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Historic Preservation - Technical Procedures

Spectitle:

Polishing Architectural Scagliola

Procedure code:

0920007R

Source:

Interior's Handbook For Historic Buildings - Jeff Greene

Division:

Finishes

Section:

Lath & Plaster

Last Modified:

02/24/2012

Details:

Polishing Architectural Scagliola



POLISHING ARCHITECTURAL SCAGLIOLA


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on polishing
         architectural scagliola using the French Polishing
         Process of applying a shellac.  NOTE: THIS IS A VERY
         SPECIALIZED AND LABOR INTENSIVE PROCEDURE.  IT SHOULD
         ONLY BE PERFORMED BY A SKILLED CONSERVATOR, EXPERIENCED
         IN WORKING WITH SCAGLIOLA AND IN PERFORMING THIS TYPE OF
         PROCEDURE.

    B.   French Polishing is a special technique for applying a
         finishing solution of resins and alcohol.  It is
         typically a three-part process and is the most frequently
         used method of polishing on scagliola and marezzo in the
         United States.

    C.   Because resins dry so quickly in alcohol (a solvent that
         evaporates quickly) it is difficult to apply them so that
         the finish looks uniform, flat and smooth.  Consequently,
         this technique was developed to enable the successful
         application of these fast drying resins.

    D.   For general information on scagliola, including its
         characteristics, uses and problems, see 09200-05-S.

    E.   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 MATERIALS

    A.   Shellac: Highest grade of blonde flake shellac.  NOTE:
         NEVER USE BLEACHED OR WHITE SHELLAC.

    B.   Cloth tampon: This is created by wadding a piece of
         cotton or wool inside a woven material such as linen.

    C.   Alcohol

    D.   Linseed oil or paraffin oil

    E.   Pumice

    F.   800-1600 grit wet and dry sandpaper

    G.   Clean, potable water

    H.   Clean, cotton cloths


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Verification of Conditions:

         1.   Determine the type of finish used before proceeding
              with this procedure.  Test a very small area in an
              inconspicuous location.

              a.   French Polish easily dissolves in alcohol.

              b.   Solvents and paint removers may be used to
                   remove types of varnishes.  CAUTION: NEVER USE
                   ALKALINE CLEANERS OR POULTICE-TYPE PAINT
                   REMOVERS ON SCAGLIOLA.  THESE MAY LEAD TO
                   DISINTEGRATION OF THE GYPSUM AND PRODUCE
                   EFFLORESCENCE.

              c.   Other types of finishes might include glue and
                   wax, various types of oil, combinations of oil
                   and shellac, pure shellac, cellulose lacquers,
                   styrene, and polyurethanes.  These may be
                   removed using chemical solvents, but should be
                   tested.  

              NOTE: CHEMICALS SHOULD ALWAYS BE TESTED, MONITORED
              AND SUFFICIENTLY NEUTRALIZED.

         2.   Determine which type of scagliola it is - true scag
              or marezzo. THIS WILL REQUIRE A CONSERVATOR'S
              EXPERTISE.  The biggest difference lies in how each
              is manufactured, applied and finished.  Marezzo is
              made in reverse order from the way true scag is
              produced and is generally a less labor-intensive
              process.  Recognizing the difference between the
              two can aid in better understanding the problem or
              failure.  The typical polish used for each type is
              also significant, as some polishes have proven to
              be detrimental to the material.

3.02 PREPARATION

    A.   Surface Preparation:

         1.   ALWAYS test polishing methods in an inconspicuous
              area to determine the effects of the finishing on
              the material and whether this procedure is suitable
              for use in this situation.

         2.   Remove dust and dirt accumulations from the surface
              prior to finishing.  See 09200-09-R for guidance on
              stain removal.

         3.   For finish removal (if required), mechanical
              methods are usually preferred over chemicals, as
              chemicals may cause the color to fade.

         4.   French Polish can be removed using alcohol and
              acetone combined with sanding screens or scotch
              brite pads.

3.03 ERECTION, INSTALLATION, APPLICATION

    A.   Using the special cloth tampon, seal the scagliola with
         a thin layer of shellac to coat any voids that are
         present below the surface.

    B.   Saturate the scagliola with linseed oil or paraffin oil.

    C.   Continuing to use the special cloth tampon, apply a
         mixture of pumice and alcohol to the surface, creating a
         paste that polishes by filling the pores in the
         scagliola.  NOTE: KEEP THE TAMPON MOVING AT ALL TIMES.
         DO NOT ALLOW IT TO STOP IN ONE PLACE ON THE SURFACE.

    D.   Using the same special cloth tampon, apply a mixture of
         pumice, alcohol and shellac to the surface. This mixture
         should consist of an increasing the amount of shellac and
         a decreasing amount of pumice supplied to the surface.

    E.   Lubricate the surface again with a small amount of
         linseed oil or paraffin oil.

    F.   Remove surface imperfections by rubbing with 800-1600
         grit wet or dry sandpaper.

    G.   Final polish with a new cloth tampon to achieve the
         desired surface sheen.

                         END OF SECTION