“In the limelight”.
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Upon reading this phrase, one might first think of Rush’s song “Limelight” in which the musician is referring to being the main event on the brightest area of the stage. But, did you know that this phrase actually refers to plaster?
First used in 1837 in London’s Covent Garden Theater, limelight became the primary method of spotlighting theater actors and actrices. Limelight, discovered by English inventor Goldsworthy Gurney in the early 1820s, was made using a blowpipe that constantly burned hydrogen and oxygen creating an extremely hot flame. When a block of lime, or calcium oxide, was put into the flame, an extremely bright light was created. This light could then be focused on far away areas such as thesbians on a stage. Thus, the phrase “in the limelight” was born!
Lime use in plaster actually goes back even further in our history. Lime wash paint was first used in Roman construction, found in the ruins of Pompeii and the Herculean ruins. It was the only indoor/outdoor paint available at the time. Although, its use disappeared after the fall of the Roman Empire and was not used again until the late Middle Ages, after the Dark Ages. Lime plaster resurfaced in history in 1212; After a disastrous fire in England, King John required that all the homes along the Thames be limewashed to prevent future damage. (Yes, it is a natural fire retardant!). Limewash reached its peak use during the Renaissance as the choice masonry paint until post WWI. By the 1950s limewash production had ended. It was not until the “green” movement beginning in 1990s Italy that the production began again and continues to be popular today!
What is the chemical composition of lime in plaster? Lime is extracted from sedimentary rocks made of carbonated lime (CaCO3). In Italy, the lime is extracted from rocks in the Dolomites. Lime can also be produced from marble. Once collected, the rock is crushed and heated to about 1470 to 2200 degrees Fahrenheit for a couple of days. The applied heat initiates a chemical reaction in the lime that alleviates the rock of carbon and oxygen, decreasing the products weight by one-third. The new material is now called active lime. In order to be ready for use in plaster and/or paint, water is slowly added to the product over a two-year time span, creating heat, and converting the active lime to inactive lime or inorganic slaked lime.
Slaked lime was probably used on the walls of your great-grandparent’s home! Beginning with a scratch coat, which is plaster mixed with materials to prevent shrinkage, such as horsehair, a brown coat or plaster mixed with sand is then applied, and the wall is finished with a fine skim coat of only plaster. Today, modern shrinkage-prevention aggregates are used (AKA no more horse hair) and the material, originally white, can come in many colors. The active lime goes through a chemical reaction when it is exposed to carbon dioxide, completing the lime cycle on the wall. This is called curing. Once cured, the lime has essentially turned back into stone! (quote from Lauren Dillon via Remodelista). Additionally, slaked lime plaster can be applied to surfaces that may come in contact with water. In high water splash surfaces, beeswax or other sealants should be applied.
These techniques and materials should not be left in the past, and here’s why…
The way in which historic structures were built necessitates specific techniques for their repair and decoration. New builds are commonly equipped with cavity walls that keep out moisture. This is misguided, as these walls can cause condensation leading to rot and decay in the structural supports. Historically, walls would be sealed with paints made of locally available limestone and sometimes tinted with earth pigments. Being highly alkaline, limewash is a naturally antibacterial material. It is also a natural insecticide with fireproofing capabilities (King John knew this). Furthermore, limewash can eliminate mosquito larvae, reduce animal odors, can neutralize surrounding soil, and, when applied to roofs, can reduce the inside temperature by up to 10 degrees. Most importantly though, limewash is highly “breathable”, allowing moisture from inside to evaporate to the outside. With the growing popularity of DIY paint projects, paints are often cheaply made with many unnatural components. These modern petroleum paints are cause for concern due to their mixture of synthetic chemicals. Additionally, research has shown that these paints put close to as much air pollutants into our atmosphere as car exhaust.
The application of limewash onto the interiors and exteriors of stone, brick, and wood buildings is both aesthetic and practical. (Think Tom Sawyer whitewashing or limewashing the fence). Limewash lightens up a structure and gives it a slight glow with its crushed crystal composition. It also slows decay of the materials to which it is applied by protecting the surfaces from weather and erosion. Rainwater will simply roll off of a limewashed surface. This occurs because the limewash is absorbed by the surface onto which it is applied and becomes part of it. From rock back to rock!
By Alexandra Masters – Master of Plaster Intern