- Last Updated: Tuesday, 28 June 2016 15:52
- Written by Graham Brooks
Limekilns have a basic function to convert limestone (Calcium Carbonate CaCO3 ) to quick lime (Calcium Oxide CaO).
The quick lime has had two major uses during history. The first use was a mortar (lime mortar) for use in building and also as a plaster (lime plaster). This use relies upon the chemistry of quick lime. If water is added to quick lime a very vigorous reaction with a large amount of heat been given off. Chemically the outcome is the production of Calcium Hydroxide (Slaked lime Ca(OH)2 ). This is insoluble in water but when mixed with a suitable aggregate either a mortar or plaster can be produced. Lime mortar has an advantage over more modern cement mortars in that it takes a long time to become very solid. This relies on the Calcium Hydroxide absorbing Carbon Dioxide from the atmosphere and been converted back to Calcium Carbonate. This slow hardening allows a building to both breath and also to move slightly without cracking.
The other main use of lime has been in agriculture as a means of reducing the acidity of the soil. Most plants grow best in a neutral soil but most soils are acidic. The lime only requires to be spread on the land were the chemical reactions take place between the acids in the soil and the lime thereby neutralizing the acids.
It is thought that the Romans introduced the making of lime to the United Kingdom and its use in building as a mortar. There have been Roman limekilns excavated through out the country. This technology does not appear to have been lost during the dark ages as mortar built buildings from that period still survive. It continued to be made and used during the medieval period. It would appear that the limestone was burnt in a sow or pye kiln. These consist on hollows in the ground into which the fuel and limestone is layered and the covered in soil and turfs and set fire to. Usually there was a flue to allow a draft to enter the kiln and a out let to allow the Carbon Dioxide to escape.
A SOW KILN AT THISTLE DOWN PASTURE INGLETON. Showing the depression of the kiln and neck of kiln for stoking and air inlet.
Most people will be familiar with the stone built limekilns that now dot the country side these appear to date from the 16th century onwards, with the majority being built for the production of lime for agricultral use, although there are examples of kilns being used for both purposes.
kilns can be found anywhere where limestone outcrops (a few kilns are not on limestone).
A typical small limekiln (Bulman Hills) built to be filled and then emptied once the kiln is full burnt out.
Larger kilns were built that could be continously kept burning. The size of the pot has to be sufficently high enough to allow the fire to move up the pot and the lime at the bottom can cool and be removed. Removal of the lime allows the fire to fall back down the pot allowing new limestone and coal to be put in on top. These kilns wered more efficent because the whole kiln did not have to be heated up from cold each time, also as the fire burntnup through the load it pre-heated the limestone above.
A diagram showing the structural function of a limekiln
Most of the larger kilns were run as commercial enterprises. To increase the production banks of kilns were constructed with a number of pots in one bank.
A large commercial kiln at Allenheads.
Large commercial kilns on the harbour at Sunderland.
During the nineteenth Century a number of paptents were taken out to design more efficent kilns and in some cases to keep the fuel seperate from the llime. One of these was the Hoffman kiln. This was a horizontal kiln consisting of a series of inter-connected chambers arranged in a ring. This allowed one chamber to be fired and the ones behind can be emptied and refilled the fire is slowly moved round the ring of chambers by the use of dampers and the introduction of fuel into the correct chamber.
Langcliffe Hoffman kiln
Another type of kiln was the side burn kiln in which the fuel was burnt in a seperate chamber and the hot gasses then passed through the limestone to cause it to be converted to quick lime.
Remains of a side burn limekiln at Coldwell lime works.
During the twentieth century other types of fuel began to be used such as gas, and the structures began to built from steel.
Modern gas fired limekilns at Shap.
Links to pages for linekilns in specific areas.