Repairing Cracks In Architectural Scagliola
- Procedure code:
- Interiors Handbook for Historic Buildings, by 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 twopanels. 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 CONSERVATOR.
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 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 tool 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 setting time.
2. Set a small amount of mix aside for use later.
3. Thoroughly wet the surrounding area of scagliola to be treated.
4. Backfill hole or crack with scrim and white plaster (Keene's cement) to about 3/16" to 1/8" from the surface.
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 surfaces.
d. Wipe the surface clean using clean, soft cotton cloths.
e. Fill small voids in the surface by "stopping back":
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 water.
4) If fissures were not completely filled, repeat the process of "stopping in". This time, strike off in a horizontal direction.
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, see 09200-07-R.