Removing Stains and Efflorescence from Architectural Scagliola
- Procedure code:
- The Interiors Handbook For Historic Buildings, Vol. II, by Jeff Greene
- Lath & Plaster
- Last Modified:
REMOVING STAINS AND EFFLORESCENCE FROM ARCHITECTURAL SCAGLIOLA
Reference: New York Landmarks Conservancy. "Restoring Scagliola to Glory", Common Bond, Vol.18, No.1 and 2, Fall-Winter 2003.
A. This procedure includes guidance on removing metallic stains and efflorescence from architectural scagliola by mechanical surface removal or poulticing.
NOTE: THIS PROCEDURE SHOULD ONLY BE PERFORMED BY AN EXPERIENCED ARCHITECTURAL CONSERVATOR.
1. A poultice is usually made by adding a solvent or chemical cleaning agent to water and blended with an inert filler to make a smooth paste. The paste is then applied over the stain using a trowel or spatula.
2. The liquid portion of the paste migrates into the scagliola where it dissolves some of the staining material. Then the liquid gradually moves back beyond the surface and into the poultice, from which it evaporates, leaving its burden of dissolved staining material in the poultice.
3. When the poultice has dried, it is scraped and brushed away.
B. Stains may be caused by the presence of water, either on the face of the scagliola or at the rear. Water can contain dirt and contaminants which can stain the surface. These contaminants can also migrate into the scagliola if the surface finish is deteriorated. Moisture within the scagliola can then cause internal metal components to oxidize, which can lead to metallic staining on the surface.
C. Excessive amounts of water present in the scagliola can result in the migration of mineral salts from adjacent masonry or from the scagliola to migrate to the surface, evident as a white powder (efflorescence).
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)
F. 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).
2.01 MANUFACTURERS AND REFERENCE SOURCES
A. Manufacturer: The Procter & Gamble Co.
B. Reference Source: American Institute for Conservation
A. Neutral pH detergent, such as "Orvus" (Procter & Gamble), or approved equal; check internet for retail/wholesale sources.
B. White absorbent material (molding plaster, untreated white flour, white tissue, paper towels, powdered chalk, talc, fullers earth or laundry whiting).
C. Mineral water
D. Plastic sheeting
E. Clean dry towels for blotting the area after treatment.
A. Sanding blocks and paper
B. Glass or ceramic container for mixing the solution
C. Wooden utensil for stirring the ingredients
D. Wood or plastic spatula
E. Masking tape
A. Verification of Conditions:
1. Determine the source of the staining. Is there evidence of moisture, leaks, etc.?
2. Determine which type of scagliola it is, e.g., true scagliola 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 scagliola 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 cleaning methods in an inconspicuous area to determine the effects of cleaning on the material and whether this procedure is suitable for use in this situation.
3.03 ERECTION, INSTALLATION, APPLICATION
A. For surface staining, the recommended procedure is to mechanically remove the surface layer of the scagliola to a clean surface layer below. See 09200-07-R for guidance on polishing the new scagliola surface.
B. For penetrating stains: Try poulticing with a neutral detergent. NOTE: NEVER USE ALKALINE CLEANERS OR POULTICE-TYPE PAINT REMOVERS ON SCAGLIOLA. THESE MAY LEAD TO DISINTEGRATION OF THE GYPSUM AND CAN PRODUCE EFFLORESCENCE.
1. Thoroughly rinse the area to be treated with mineral water.
2. Mix the liquid solution to be used in a glass or ceramic bowl.
3. Thoroughly moisten the stained surface with this liquid. Be sure to dampen well beyond the stain.
4. Mix the remaining liquid with the white absorbent material to form a paste the consistency of oatmeal or cake icing. (Approximately one pound of paste is needed for every square foot of surface area to be treated).
5. Using a wooden or plastic spatula, apply the paste to the stained surface in layers no more than 1/4 inch thick. The poultice should extend well beyond the stain to prevent forcing the stain into previously clean scagliola.
6. Check the coating for air pockets or voids.
7. Cover the poultice with plastic sheeting and seal with masking tape.
8. Let set for 48 hours (unless otherwise specified).
9. After set period, dampen the poultice with mineral water.
10. Remove the poultice with a wooden or plastic spatula to avoid scratching the surface.
11. Again, thoroughly rinse the cleaned area with mineral water, blot with clean towels and allow the surface to dry.
12. Once the surface has dried completely, check for remaining residue and repeat the treatment if necessary.