Repairing Cracks In Architectural Scagliola
- Procedure code:
- Interior's Handbook For Historic Buildings - Jeff Greene
- Lath & Plaster
- Last Modified:
REPAIRING CRACKS IN ARCHITECTURAL SCAGLIOLA
A. This procedure includes guidance on stitching cracks in
architectural scagliola caused by movement between two
panels. The stitching of cracks is typically performed
when it is not possible or practical to remove and reset
the panels, or face-pin the pieces.
NOTE: THIS IS A VERY SPECIALIZED AND LABOR INTENSIVE
PROCEDURE. IT SHOULD ONLY BE PERFORMED BY AN EXPERIENCED
B. For general information on scagliola, including its
characteristics, uses and problems, see 09200-05-S.
C. 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. Keene's Industrial Quick Set Cement - USG Corporation
B. Limeproof fresco pigments
C. Sandpaper (must be glued to sanding or blocks)
D. Clean, potable water
E. Clean, soft cotton cloths
A. Electric dremel tool or pneumatic die grinder
A. Verification of Conditions:
1. Determine the source of the crack. Is there
evidence of movement between sections?
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: ALWAYS test compatibility of repair
mixes, including color and composition.
3.03 ERECTION, INSTALLATION, APPLICATION
A. Rout out crack using an electric dremel too or a
pneumatic die grinder.
B. Channel crack in an inverted "V".
C. Cut slots into crack - about 1" long and 1/2 the depth of
material to be fixed.
D. Inject epoxy adhesive into slot and insert headless
stainless steel screw into slot. Leave approximately
3/8" space on top for backing and touching in.
E. Touching In:
1. Prepare plaster mix using Keene's Industrial Quick
Set Cement and limeproof pigments. Mix enough for
the entire touch in.
a. DO NOT MIX THE PLASTER INTO THE WATER. Put
the water in the container and sprinkle the
plaster around the edges.
b. Carefully mix in selected pigments. DO NOT
BEAT THE MIXTURE, as this accelerates the
2. Set a small amount of mix aside for use later.
3. Thoroughly wet the surrounding area of scagliola to
4. Backfill hole or crack with scrim and white plaster
(Keene's cement) to about 3/16" to 1/8" from the
5. Mix dry pigment into the plaster mix on the palette
to achieve the desired color.
6. Trowel pigmented plaster into the crack or hole to
match the existing surfaces.
7. To give the appearance of veins:
a. Pull a needle threaded with pigmented silk
through the mix.
b. Build up plaster 1/8" above the surface.
Allow to cure.
c. Sand the surface with wet or dry sandpaper so
that it is uniform with the surrounding
d. Wipe the surface clean using clean, soft
e. Fill small voids in the surface by "stopping
1) Apply a very thin mixture of plaster over
the surface; mixture should be pigmented
a neutral color.
2) Before the mixture cures, strike it off
in a vertical direction using a wood or
plastic spatula. Small fissures should
remain filled from this process.
3) Rewet the surface with clean, clear
4) If fissures were not completely filled,
repeat the process of "stopping in".
This time, strike off in a horizontal
5) Rinse the surface again with clean, clear
water and dry with clean, cotton cloths.
6) Carefully sand the surface again.
8. For guidance on polishing the scagliola surface,
END OF SECTION