Historic Preservation - Technical Procedures
Polishing Architectural Scagliola
Interior's Handbook For Historic Buildings - Jeff Greene
Lath & Plaster
Polishing Architectural Scagliola
POLISHING ARCHITECTURAL SCAGLIOLA
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
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
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
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. 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.
D. Linseed oil or paraffin oil
F. 800-1600 grit wet and dry sandpaper
G. Clean, potable water
H. Clean, cotton cloths
A. Verification of Conditions:
1. Determine the type of finish used before proceeding
with this procedure. Test a very small area in an
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
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
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.
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
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
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