Scagliola (from the Italian scaglia, meaning "chips") is a technique for producing plasterwork columns, sculptures, and other architectural elements that resemble marble. Batches of pigmented plaster (ground alabaster or gypsum) modified with animal glue are applied to molds, armatures and wall planes in a manner that accurately mimics natural stone and marble. In one technique, veining is created by drawing strands of raw silk saturated in pigment through the plastic mix. Another technique involves trowelling on several layers of translucent renders and randomly cutting back to a previous layer to achieve colour differential similar to hewn marble. When dry, the plaster is polished until it resembles stone; beeswax is sometimes used for this purpose. Because the colours are integral to the plaster, the pattern is more resistant to scratching than with other techniques, such as painting on wood.
There are two scagliola techniques: in traditional scagliola coloured batches of plaster of Paris are worked to a stiff, dough-like consistency. The plaster is modified with the addition of animal glues such as isinglass or hide glue. Marezzo scagliola or American scagliola is worked with the pigmented batches of plaster in a liquid state and relies mainly on the use of Keene's cement, a unique gypsum plaster product that sets to an exceptionally hard state. It is typically used without the addition of animal glues. Marezzo scagliola is often called American scagliola because of its widespread use in the United States in the late 19th and early 20th centuries.
While there is evidence of scagliola decoration in ancient Roman architecture and craft, scagliola decoration became popular in Italian neoclassical buildings in the 17th century, and was imitated throughout Europe until the 19th century. In the United States it was popular in the 19th and 20th centuries. Important buildings featuring scagliola include Syon House in Brentford, London and Allen County Courthouse in Fort Wayne, Indiana.
Scagliola has historically been considered an ersatz material and an inexpensive alternate to natural stone. However, it has eventually come to be recognised as an exceptional example of the plasterer's craft and is now prized for its historic value as well as being used in new construction because of its benefits as a plastic material suited to molding in ornate shapes.
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