The Venetian plaster process dates its roots to the 1st century B.C. and is described by then architect and engineer Vitruvius as a seven step process where several layers of coarse plaster would be used to level the rough Roman masonry of that day. This leveling layer would act as a base for applied finishing layers which included finely ground marble dust.
After the final layers were applied the walls were waxed to protect them. The finished product would be a marble hard finished coating with a fine sheen called opus marmoratum. Today’s Venetian plasters come in two varieties, Lime-Based and Synthetic and can be applied in just a few coatings instead of the original seven coats used long ago.
Lime-Based Venetian Plaster
Lime based plasters are the purist’s forms of plaster as limestone was (and still is) abundant and widely used for wall coverings. Since modern construction usually has a flat sheet rock base, it takes fewer coats to complete Venetian plaster.
The first base coat has a fiber material and a little acrylic binder. Scratch coats like these add mass and stiffness and will help support the later coats. These preparatory layers should a total of one eighth or one quarter of an inch.
After drying, the final coats of lime and marble dust can be applied. To the mix of lime and dust add a lime-proof mineral pigment for color. A small amount of water is then mixed in to aid in color dispersion.
Next add linseed oil and soap to impart luster and make it more water resistant. These final coats are applied in thin layers with a small spatula or trowel and must be applied wet on wet to guarantee adhesion.
Burnishing, or sanding the coat between layers ensures a shine from the limestone. The next to last step a thin coat of olive-oil soap is put on. This soap coat seals the plaster, adds water resistance as well as evens out the last coat which is the wax. The wax enhances the color and increases protection from water and wear. The finished product is a hard shiny wall cover that will send your room back to the times of ancient Rome.
Synthetic Venetian Plaster
In the 1950’s lime was replaced with resins to get a more intense color that is usually not possible with lime plaster. It is a calcium carbonate in a latex binding agent. It is like adding marble dust to latex paint. The application of synthetic plaster is the same as lime base but there is no need for a base coat. It can be applied directly over painted walls.
Burnishing can happen between any layer and up to a week after the coats are applied. Finish it like lime based plasters, with soap and wax. The finished wall will not be as thick and durable as a lime based plaster but it will have the same beautiful effect.
The sheen is what gives Venetian Plaster is popularity. It is up to you how you want to go about it. If you are stickler to detail with multiple layers, then use lime based plaster. It is more time consuming but if you are afraid of a little elbow grease, go the route of old Rome. If you want it done quickly then today’s synthetics are the way to go.
photo by Andy Weit -CreativeCommons Attribution