The Original Venetian Plaster was “Lime Plaster”
The best part of lime plaster (plaster made from limestone) is the look, and this is what is driving the surge in interest in lime plaster – Venetian Plaster – finishes in the United States.
The look of lime is natural. The light seems to go deeper into the plaster than with Portland cement mixes and acrylic finishes. Common modern building materials seem to have a flat, synthetic look to them but lime finishes have an iridescence, a translucency that makes the finishes look richer. And when the light bounces off a lime building and enters your eyes, it evokes emotions tied to tiny European towns, built house by house by artisans trained over generations in the family business.
So what’s so different about lime to explain all this emotional response? After all, it’s only limestone, right? Simply put, lime is Calcium Hydroxide that, when exposed to the CO2 in the atmosphere, becomes limestone. And, if the manufacturer has mixed the right pH-tolerant pigments in with the lime mix, when the product reverts to limestone, it will look like a rock born from the earth. This is why earth tones work so well in lime plaster mixes.
The depth of color saturation and surface translucency is there immediately after the completion of plaster work. So, the building looks like an Italian villa that has seen generations and families come and go over decades or centuries. On the interior, ceiling pot lights bring out the glow in the lime in the evening. But on the exterior is where lime finishes really shine – the light plays off the aggregate (typically marble dust) and penetrates into the colored limestone and returns to the viewer warm and inviting.
What’s in Venetian Plaster – Lime Plaster?
Today’s lime plasters are typically composed of lime, marble and quartz aggregate, and water. When it’s made the old way, the lime is fired in a kiln at about 900 degrees Celsius for a week to create quick lime. For the Chemistry major, that is Calcium Oxide. In a quaint nod to European environmental sensitivity, the traditional Italian lime kilns are fired with sawdust from nearby furniture manufacturers. Coal could not be used because of the Sulfates it would introduce into the lime. The quick lime thus produced is evenly calcined and incredibly thirsty for water. So much so, in fact, that the simple act of adding water to it creates a violently exothermic (releases heat) reaction that creates a huge cloud of steam. Through this reaction, the baseball-size lime rocks are transformed into a liquid slurry called Latte di Calce (lime milk), made up of water and fine lime particles on the order of 1 micron. Grinding the lime is not necessary to achieve the fineness needed for fine plaster finishes.
The product of the slaking process depends on the production process. In the traditional method of making “seasoned slaked lime”, additional water is added in the slaking process to create a lime putty called Grassello di Calce, technically Calcium Hydroxide plus free water. This putty is aged in a remarkable transformation in which the water content, workability, plasticity, crystal structure, and particle size develop into ideal plaster materials. After three months of slaking, lime is somewhat plastic and spreadable and provides good performance in mortars. After two years, it is striking how much more plastic the seasoned slaked lime has become and how much more body it imparts to the product. During this time, the water content has increased significantly. Under the surface, even more, is different after two years, as the particles have morphed from prism-shaped particles into flat plate-like crystals in an orderly crystalline structure that react differently to light. It is this long slaking process that gives the best lime plasters and Venetian Plasters their difference in workability and aesthetic impact, so ask your manufacturer about how their lime is made and slaked, and for how long.
In the modern high-volume lime production process, just enough water is added during the slaking process to hydrate the lime into a dry powder state. Technically, this is also Calcium Hydroxide, but the free moisture is almost zero. This allows for ease of transportation and handling in many industrial processes, from agriculture to wastewater processing, gold mining, and also our mundane world of building materials manufacturing. Adding water to bag lime creates a plastic putty with many of the same characteristics as the seasoned slaked lime – plasticity, water retention, flexibility, workability, and decreased water penetration. Because the slaking of the lime occurs so quickly in the modern industrial process, bag lime can contain a slightly higher percentage of unhydrated Oxides than lime slaked for 2 years. This can cause “pops” and “pits” in the wall when the lime particles pop off the wall. If your lime plaster product comes dry in a sack, then it is made with hydrated (“bag”) lime.
Whatever lime is used, it needs to be mixed with clean, properly graded, and shaped aggregate to produce a durable plaster finish. The European tradition is to use Carrara marble aggregate, which is chemically Calcium Carbonate or limestone. The geologic process has smiled on all marble, though, because it is strong and creates beautiful effects with light. Artisans from generations ago used hair to give strength and crack resistance. And other creative additives from oils to eggs were used at one time or another to achieve the plasterer’s vision. Today’s Venetian plaster finishes borrow from the knowledge gained by generations of plasterers and are designed to showcase the beauty of lime and marble, with the performance-enhancers to engineer around some of lime’s inherent limitations.
The Modern Lime Plaster Finish
Whereas the traditional lime plaster system required months to cure out in three coats, today’s applications provide all of the beauty and function of these traditional systems with much more practical working conditions. On the interior, lime finishes can go over Level 3-5 drywall – Level 3 if you’re putting on an aggregated lime plaster such as Rialto Antiqua I or Antiqua II. You will need Level 4 or Level 5 drywall, if you’re applying a finer lime plaster to achieve a shiny marble effect, such as Rialto Epoca Spatolato. Typically, a primer coat is needed to bridge the interface between gypsum wallboard and lime plaster. These are commonly acrylic or silicate primers with marble aggregate in them to provide for good adhesion to the substrate, a good key for the plaster finish coat, superior crack resistance, and mold- and mildew resistance. The finish coat is troweled on in two or three coats. This is necessary to achieve the full depth of color and mottling that only true lime products can produce.
