION

    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
              be duplicated.

         2.   Patch the fragment or fill in any damaged portions
              of the pattern using spackle or other patching
              compound.

         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
              curing process.

    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
              as possible.

    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