is a mixture of calcium hydroxide
(or other inert ). Carbon dioxide in the atmosphere causes the plaster to set by transforming the calcium hydroxide into calcium carbonate (limestone
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. Modern stucco, which incorporates Portland cement is incorrectly also referred to as lime plaster.
Lime-and-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 entering 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, 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”.
"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.
Hydraulic Lime sets as a reaction with water and, as a generalisation, is used in areas likely to be exposed to weathering before full setting could otherwise take place. Hydraulic Lime has an initial set with water, much like cement, and a second set through carbonation, like non hydraulic lime . This allows for simplicity in application, identical to ordinary stucco.
Non-hydraulic lime can only set through carbonation (re-absorption of CO2). It is often sold in builder's merchants as 'bagged' or hydrated lime or is available as lime putty (or as quicklime to be made into lime putty), lime putty generally being considered more suitable for pure lime application. Non-hydraulic lime is the most commonly used and known lime, also called (high) calcium lime or air lime, as it sets only by reaction with CO2 in the air and will not set until dry. This causes limitations in construction use as the lime can remain soft for months or years.
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.
Hydraulic and hydrated limes must not be confused. Hydrated lime is merely a form that lime can be supplied in (as opposed to quicklime or lime putty) while 'hydraulic' refers to a characteristic of the lime.
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.
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.
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 el-Amarna
, 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 Amenhotep IV
. Ancient Chinese used Suk-wui (the Chinese word for slaked lime) in the construction of The Great Wall of China.
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,
General Guidelines for Working with Lime Mortar and Limewash, Ellen Hagsten,Traditional & Sustainable Building, March 2007
British Museum: Lime Plaster Statues
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