Lime plaster is type of plaster composed of sand, water, and lime, usually non-hydraulic hydrated lime (also known as slaked lime, high calcium lime or air lime). Ancient lime plaster often contained horse hair for reinforcement and pozzolan additives to reduce the working time.
The most commonly-used type of lime for lime plaster—non-hydraulic hydrated lime—only sets through carbonatation of CO2 in the air, and will not set until dry. The water added to the mix is purely to improve workability and stickiness. This causes limitations in construction use as the lime can remain soft for weeks or months. The curing time of lime plaster can be shortened by using natural hydraulic lime or adding pozzolan additives, transforming it into artificially hydraulic lime. In ancient times, Roman lime plaster incorporated pozzolanic volcanic ash; in modern times, fly ash is preferred. Non-hydraulic lime plaster can also be made to set faster by adding small quantities of gypsum plaster.
Lime plaster sets up to an extremely hard and solid mass that is very durable; lime-plastered walls have lasted for thousands of years. Lime plaster is unaffected by water and will not soften or dissolve like gypsum plaster or drywall. Unlike gypsum plaster or drywall, lime plaster is sufficiently durable and resistant to the elements to be used for exterior plastering. Compared to cement plaster, lime plaster is less brittle and cracks much less, requiring no expansion joints as a result. The elevated pH of the lime in the plaster will also act as a fungicide; mold will not grow in lime plaster or on nearby wood.
Lime plaster sets slowly and is quite caustic while wet, with a pH of 12. Plasterers must take care to protect themselves while working and keep around mild acids (such as vinegar or lemon juice) to neutralize any chemical burns. Due to the extended working time, this caustic environment can persist for weeks. When the plaster is finally dry, the pH falls to about 8.6. Because of these complications, in modern times, lime plaster is more expensive to apply than gypsum veneer plaster and especially drywall. The number of qualified tradesmen capable of plastering with lime is in decline due to widespread adoption of drywall and gypsum veneer plaster.
Historical 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.
Historical uses in building
- Lime plaster was found to have been a multi-purpose material at the archaeological site of 'Ain Ghazal in modern day Jordan. The site dates human occupation from 7200BCE to 5000BCE. Lime plaster is believed to have coated internal walls of buildings and to have been used as the main component of some anthropomorphical figurines discovered at the site.
- 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.
- Ancient Romans used hydraulic lime (added volcanic ash, an activated aluminium silicate) to ensure hardening of plaster and concrete in cold or wet conditions.
- The Aztec Empire and other Mesoamerican civilizations used lime plaster to pave streets in their cities. It was also used to coat the walls and floors of buildings.
- This material was used in the San Luis Mission architecture.
- Hagsten, Ellen. General Guidelines for Working with Lime Mortar and Limewash, Traditional & Sustainable Building, March 2007
- "Lime plaster statues". britishmuseum.org. British Museum. Retrieved 21 November 2013.
- 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
- ISBN 978-0-500-28714-9
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,