This week is National Science and Engineering Week at the Roman Baths and I have been charged with donning my other hat and talking to you all about Lime (not the little green citrus fruit)…..
So what is Lime?
Lime is a general term given to a number of compounds containing Calcium. This blog will be focusing on Quicklime and Slaked Lime. Quicklime is a white caustic alkaline substance consisting of Calcium Oxide, which is obtained by heating Limestone (Calcium Carbonate). Slaked Lime is a white alkaline substance consisting of Calcium Hydroxide, made by adding water to Quicklime and it is this form that is predominantly used in traditional building methods to make plaster, mortar and limewash.
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| Calcium Oxide Molecule |
The diagram below shows clearly how Limestone or Chalk - Calcium Carbonate - turns into Quicklime (Calcium Oxide) after burning or heating. If water is then added it turns into Hydrated Lime or Slaked Lime (Calcium Hydroxide). The cycle is complete when it reacts with Carbon Dioxide from the air around it, turning it back into Limestone. This process is known as the Lime Cycle.
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| The Lime Cycle |
Buildings pre 1900 would not have been built with modern cement (with few exceptions in the late 19th century), but with a lime mortar. In order to conserve and repair these buildings, it is essential to use similar materials such as lime mortars, lime plasters and renders.
Does it have to be Lime?
Modern buildings generally rely on an outer layer to prevent moisture penetrating the walls, whereas buildings constructed before 1900 generally rely on allowing the moisture, which has been absorbed by the fabric, to evaporate from the surface. In essence, old buildings exposed to the elements are continually absorbing moisture, and the ability for the moisture to evaporate again is crucial to the welfare of the structure. Lime based building material is perfect for this two way exchange.
Using modern cement based mortars and plasters in traditional buildings risks locking-in the moisture, which could result in dampness internally and spalling of brick and block externally, as a result of freeze and thaw.
Pop along to the Roman Baths today, Wednesday 14th March 2012 between 4pm and 8pm and see me in action. I will be on hand to answer any of your lime based queries and demonstrating how we use lime based building material on-site.
For more specific information on applied lime based building material here at the Roman Baths, take a look at my previous blog – It’s all in the mix.
http://bathsbloggers.blogspot.com/2011/05/its-all-in-mix.html
Helen Harman - Collections Assistant
Just after the Temple Precinct was open to the public, I was down there applying a hot lime-poultice to the portal blocks of the Great Bath entrance, and I saw a tour guide wondering what the hell I was doing as she approached with her group. "Here is an archeologist taking a plaster-cast of one of the stones", she said uncertainly. I shook my head, so she eventually asked and I told her. They were not so well informed in those days!
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