Depth and Nuance
With Venetian plaster, the usual choice applies: costly but beautiful historic materials or quick, cheaper synthetics?
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By James Gloria
Venetian plaster is a material resembling thick paint that can be tinted with pigments and layered over other faux finishes, resulting in a more tactile surface than paint gives. All photos: James Gloria
As an instructor and sometime student of decorative techniques, it has become clear to me that often the best part of taking a class is the interaction among skilled artisans. Many insights into the arcane knowledge of one craft can be gleaned while studying another.
One technique that was always talked of by my scagliola students was Venetian plastering. As a decorative painter, I was aware of a material resembling thick paint that could be troweled on a wall and burnished to a high sheen. Tinted with pigments and layered over other faux finishes, the look and feel of this “plaster” was much more tactile than paint.
It did not take long to learn that this ready-made material was a mere imitation of a surface that dates at least to the fourth millennium B.C. I needed to know more. It was my good fortune to have as a student artist and sculptor Orazio De Gennaro, a specialist in traditional Venetian plasters. It so happened that we were both planning a trip to the Venetian School for Architectural Heritage on the island of San Servolo, Italy, to study scagliola. I received more than my tuition’s worth there. A communal meal in the company of several plasterers, a boat ride away from the finest examples of stucco work in Europe, was an incomparable experience. I ended up learning as much about Venetian plasters as scagliola. Upon my return home, I hit the books to study and called up Orazio to fill in the details of the process.
There are many steps involved in producing Venetian plaster, with different materials and tints possible.
Limestone is one of the world’s most common materials, readily available worldwide. It has been used for thousands of years as the main component of stucco, mortar and plasterwork. The pyramids of Egypt contain plasterwork from at least 3000 B.C., showing a remarkable level of refinement. According to 19th-century authority William Millar, in his book Plastering Plain and Decorative, in Pompeii and later in ancient Rome, the craft of plastering reached new heights with the introduction of multi-layered plaster using fine marble dust as an aggregate. The final coats were burnished and colored either integrally, or a fresco. Architect and engineer Vitruvius described a seven-step process for producing a finished wall in the first century B.C. Several layers of coarse plaster leveled the rough Roman masonry and acted as a base for the finer finishing layers that included finely ground marble dust. The walls were then soaped and waxed to protect them. The finishes were called opus marmoratum, owing to their marble-like hardness and beauty.
The process remained essentially unchanged through two millennia. It was rediscovered during the Italian Renaissance, acquiring the name “marmorino.” “Pietra d’Istria” was its moniker when revived again by Palladio in 18th- and 19th-century Venice. The latter name referred to the pure white marble that was used as the aggregate. The processes remained faithful to the ancients through the 1950s, when it had another revival by the Italian architect Carlo Scarpa. However, he also introduced some significant changes, diverging from the traditional craft, and creating a new type of finish.
Vitruvius described a seven-step process for making Venetian plaster, where several
layers of coarse plaster leveled the rough Roman masonry and acted as a base for the finer finishing layers that included finely ground marble dust.
In conjunction with his craftsman Eugenio De Luigi, Scarpa created “Pastellone Scarpiano,” also known as Stucco Lustro. This was the precursor to today’s synthetic Venetian plaster. It was made with fine marble dust, as per the ancients, but replaced the lime matrix at first with hide glues and other materials and, later with acrylic resins. This substitution was an elemental change, because lime-based plasters take advantage of what is known as the “Circle of Lime.” This is the unique ability of limestone to transform from raw stone into kiln-fired quicklime through slaking with water into a thick putty. Once applied to a wall and combined with marble dust, it converts back into limestone. Lime-plastered walls re-crystallize over time by absorbing carbon dioxide from the air, chemically changing back into limestone, while acquiring the hardness and crystalline beauty unlike that of any other finish.
Resin-based plasters, while superficially resembling lime plaster, betray the qualities of acrylic resins with a synthetic sheen and reduced density and hardness.
Top: After a trowel is loaded with a colored plaster and two base coats have dried, the first of the finish coatings, or "intonaco", is applied.
Above: The intonaco is then burnished to provide a high sheen.
Lime-Based Venetian Plaster
Where Vitruvius described seven layers of stucco to create a completed wall, the modern plasterer generally uses three to five coats. The Romans constructed their buildings of rough masonry, necessitating several preliminary coatings of coarser plaster in order to even out the surface of the wall.
