Polished plaster

Polished plaster is a term for the finish of some plasters and for the description of new and updated forms of traditional Italian plaster finishes. The term covers a whole range of decorative plaster finishes, from the very highly polished Venetian plaster and Marmorino to the rugged look of textured polished plasters. Polished plaster itself tends to consist of slaked lime, marble dust, and/or marble chips, which give each plaster its distinctive look. A lime-based polished plaster may contain over 40% of marble powder.

Polished plaster is mainly used internally, on walls and ceilings, to give a finish that looks like polished marble, travertine, or limestone. Such plasters are usually applied over a primer and basecoat base, from one to four layers. They are finished (burnished) with a specialised steel trowel to a smooth glass-like sheen. Polished plaster is usually sealed with a protective layer of wax.

History
The history of polished plaster can be traced back to ancient times, with evidence of its use in ancient Egyptian, Roman, and Greek architecture. The technique was highly valued for its durability and aesthetic appeal, and it has continued to be used and refined throughout history.

Throughout ancient times, lime was a widely employed material for constructing plaster on both interior and exterior walls. The Greeks, in particular, made a remarkable discovery regarding the production of a special adhesive by subjecting limestone rocks to intense heat within expansive ovens. Nevertheless, this transformative process, which involved converting limestone into calcium oxide, carbon dioxide, and steam, posed significant challenges due to the requirement of extremely high temperatures, reaching approximately 2200 °F. The resulting substance, known as quicklime or lump-lime, was subsequently pulverized into a fine powder and combined with water in a process called "slaking." Through this procedure, a fundamental binding agent called "lime putty" was created and utilized for plastering purposes. The slaked lime, a dense and moist substance, would then be stored in a designated pit for several months, or even years, to ensure complete hydration. Historical accounts suggest that the Romans enforced a regulation stipulating that slaked lime could only be employed if it had aged for a minimum of three years.

Venetian plaster, a distinctive type of wall covering, boasts a rich historical legacy that traces back to ancient times, with its origins linked to Pompeii and the subsequent Roman Empire. Vitruvius, who lived around 80-70 B.C., documented the process of manufacturing lime plaster in his renowned work "De architecture" or "Ten Books of Architecture." These methods were further elaborated upon by Pliny the Elder in his book "Natural History," dating back approximately 2,000 years. The Romans referred to the finished product as "Marmoratum Opus," meaning "smooth marble." The rediscovery of Venetian plaster can be attributed to the Renaissance period, characterized by a renewed interest in the ancient techniques of Rome. Palladio, a renowned Renaissance architect, referred to the process as "Pietra d'Istria" since the plaster bore a striking resemblance to natural rocks such as marble, granite, and travertine commonly found near Venice. Palladio's architectural creations, although seemingly constructed from stone, were in fact composed of brick and stucco. The plastering process involved the initial application of a coarse layer of plaster known as "arricio," followed by subsequent layers of lime putty blended with powdered marble to achieve a smooth and polished surface. On occasion, pigments were added to the wet plaster to introduce vibrant hues.

During the Baroque period, Venetian plaster experienced a decline in popularity, echoing the diminished prominence witnessed after the fall of the Roman Empire. However, in the 1950s, a notable Venetian builder named Carlo Scarpa played a pivotal role in revitalizing the use of Marmorino in contemporary construction.[9] Scarpa not only adhered to the methods outlined by Vitruvius and Palladio but also introduced innovative techniques involving the utilization of animal hides and acrylic resins.

Venetian plaster
Venetian plaster is a wall and ceiling finish consisting of plaster mixed with marble dust, applied with a spatula or trowel in thin, multiple layers, which are then burnished to create a smooth surface with the illusion of depth and texture. Venetian plaster techniques include marmorino, scagliola, and sgraffito. When left un-burnished, Venetian plaster has a matte finish that is rough and stone-like to the touch. Un-burnished Venetian plaster is also very brittle and damages rather easily.

When applied correctly, Venetian plaster can be used to create a highly polished, rock-hard, marble-like finish. Venetian plaster is especially useful on surfaces where marble panels could not be installed easily, and on surfaces that would be too expensive to have carved from real marble such as columns, corbels, and curved walls.

Venetian plaster can be tinted, or colored using natural or synthetic colorants. The ability to tint Venetian plaster is especially helpful when a specific color of "marble" is desired, or when a color that does not exist naturally is wanted. Through the application of a top layer of wax sealant, Venetian plaster can also be rendered waterproof.