Lime plaster
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Lime plaster is a mixture of calcium hydroxide and sand (or other inert fillers). Carbon dioxide in the atmosphere causes the plaster to set by transforming the calcium hydroxide into calcium carbonate (limestone). Whitewash is based on the same chemistry.
To make lime plaster, Limestone (calcium carbonate) is heated to produce quicklime (calcium oxide). Water is then added to produce slaked lime (calcium hydroxide), which is sold as a white powder. Additional water is added to form a paste prior to use. The paste may be stored in air-tight containers. Once exposed to the atmosphere, the calcium hydroxide turns back into limestone, causing the plaster to set.
Lime plaster is used for true frescoes. Pigments, diluted in water, are applied to the still wet plaster.
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[edit] Lime Mortar
Lime-sand only mortars are more plastic and better accommodate any settling or movement in the wall than cements, which do not adjust to changes around them once they set. Although lime stuccoes and limewashes are more breathable, they also have better water shedding characteristics. Cement stucco is likely to crack under stress or movement, allowing a route for water infiltration into the interior where it will be trapped. In contrast, lime stucco can better adjust to early movements in the building because it does not set fully all the way through immediately, but only as the interior more slowly carbonates. Any tiny cracks that open can be resealed as acidic rainwater enters those cracks either draws some of the remaining calcium hydroxide into the crack or as the slightly acidic rainwater partially dissolves calcium carbonate along the edge of the crack temporarily creating calcium bicarbonate and re-deposits it toward the front of the crack as calcium carbonate again. This self-healing characteristic of lime is well described in the literature as“autogenous healing.”[1]
[edit] Limewash
"Unlike modern paints, which lay on the surface of the substrate, limewash instead acts like a stain by penetrating deep into the pores of the underlying material. This process creates a peel-free, breathable surface, and the limewash remains vapor permeable after it cures. It is a beautiful, traditional material that mellows while it gradually wears away, and over time it develops the weathered patina that characterizes the Old-World charm of Europe."[2]
[edit] Hydraulic Lime
Hydrated Lime is the most commonly used and known lime, also called (high) calcium lime or air lime. It can only set through carbonation (re-absorption of CO2), and has very strong limitations in construction use. Hydraulic Lime has an initial set with water, much like cement, and a second set through carbonation, like hydrated lime . This allows for simplicity in application, identical to ordinary stucco.
Some kind of Hydraulic Lime was used for most of our old structures—many dating back several centuries. Its durability or longevity has been unsurpassed by any modern material, including cement stucco or “lime-based plasters.” This is simply due to its composition (calcium carbonate or limestone), resistance to salts (no sulphate attack or alkali-silica reactions), elasticity (reduces the risk of cracking and water intrusion), and breathability (does not trap water and allows its elimination through vapor exchange). A true lime plaster has the unique quality of reflecting multi-nuances of color, enhanced by the varied angles of sunlight reflected throughout the day.
[edit] Environmental Impact
The use of lime plaster saves approximately 80% of the CO2 release compared to ordinary stucco. One single residence will save between 5,000 and 10,000 lbs of CO2 emissions. Each year in the U.S. alone, environmentally conscientious builders are saving several millions of pounds of CO2 release by avoiding the use of cement-based products, and choosing Natural Hydraulic Lime instead.
[edit] Safety issues
Lime is an extremely caustic material when wet, with a pH of 12. (Lime becomes pH neutral when carbonated). Protective goggles and gloves should be worn at all times. Additionally, protective clothing should be worn where risk of splatter on to bare skin is present.
Clean water should always be at arms length if lime gets in someone’s eyes or on their skin. Skin can be neutralized with a very mild acid such as vinegar or lemon juice. Repeatedly flush eyes with fresh water for several minutes and consult medical advice. [3]
[edit] Use in the arts
One of the earliest examples of lime plaster dates back to the end of the eighth millennium BC. Three statues were discovered in a buried pit at Ain Ghazal in Jordan that were sculpted with lime plaster over armatures of reeds and twine. They were made in the Pre-Pottery Neolithic period, around 7200 BC. The fact that these sculptures have lasted so long is a testament to the durability of lime plaster.[4]
[edit] Use in architecture
Some of the earliest known examples of lime use for building purposes are in early Egyptian buildings (primarily monuments). Some of these examples in the chambers of the pyramids, which date back to around 2000 B.C., are still hard and intact. Archaeological digs carried out on the island of Malta have shown that in places like Tarxien and Hagar, lime stucco was also used as a binder to hold stone together as well as for decoration at sites dating back as far as 3000-2500 B.C. At Tel-el-Armana, a large pavement on brick was discovered that dates back to 1400 B.C. It was apparently the floor of part of the harem of King Amenhoted IV.[5] Ancient Chinese used Suk-wui(the Chinese word for slaked lime)in the construction of The Great Wall of China.
[edit] Misconceptions
There is a lot of misconception or misunderstanding concerning the definition of what is actually a lime plaster . Lime Plaster refers to a mortar using only pure lime as a binder and sand as an aggregate. Too often common mortars made out of lime, cement, and sand are improperly called lime plasters simply because they include some kind of lime . The correct name for such a product is stucco, cement stucco, or lime-based stucco. The addition of cement to lime dramatically changes the physical, mechanical, and chemical properties of the lime by reducing its elasticity, breathability (vapor exchange), and durability as well as altering the color rendering. As a matter of fact, lime is commonly added to cement in order to mitigate these properties. Mixing Hydraulic or Hydrated Lime, or both, with aggregates, achieves a true lime plaster.[6]
[edit] See also
[edit] References
- ^ Hagsten, Ellen. General Guidelines for Working with Lime Mortar and Limewash, Traditional & Sustainable Building, March 2007
- ^ John Speweik, U.S. Heritage group
- ^ Hagsten, Ellen. General Guidelines for Working with Lime Mortar and Limewash, Traditional & Sustainable Building, March 2007
- ^ J.N. Tubb, Canaanites, London, The British Museum Press, 1998
- ^ Cowper, Ad. Lime and Lime Mortars, first published for the Building Research Station by HM Stationery Office, London, 1927
- ^ Michel Couvreux, Building Green
[edit] Further reading
Cedar Rose Guelberth and Dan Chiras, The Natural Plaster Book: earth, lime and gypsum plasters for natural homes'
J.N. Tubb, Canaanites, London, The British Museum Press, 1998
Stafford Holmes, Michael Wingate, Building With Lime: A Practical Introduction, Intermediate Technology Publications Ltd,
[edit] External links
British Museum: Lime Plaster Statues
[http://www.jstor.org/pss/530161 Yuval Goren, Paul Goldberg, Peter W. Stahl and Udo H. Brinker Journal of Field Archaeology, Vol. 18, No. 1 (Spring, 1991), pp. 131-140, Published by: Boston University]
