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Introducing Scagliola

Scagliola is a venerable technique that has plaster posing as marble.
Scagliola in Toronto’s Romanesque Old City Hall

Scagliola in Toronto’s Romanesque Old City Hall, 1889–99, designed by E.J. Lennox. 

A Brief History. Materials similar to scagliola can be traced back to ancient civilizations: in Egypt, for coating walls in tombs; in Greece, for lining aqueducts; in India, for finishing the interiors of domes.

In its modern iterations, scagliola is associated with Italy, where a key ingredient—selenite, a crystalline form of gypsum—is mined in the Apennine Mountains. Scagliola, the use of plasters to imitate marble, is attributed to Guido Sassi of Cari, Italy, who, working in the early part of the 16th century, introduced more colors as well as pieces of marble and alabaster to make the material harder.

The Material. Scagliola may be mistaken for fine stone, such as marble, lapis lazuli, or malachite. It can, however, be distinguished by its temperature when touched, as it feels warmer than stone. It sounds more hollow than stone when tapped. Scagliola is made from a mix of Keene’s cement (industrial, double-fired, quickset gypsum) and “size water,” a Scotch glue and water solution, to which lime-proof fresco pigments are added to provide color. The cement and glues both set up extremely hard, resulting in the final material being stone-like in composition and appearance. There are two types of scagliola: traditional scagliola, and marezzo, or American, scagliola.

Traditional scagliola starts with placing knobs of the pigmented plaster on a stone working surface.

It is then sprinkled with veining colorant composed of dry and crumbly gauged plaster mixed with stone dust. These materials are repeatedly lumped together and cut, resulting in the appearance of veining. The final lump is pressed tightly into a mold to form a loaf, and sliced. These slices are applied over a base of brick, wood, or plaster, about 1/8" more than the desired final thickness to allow for dimensional loss during polishing. To ensure its keying into the substrate, the slices are beaten onto the base with a flat mallet and gauging trowel. The surface is finished to its final smooth appearance by the polishing process described below.

Scagliola, Old City Hall

Note in the closeup that the base has joints, indicating it is made of real stone. The scagliola is missing material from an impact; the cracking shows it is scagliola, not stone.

Marezzo scagliola dates to the 18th century, and often is used for covering large, flat areas. This process involves silk threads soaked in colored dyes, placed on a glass sheet over which wet pigmented plaster is placed. Veining is created by the threads being drawn through the plaster, leaving colored trails that mimic marble veins. Once the plaster is partially set, it is transferred to the base by pressing the plate to the wall, leaving a smoother top surface than does traditional scagliola.

Both techniques employ a polishing process, which starts by honing with pumice stones and continues with several rounds of honing with a fine-grit Water of Ayr stone (or its modern equivalent) and damp sponges.

The next stage is “stopping,” during which a slightly set gauged cement mix is brushed over the surface to fill tiny holes and voids. As it dries, it is scraped off with a sharp-edged wood scraper. This is repeated several times. Once the last stopping coat is fully hard, a final application is wiped off with a damp rag. The final step is polishing with putty powder (oxide of tin) using rags and pure linseed oil and, sometimes, wax. The traditional finish is easy to maintain and will withstand gentle wear and tear.

It is important to differentiate between the methods only when repairs are required, since each requires a different approach.

Assessment. Prior to repairs, engage a qualified consultant. Only professionals can differentiate between traditional and marezzo, identify the nature and cause of deterioration, and perform repairs correctly. Drawings made to itemize all repairs should illustrate patterns of failure to inform causes and solutions. As always, the underlying cause of failure must be addressed before repairs are made.

To ensure a correct color match, clean a small area in an inconspicuous location to reveal the true color and. A set of samples targeting the original appearance should be prepared on boards, cured and polished, to compare with the cleaned original. Each sample will have its own recipe, and the final repair will use the exact recipe from the sample selected.

Repair. While durable, scagliola requires expert craftspeople to repair it. Repair materials must be compatible with the substrate, or the scagliola could be further damaged by new stresses, chemical reactions, and staining.

Scagliola repair

Ports are drilled into scagliola for the injection of adhesive, which will secure the scagliola finish to the substrate. The Co-Cathedral of St. Joseph in Brooklyn was built in the Spanish Colonial Revival style in 1912. 

To address missing material, clean away any damaged area, leaving the perimeter beveled towards the missing area. Rout out cracks with the floor of the gouge wider than the top. In both cases this slanted sidewall permits a good key between old and new material. For traditional scagliola, the material is applied to the crack in loaf form, ensuring that the grains match and the surface levels are marginally proud of the adjacent material.

For marezzo, the crack ispartially filled to within 3/16" of the finished surface and roughed up. Then colored threads are laid into the crack, matching the adjacent veining, and colored plaster is applied to slightly proud of the finished surface. The threads are drawn through the wet plaster to create veining, before curing.

Where scagliola has separated from the substrate, acrylic lacquers applied behind the scagliola, or injection of epoxy resins through the front face, can reattach it to the substrate. This work should be undertaken only undertaken by professionals.

Cleaning. Hand grime, or yellowing caused by a subsequent coating, frequently mar the appearance of scagliola. Removing grime and restoring the original finish is a simple if labor-intensive process. Scagliola historically had a finish coat of linseed oil or, in more recent installations, blond flake shellac, paraffin oil, styrene, or polyurethane. Modern coatings are not advisable, as they are difficult to repair later, and those repairs result in the removal of more historic material when they fail. When removing an inappropriate coating or deteriorated finish, use careful, mechanical methods in lieu of solvents, which can harm the scagliola. Once the coating is removed, a new stopping coat will restore the original luster. 

impasti di colore

The impasti di colore: knobs of pigmented plaster are sprinkled with veining colorant.

Damage & Deterioration

* While scagliola is remarkably durable, its shallow depth makes it fragile to impact and movement.

* A blow or impact may break off a portion of the material.

* If the substrate moves or cracks (due to seismic activity, building settlement, etc.), the scagliola may crack, just like finish plaster.

* Excessive moisture or humidity causes delamination. Because scagliola contains gypsum, it is hygroscopic. Moisture causes swelling, which may force the material to separate from the base. Moisture may cause portions of scagliola to dissolve and flake or effloresce.

* The surface may yellow due to hand soiling, exposure to ultra-violet light, or coatings later applied.

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