Polishing Architectural Scagliola
- Procedure code:
- Interior's Handbook For Historic Buildings - Jeff Greene
- Lath & Plaster
- Last Modified:
New York Landmarks Conservancy. "Restoring Scagliola to Glory", Common Bond, Vol. 18, No.1 and 2, Fall-Winter 2003.
- 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.
- 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.
- 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.
- For general information on scagliola, including its characteristics, uses and problems, see 09200-05-S.
- See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- 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).
- Shellac: Highest grade of blonde flake shellac. NOTE: NEVER USE BLEACHED OR WHITE SHELLAC.
- Cloth tampon: This is created by wadding a piece of cotton or wool inside a woven material such as linen.
- Linseed oil or paraffin oil
- 800-1600 grit wet and dry sandpaper
- Clean, potable water
- Clean, cotton cloths
- Verification of Conditions:
- Determine the type of finish used before proceeding with this procedure. Test a very small area in an inconspicuous location.
- French Polish easily dissolves in alcohol.
- 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.
- Other types of finishes might include glue and wax, various types of oil, combinations of oil and shellac, pure shellac, cellulose lacquers, tyrene, and polyurethanes. These may be removed using chemical solvents, but should be tested. NOTE: CHEMICALS SHOULD ALWAYS BE TESTED, MONITORED AND SUFFICIENTLY NEUTRALIZED.
- Determine which type of scagliola it is - 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.
- Surface Preparation:
- 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.
- Remove dust and dirt accumulations from the surface prior to finishing. See 09200-09-R for guidance on stain removal.
- For finish removal (if required), mechanical methods are usually preferred over chemicals, as chemicals may cause the color to fade.
- French Polish can be removed using alcohol and acetone combined with sanding screens or scotch brite pads.
3.03 ERECTION, INSTALLATION, APPLICATION
- Using the special cloth tampon, seal the scagliola with a thin layer of shellac to coat any voids that are present below the surface.
- Saturate the scagliola with linseed oil or paraffin oil.
- 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.
- 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.
- Lubricate the surface again with a small amount of linseed oil or paraffin oil.
- Remove surface imperfections by rubbing with 800-1600 grit wet or dry sandpaper.
- Final polish with a new cloth tampon to achieve the desired surface sheen.