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Natural vs. Synthetic Plasters

 Over the past fifteen years there has been a growing popularity for decorative plasters in the high-end market. Skillfully applied, these plasters lend a beauty to a surface that cannot be matched with just paint or glazes and they become more beautiful with age. Unfortunately, some of the characteristics of these plasters can also be duplicated with thick, flat paint, hence there is a increasing confusion over the differances between real, natural, synthetic or substitute plaster and how to tell the difference.

The types of plasters I am writing about are the paste plasters with no sand or aggregate in them and commonly called “Venetian plaster”. This term is a generic marketing name made up in North America referring to a smooth polishing plaster/paint that is applied in thin layers and burnished to a marble-like finish. The Italians refer to this as “grasiello” or “marmorino” depending on the type, size and amount of marble flour in it.

To capture the DIY market, many manufacturers are jumping on the bandwagon with alternatives that are supposedly easier to apply than the traditional plaster but for many reasons as you will see, they are actually a lot more expensive in the long run.

One can take any inexpensive flat paint or patching material that has a large chalk content (calcium carbonate), apply it to a flat surface, let it dry and burnish it with a cotton cloth to a shiny surface. A synthetic plaster/paint formulation with a large chalk content like this will always remain fairly soft because of the small amount of binders in it (binders are expensive) and will wash right off the surface with water. Sealing is not an option but a neccesity. Since chalk is fairly opaque, there is little translucency and only a small amount of visual texture remains after all the work of applying and burnishing.

On the other hand, very few to none of the natural decorative plasters being sold commercially in N. America are purely natural. A plaster that has no man-made (synthetic) ingredients in it has to be applied over an absorbent substrate like unsealed stucco, brown coat, plaster or masonry. Any painted surface needs to have the paint sandblasted or chemically removed or the plasters will not adhere.

What people are now calling a natural plaster is actually a misnomer, as they are not purely natural; they have been mixed with man-made polymer resins in order to enable adhesion when applied directly onto a sealed surface like primed drywall. The market standard for a mineral plaster to be called natural is that it should be at least 95% natural ingredients. The remaining 5% can be man-made like acrylic resins, mildewcides, etc.

What makes a product like TexSton's Veneciano different (and more natural) than the synthetics is the addition of aged (at least one year) slaked lime in the manufacturing process. This makes it a 'chemical' set plaster, meaning that the material will go back to its original state of rock as it sets. This is a process that can take months to achieve and the plaster to reach its fully hardened state. The term "Natural Plaster" is a marketing term and it would be more accurate to call these lime-based plasters.

A few key observations:

Lime-based plasters

Lime-based plasters dry hard and continue to cure for months afterwards as the lime crystallizes and interlocks from the reaction between humidity in the air and the lime. The process is called carbonization as the slaked lime adsorbs carbon from the air and reverts back to it's original limestone state.

  1. These plasters are washable but they are porous. Sealing them is an option, but not necessary for most interior applications. Wax is mainly used for aesthetic purposes but will provide limited protection from water penetration. The best sealers to use are impregnator sealers because they will not change the look or sheen of the plaster.
  2. Lime-proof pigments should be used although a lot depends on the actual ph of the plaster. Texston paste plasters are not so “hot” that you cannot use other pigments, but they should only be used indoors.
  3. Stainless steel should be always used for any finish work.

 

Synthetic plasters

•  Synthetic plasters dry by evaporation and only the polymers 'cure' to give the plaster surface strength. The hardness of the plaster will be determined by the type and amount of resins used in the manufacture. Cheaper plasters will have less resins, never harden and will wash off with water. These plasters look and feel more like a thick paint or gel than a plaster. With most of the synthetic plasters, a clear gel topcoat is not an option but a necessity.

•  As with most things, price is the best indicator of quality ingrediants.

•  You can use carbon or blue steel trowels and spatulas to apply and polish synthetic plasters


All of the smooth polished plasters are fairly easy to repair before they are sealed or top coated. It becomes a problem with varying degrees of difficulty after they are sealed, depending on what is used to seal the plaster. Wax can be removed, colored or clear gels can not.

Another good reason I prefer using the lime-based products over the synthetics is the lack of a chemical smell. I frequently had a headache when I used the synthetic plasters for any length of time. We carry Texston's lime-based plasters because of the quality of the lime they use (that's another article), the sophisticated range of colors they offer and the professional support of the staff.

Dean Sickler, 1998

(edited January, 2004)

The Keys to Color theory book Dean's new book "The Keys to Color; A Decorators Handbook for coloring Paints, Plasters and Glazes is now available at Amazon and at the blog site here

Please write with any questions/comments.

— Dean Sickler

Pingo Ergo Sum

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