It was history before time. Seven thousand years before Christ walked the earth, a settlement of villagers in what is now northeast Jordan smears a concoction of lime-plaster on the floors, walls and ceilings of their simple mud huts. The result was pleasant, with a starch white finish that made the room much more livable. The knowledge was passed down through the generations. How the technology was derived remains a mystery.
During the Bronze Age (5600 BC), the early Greeks became more proficient at the task, mining quarries of limestone that they had discovered and placing the rocks in large fire pits. At nearly 2,200 degrees Fahrenheit this broke down the limestone rock (calcium carbonate) into its basic component parts, through the release of carbon dioxide and steam, leaving behind lumps of calcium oxide. This remaining crumbly, friable material was allowed to cool and then it was pulverized.
This material now known as “quick-lime” or “lump lime,” was recombined with water in a process now referred to as “slaking” to make the basic binder or lime putty for the plaster. This chemical (exothermic) reaction creates a violent liberation of heat which dissipates over time. For this reason the slaked lime, which took on the consistency of heavy cream, was stored and covered in a pit and aged for months or even years. This also prevented the lime from being exposed to air, at which time it would begin to cure back into calcium carbonate by a process known as carbonation, in which the lime putty sequesters carbon dioxide back from the atmosphere.
At the time of the first Roman Empire, Caesar Augustus entrusted the charge of codifying building laws and principles of the Greek and Roman civilizations to one of his senior military engineers, Marcus Vitruvius Pollio. Astute in the workings of the great army’s artillery such as the Ballistae, Catapult and Trebuchet, Vitruvius shared Augustus’s passion for ensuring that knowledge that had been gained through the ages, was not lost. By Augustus’s directive, Vitruvius engaged in writing a great treatise that encompassed the disposition of ten books on the science of building. It is in Vitruvius’s second book on materials that he elaborates on the principles of producing a finished wall. Here are some of his words from over 2,000 years ago:
“When the lime is rich and properly slaked, it will stick to the tool like glue, proving that it has been completely tempered.”
“When not less than three coats of sand mortar, less the rendering coat have been laid on, we must make the mixture for the layers of powdered marble.”
“After this powdered marble has been spread on and gets dry, lay on a second medium coat. When that has been applied and well rubbed down, spread on a finer coat.”
“These colors, when they are carefully laid on stucco still wet, do not fade but are permanent.”
“Owing to the solid foundation given by thorough working with polishing instruments, and the smoothness of it, due to the hard and dazzling white marble, will bring out the brilliant splendor of the colors which are laid on at the same time with polishing.”
In effect, Vitruvius was identifying a seven step process which included: a dash coat, a scratch and brown coat; the application of three coats of finish plaster and final polishing. The Romans referred to the finish as Marmoratum Opus meaning “marble capable of taking a high polish.”
As might be expected, polished plaster was seen as an unnecessary step for most common utilitarian uses. While lime-based plaster flourished through the ages in creating wall planes that were monolithic, clean and vermin proof, the processes of creating beautiful polished plaster walls became forgotten over the centuries.
The Renaissance spirited in an age of enlightenment. The re-discovery of ancient classical authors such as Plato, Cicero and of course Vitruvius ushered in a new found awareness in the “antiquities” as they were known. In 1414, Vitruvius’s “10 Books on Architecture” reintroduced the processes of creating polished plaster to the Renaissance world. Many involved in the medium today attribute polished plaster’s rebirth to Renaissance architect “Palladio” who described the process as “Pietra d’ Istria,” which loosely translated refers to the resemblance of the plaster to natural stone formations of marble, granite and travertine that surround the region of Venice. To the untrained eye, much of Palladio’s work appears to be stone, but look closer and you will find that they are actually of brick and stucco (lime-based). The first layer of coarse plaster was referred to as arricio, this was followed by several layers of lime putty with powdered marble pigmented integrally or a fresco (plaster still wet) to give a smooth surface or intonaco. Marmoratum Opus became transcribed to the Italian Marmorino or “little marble” by virtue of the marble dust that was added to the lime putty to impart the polished stone look of the plaster.
Much like what happened as a result of the fall of the Roman Empire before, polished plaster’s popularity waned as the Renaissance transitioned to the Baroque period. In the 1950s, Venetian architect Carlo Scarpa is largely attributed as the person who was most influential in reviving the craft of Marmorino in many of his contemporary designs. Scarpa not only looked to those processes identified by Vitruvius and translated by Palladio, he innovated in coming up with modern compositions that included animal hide glues and later acrylic resins.
You may have noticed in reading this article to this point that with the exception of the title, “Venetian” plaster has generally been avoided. That is because the term in actuality is a distinctly modern American name that invokes the image of Old World decorative plastering technique. By and large it recognizes the processes described by Vitruvius so long ago, but it also encompasses a variety of newer techniques that emulate the look of leather, suede, honed stone or sandstone. Polished plaster in the vein of Old World methods is probably better described by calling it Marmorino or even Pietra d’ Istria as coined by Palladio.
Interior: While there are today many derivative products that aspire to the look of Marmorino, there are also many purists that insist on the true descendants of the ancient Roman compositions and methods. However even the purists are resigned to the fact that the economy of Venetian plasters are best suited to drywall applications. In this respect, most manufacturers require a Level 3 to 5 drywall finish. Typically, per modern methods a sanded (marble dust) acrylic primer or bonding agent is applied to the prepared drywall to provide some “tooth” to the surface. The finish plaster coat is then troweled on in typically three applications and burnished with a steel trowel for the polished effect. In some cases an additional wax coat is applied to further bring out the sheen of the polished plaster and to impart better wear and durability.
Exterior: Some manufacturers allow for their products to be installed over a base of Portland cement (stucco), cement board, properly prepared masonry or even EIFS. The manufacturer of the Venetian plaster product should be consulted to determine whether an application is viable for the climate conditions in which it is intended and whether any warranty for any other related product may be affected by its application.
Venetian plasters are naturally less prone to cracking than other plaster materials.
Lime-based Venetian plasters have been known to go through a process known as autogenously healing where small cracks actually fill in over time.
Venetian plasters are vapor permeable.
Venetian plasters perform well in wet climates.
Because of their higher alkalinity level (above neutral pH) lime-based Venetian plaster is a natural mold inhibitor.
Time tested, in millenniums: Lime-based plasters have witnessed and survived the rise and fall of the Greek, Egyptian and Roman empires.
Many Venetian plasters are based in natural and abundant occurring minerals of lime and marble.
Venetian plasters actually attract and sequester carbon dioxide in a process known as “carbonation.”
Without question, Marmorino is the most recognized of the Venetian plaster names; however different regions of Italy have inspired yet further formulations that are closely guarded by generations of craftsman. Some of these formulations may also include quartz, kaolin or other pulverized minerals in their makeup. Similarly these visages have spawned modern polymer formulations that emulate or have creatively challenged some of these more traditional methods. Venetian plaster, it should be understood, encompasses not only Marmorino, but many old and new decorative plastering materials and techniques. W&C