Scagliola

The highest expression of the plasterer’s art has been created, lost and rediscovered. Yet for the past two millennia Scagliola has never ceased to fascinate nor witness its aura of mystery diminish.

Part sculpture, part science, the sophisticated process that gives birth to Scagliola demands the focused mind and precision of a chemist, the hands of an experienced plasterer and the subtle, sensitive eye of an artist.

History

Scagliola results from a meticulously programmed sequence of tinting, blending and arranging plasters to imitate marble. Archaeologists have discovered that the Romans and the Egyptians employed methods, long since forgotten, to imitate marble in plaster. However, it was in the 16^th century Italian Renaissance that the contemporary approaches to Scagliola were conceived and perfected to effect complicated inlays in furniture surfaces.

By the early renaissance many desired marbles were rare or had been exhausted. Scagliola could imitate such marbles as well as create colours and patterns that did not exist in nature. The use of Scagliola soon expanded to casting in ornament, column shafts and even entire walls, a process that became known as Stucco Marmo.

The use of Stucco Marmo expanded throughout continental Europe and finally to Britain in the 18^th century. Prominent uses of Stucco Marmo Scagliola in England include column and pilaster shafts at Buckingham Palace and the Syon House by Robert Adam. A significant breakthrough in plaster technology was achieved in the mid 19^th century with the advent of Keen’s cement. Keen’s paved the way for a new method for producing Scagliola called Marezzo, known in the United States as American Scagliola due to its ready acceptance and prominent use from the mid 1800’s until the Great Depression.

There are countless recipes, historical and contemporary, in all cases secret and proprietary for the ingredients and mixing of Scagliola. Let’s attempt to at least partially pull back the curtain of secrecy with a basic explanation of Scagliola manufacture.

Manufacture

Traditional Scagliola can be done in situ (in place) or on a bench. Work performed in situ requires several precautions in preparation of the substrate. In all cases the work environment should be clean, warm and dry.

Finely ground Plaster of Paris is used as the base material. Animal hide glue high in collagen such as rabbit or isinglass is prepared the day of manufacture to retard the plaster and add strength to the work. Dry mineral pigments can be introduced directly into the dough, mixed with dry gypsum, or emulsified depending on desired effect. Optional ingredients include whiting or selenite (ground gypsum) as filler, linseed oil to complement the glue as a retarder and aid in workability, and marble chips for decorative effect.

Much like a bread maker working with flour, yeast and water, the artisan kneads Plaster of Paris and glue water to the consistency of firm dough. This is best accomplished by forming a ring of dry plaster surrounding a central "castle" of plaster. The "moat" is filled with glue water and the process of cutting and kneading begins.

Through a series of slicing, addition of pigments, folding and re-joining the whole of the mass is mixed and set aside as large balls placed in ratios and arrangements that achieve the desired result: a counter-type of true marble or a fanciful creation. Depending on the desired outcome, from the initial mixing colored slurries and other preparations are set aside for decorative effects. Much of the artisanship lies in a process of mental reverse engineering. One must conceive the desired outcome, have all materials on hand and systematically take steps to accomplish the effect.

Typically the plaster is built up to 1/2” to 5/8” thickness, leaving 1/8” for cutting of the surface. Once the material has achieved an initial set it can be planed with an appropriate cutting tool such as a Berthelet or French razor, removing the 1/8” excess to realize a flat surface. At this point the material is still malleable and can be allowed to cure as a flat panel. Alternatively, slices of Scagliola can be pressed into a mould or directly on a keyed plaster substrate in situ. For ornamental work such as balusters, urns and column shafts, the Scagliola can be wrapped around an appropriate base and turned on a lathe.

Once the Scagliola has been allowed to set and dry naturally the work of polishing can commence. Traditionally, after cutting the Scagliola natural pumice stones and damp sponges are used to smooth the work. The final smoothing and polishing historically was achieved with Water of Ayr, a natural snake stone from Scotland renowned principally as a hone for polishing barber straight razors. Modern polishing techniques arrive at a similar result with increasingly fine grit wet/dry sandpaper. The finished surface can be rubbed with linseed oil to increase luster, hardness and add a measure of protection from stains.

The Marezzo or “American Scagliola” technique was a true innovation that followed a distinct methodology. Cutting of the surface is not required as the veining and coloration is done on the face of the mould in a thin layer for ornamental pieces. Flat panel work is typically carried out on thick plate glass so the patterns created can be seen from below.

The Marezzo plaster mix is based on Keen’s cement, a slow-setting gypsum cement that does not necessitate the use of retarders or hardeners. Silk threads are used for veining and dry mineral tints can be used to provide color.

Conclusion

Scagliola has enjoyed a rich history adorning many of Europe’s most prestigious works of architecture from the Renaissance through the Neoclassical periods. Likewise in the United States Marezzo figured prominently in many of our architectural triumphs of the 19^th century and still is to be admired in the grand entrances of court houses, state capitols, railway stations and fine hotels across the country.

This article was coauthored by Patrick Webb and Sloan Houser

Photo by Walter Cipriani