What is lime?
Lime is made from calcium carbonate. This can be found in a variety of forms for example chalk, limestone or sea shells. In the British Isles sources are almost invariably chalk or limestone. In the case of Singleton Birch, it is mainly chalk quarried in Lincolnshire.
To convert the raw material into lime, heating to temperatures between 800C – 1000C is required. At these temperatures the chalk breaks down by giving off carbon dioxide leaving calcium oxide which is known as quicklime.
Quicklime is unstable and reacts, often very rapidly, with water to form calcium hydroxide. This process produces heat and is known as hydration. When exposed to the atmosphere this calcium hydroxide can react again by absorbing carbon dioxide from the atmosphere to once again become calcium carbonate.
It is calcium hydroxide which, when mixed with sand to make a mortar, builders have used over the millennia. The setting process is the re-absorption of carbon dioxide from the atmosphere called carbonation.
Calcium hydroxide can be used by Builders in two forms, firstly, if it is hydrated with an excess of water, lime putty is produced or if the water quantity is reduced a powder is formed which can be bagged. All sources of pure calcium carbonate produce a similar result, however the Romans discovered that the calcium hydroxide would also react with volcanic ash, essentially a mixture of silica and alumina. The resulting mixes set very much harder and for the first time enabled structures of high strength to be built. By varying the quantity of silica and alumina, mortars (and lime concretes) of different strengths could be produced. These reactions are referred to as pozzolanic, from the town of Pozzuoli in Italy from where the original material came.
The processes described so far use chalk or limestone that is relatively pure containing over 95% calcium carbonate.
However some limestones are less pure and can contain quantities of silica and alumina which when heated produce very similar results to the Roman mixes of pure lime with volcanic ash. These impure limestones if they contain quantities of silica and alumina are referred to as Natural Hydraulic Limes (NHL).
Hydraulic in this context refers to the ability of the material to set under water as opposed to pure air limes which can only set when exposed to atmospheric carbon dioxide. As with addition of pozzolanic silica and alumina, the strength of NHL’s depends upon the quality of these materials in the limestone. As the quantities increase so does the strength but also the free lime (calcium hydroxide) in the mortar reduces so that the set depends more on chemical reaction and the process of carbonation reduces.
Singleton Birch have worked to develop a range of NHL products by sourcing the appropriate chalk or limestone for burning, based on its’ geology and mineralogy. At We Sell Lime we now stock and sell a wide range of Singleton Birch lime products.
More about Lime…
Haired Lime Plaster
Natural Hydraulic Lime (NHL)
To achieve greater tensile strength in plaster hair (usually goat hair) is added to the mix. This is particularly important when plastering onto laths and is quite usual for any internal plastering onto a solid background.
Non-Hydraulic Lime or Lime Putty
Quicklime is the raw material in the production of lime putty.
This is added to water and a chemical reaction occurs which is termed as slaking. The resulting mixture is sieved and left to mature for at least 3 months. During this time the liquid slaked lime thickens to the consistency of toothpaste and is pure white in colour and is called lime putty.
Lime putty based products harden by absorbing carbon dioxide from the air to revert back to calcium carbonate (carbonation).
This is a topcoat plaster, which is made with fine sand.
Coarse sand is mixed with lime putty to produce a mortar suitable for rendering and pointing.
Lime wash is made by diluting lime putty with water to create a wash. Adding suitable pigments produces many different colours.