Although certainly closer to the plasterer of past millennia, De Gennaro must accommodate modern construction practice in his quest for authenticity. Drywall surfaces have to be considered. He therefore needs only two base coats of lime and sand. The base or scratch coats add additional mass and stiffness to the wall that will help support the pressure of later burnishing. Mass also allows for more open time in the "intonaco," or top coat.
The first base coat is reinforced with fiber and a small amount of acrylic binder to aid adhesion to the drywall. Slicker surfaces are first treated with an additional binding agent. The acrylic binder on the wall knits with the binder in the plaster to help adhesion. The total thickness of these preparatory layers is 1/8 to ¼ in.
After each of the first two base coatings is dry, the finish coating can begin. These final coats are composed mainly of lime and marble dust. Finely ground lime-proof mineral pigments are added to color the material. They are then mixed with a small amount of water to aid dispersion. Linseed oil and soap are also added to impart luster, improve workability and increase water resistance. The topcoat is applied in very thin layers with a trowel or small spatula. Successive coats must be applied wet onto wet, to ensure adhesion.
When this material sets but before it is completely dry, the final step is to burnish the entire surface with the edge of a mason’s trowel. The trowel is held at a 30-degree. angle to the wall and rubbed over the surface in a circular motion.
Lastly, a solution of olive-oil soap is applied as a thin liquid. This serves two purposes: to penetrate the plaster, sealing it, and to even out the absorption of the top coating of wax, ensuring a uniform depth to the final appearance of the color. The acidic soap also bonds with the alkaline lime, forming a new compound that is more water resistant.
The wax, applied with a trowel, greatly enhances the depth of color. It provides additional sheen, as well as another layer of protection from water and incidental wear.
Top: A solution of olive-oil soap is then applied as a thin liquid. This seals the plaster and evens out the absorption of the top coating of wax, ensuring a uniform depth to the final appearance of the color.
Above: The wax, applied with a trowel, greatly enhances the depth of color. It provides additional sheen and another layer of protection from water and incidental wear.
Synthetic Venetian Plaster
As stated above, Scarpa first introduced a synthetic material in the 1950s. He replaced the lime with resins to achieve more intense color effects not possible with lime plaster.
Today’s synthetic Venetian plasters consist of calcium carbonate particles in an acrylic latex binder. Essentially, these mixtures are latex paint with added marble dust. A look at ingredient labels will reveal pre-mixed synthetic Venetian plasters partly consisting of acrylic latex, propylene glycol (increases open time), silica (sand) and calcium carbonate (marble dust). The marble dust is what forms the acrylic polished surface. Instead of a lime matrix to contain it, the aggregate is suspended in resin.
Like the traditional plasters, the synthetics are tintable with mineral pigments. They are applied with a trowel or spackling blade in several thin layers directly over existing painted walls. No scratch coats or other preparatory layers are necessary. Once a layer dries, usually from two to four hours depending on the thickness of the plaster, it can be burnished. Faux glazes applied in between layers and worked into the surface can also enhance the depth. Unlike true Venetian plaster, burnishing can happen at any time from the initial set to upwards of seven days.
The result is a polished sheen that is smooth to the touch, with some depth and nuance, but without the density, thickness and durability of a lime finish, not to mention limestone’s crystalline beauty. Though not necessary, a layer of wax will contribute added depth and soften some of the synthetic sheen. Mica powders can impart a further pearlescent dimension when added to the wax layer.
There is a trend in culinary circles called the “slow food” movement, in which attention to raw ingredients and thoughtful preparation preempts utility and speed. There is an equally fastidious, process-oriented movement in the decorative arts, using pre-industrial techniques, in which off-the-shelf products are shunned. The inherent beauty of materials and careful application is at the heart of the traditional Venetian plaster.
When considering both approaches, the flexibility and expediency that come with using acrylic-based solutions is seductive. Cost is also a factor, as a lime-based plaster is both labor intensive and unforgiving to the unskilled. However, those desiring subtlety, depth and durability would not be satisfied with anything less than a true Venetian plaster.
(For further reading, William Millar’s Plastering, Plain and Decorative and Kurt Wehlte's Materials and Techniques of Painting are invaluable. Both are available at Kremer Pigments in New York, NY, along with many of the materials used in the techniques described.)
James Gloria teaches scagliola, fresco, murals and other decorative techniques to both beginners and professionals. He currently offers weekend and weeklong classes in his studio in Bangor, PA. His decorative work, scagliola and other projects, along with class schedules, can be viewed online at www.jamesgloria.com.
Click here for materials and contractors for creating Venetian plaster surfaces and finishes