In exterior applications, lime products can be matched up with modern materials to create an easy-to-apply, durable, attractive finish. In traditional stucco market areas such as the Southwest, lime plaster is usually applied over lath, scratch, and brown coats of Portland cement and sand, just as with normal stucco products. This system gives the fast completion time that builders demand from today’s materials, but still achieves the look of lime that takes yesterday’s architecture to a higher level. The brown coat should be given additional care to ensure that no additional lime plaster material is used to fill in a rough brown, as these materials are expensive. As with interior plaster applications, a primer coat is commonly employed to bridge the transition between Portland cement and lime. Then, two coats of lime plaster are troweled on, as with a Santa Barbara Finish stucco product. In other regions, lime plaster can be applied over properly prepared masonry or foam substrates, with a base coat of cement plaster, cement board sheathing, or other accepted cementitious substrate.
Features of Venetian Plasters / Lime Plasters
Lime plasters have gained their niche in the U.S. market because of how they look. But they have important performance advantages as well:
- Crack resistance: lime is far more flexible than Portland cement, so you can expect greatly reduced cracking with lime plaster;
- Very breathable;
- Wets out and dries out FAST: Lime plasters allow water to move through them as if they were not there, so they work well in wet climates such as in Europe, where it rains a lot. They can also be used on canal sides because the water that is absorbed by the lime plaster does not stay inside the wall but quickly evaporates from the surface. Lime plasters are the only plaster materials that can tolerate rising damp and canal-side applications.;
- Self-healing: during construction, when buildings are most likely to be under stress, the fresh lime has been known to actually fill in small cracks that form due to stress. This may explain in part why lime plasters show far fewer cracks than Portland cement plasters. In fact, “ancient buildings have had lime mortar deep in walls which started to set when archaeologists exposed it”;
- Naturally mold-resistant: due to its high pH (between 11 and 12), lime is a natural biocide
- Time tested: from early Egyptian, Greek, and Roman efforts with lime through the golden age of lime in the 1800s, lime plasters have served as durable, beautiful finishes all over the world;
- Natural and green–lime plasters are sustainable materials produced without plastics and harmful chemicals in keeping with traditional plastering methods
- Easy to patch and maintain – fresh lime plaster can be “brought back” with water spray between 2 to 4 hours after application. This gradual setting process allows for relatively easy repairs, compared to Portland cement and especially acrylic plasters.
Additional Considerations When Using Lime
If you’re like most American construction professionals, you have heard stories (maybe from your uncle or grandfather) of lime plasters that were slaked in holes in the ground on the jobsite, and used after several months to plaster the interior. But there is a certain fear factor in bidding work using products that are new to someone, no matter how old and time-tested the product might be. Everyone new to using lime thus needs to keep the following things in mind when starting to take on lime projects:
- Added Expense: authentic materials cannot be made from “bag lime” and so must come from Europe. Labor cost should be roughly the same as smooth Portland cement plaster finish, but materials could run an extra $10-20 per square yard. Figure on the same applied cost as painted smooth stucco, and you’re probably in the ballpark, but your manufacturer can give you the guidance you need to bid jobs;
- Pay special care to roof and deck runoff if you don’t want that kind of character on your building; gutters are a good idea and designers need to realize that water running down a lime plaster will create an old-world look that not all homeowners appreciate.
- Extended cure time means extended vulnerability to freezing, so be sure that temperatures will not drop below freezing for about one week after application
Venetian Plaster and Lime plasters have arrived in the United States to support the demand for European architecture and high-end aesthetics. Their performance advantages are likely to keep them around for generations to come.
There are a lot of product offerings flooding the market these days, so be sure to ask the right questions about how their lime is produced. The real test is putting the materials on the wall, standing back, and enjoying the warm, natural glow of the lime. With education and a little trial and error, lime plasters can elevate the appearance of a building and enable construction professionals to differentiate their work, recapture the tradition of artisans from generations before, and create art on the wall.
INTERIOR PLASTER Evolution to Modern Products
The wonderful classic and timeless limestone-based Venetian Plasters that evolved in Europe that we have discussed in our Learning Center is still available and indescribably beautiful, but are there available options, some of which are more affordable, but still achieve the classic Venetian plaster aesthetic.
What Are the Other Types of Interior Plaster?
Gypsum Plaster is the same plaster that was likely used In Your great-grandparents house probably over “wooden Lath” – Gypsum Plaster can include integral color and be warm and beautiful.
All Venetian Style Plasters Don’t Need To “Break the Bank”
New technology has produced more affordable Acrylic-Based Interior Plaster that can achieve stunning results without busting budgets. Many of these products can rival the powerful aesthetic of Classic Venetian Plaster.
Apptek has expertise in these new products and can guide decision makers through the process of selecting appropriate products and achieving budget objectives to create spectacular results.