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Historic Preservation - Technical Procedures

Spectitle:

Architectural Scagliola: Characteristics, Uses And Problems

Procedure code:

0920005S

Source:

Interior's Handbook For Historic Interiors - Jeff Greene

Division:

Finishes

Section:

Lath & Plaster

Last Modified:

02/24/2012

Details:

Architectural Scagliola: Characteristics, Uses And Problems



ARCHITECTURAL SCAGLIOLA: CHARACTERISTICS, USES AND PROBLEMS


This standard includes general information on the characteristics
and common uses of architectural scagliola and identifies typical
problems associated with this material along with common causes of
its deterioration.


INTRODUCTION

Characteristics of scagliola:

-    Invention of modern scagliola is credited to Guido Sassi and
    Enrico Hugford in Lombardy in the 16th century

-    However, pigmented plaster, designed to imitate marble and
    other precious stones, was used much earlier - common on
    Egyptian tomb walls.    

-    Commonly referred to as "scag"

-    Traditionally, scag was made from a calcium sulfate
    composition (gypsum, animal glue and natural mineral earth
    colors), finely ground and mixed with water

-    In the United States, scag was also made from calcium sulfate.
    However, Keenes Cement was most commonly used for making scag,
    because this material had slow-setting properties and,
    therefore, didn't require the retarder component

-    Different formulas for scagliola are appropriate - one type
    may be better than another for a particular situation; an
    experienced scagliolist should be able to select the most
    appropriate mixture of materials


TYPICAL USES

Typical historical uses for scagliola include:

-    Covered Egyptian tomb walls

-    During the 16th century, used in a marquetry design for
    altars, tabletops, etc.

-    Architectural potential not fully realized until the 17th and
    18th centuries, when the craft was imported to Great Britain,
    Austria and Germany and used in Baroque churches

-    Architectural uses for scagliola in the churches included
    columns, altar pieces and ethereal clouds

-    In Europe, scagliola was common in churches and palaces

-    In the United States, scagliola was most commonly used in
    public buildings, state capitols, railway stations and
    theaters

-    Marezzo was also commonly used in the United States as a less
    expensive, less elaborate alternative to scag


NATURAL OR INHERENT PROBLEMS

-    Cracks: Often caused by differential movement between
    sections.  See 09200-13-R for guidance on repairing cracks.

-    Delamination: Typically a flaw in the manufacture of the scag
    and can be as subtle as the placement of the burlap
    reinforcement, the type and gauge of plaster used for the
    backing, temperature fluctuations and moisture. See 09200-06-R
    for guidance on consolidating delaminated scagliola.

-    Warped or bowed panels: May be caused by exposure to moisture,
    or by the natural tendency for these panels to contract.


VANDALISM OR HUMAN-INDUCED PROBLEMS

-    Dirt: Accumulation of surface dirt can result from lack of
    routine cleaning and maintenance.  See 09200-09-R for guidance
    on removing stains and efflorescence.

-    Staining: Can result from the presence of water on or behind
    the surface and can cause internal metal components to oxidize
    and produce metallic staining.  See 09200-09-R for guidance on
    removing stains and efflorescence.

-    Finish Failure: This may include surface abrasion, small
    nicks, scratches, and discoloration (yellowing) of the finish
    over time. See 09200-07-R for guidance on finishing and
    polishing.

-    Efflorescence:  Can result from the presence of excessive
    amounts of water, causing salts from adjacent masonry or
    withing the material to migrate to the surface.  See 09200-09-
    R for guidance on removing stains and efflorescence.

-    Fading: Inappropriate use of chemicals for cleaning can lead
    to color fading.

-    Bubbles or surface irregularity: Typically occurs when the
    retarder component of the scagliola mixture is too strong or
    too weak, causing inappropriate drying of the color coating
    and backing.

-    Other problems included missing or broken pieces, open joints,
    loose sections, improper repairs or maintenance, and
    inappropriate finishes.

                         END OF SECTION