Look - Click Here for -

Lime Kiln Operation

by Luke Toft

Back to - Limestone Quarrying & Lime kilns

Extracted from

‘Lime Burning on the Gower Peninsula's Limestone Belt,’

by L. A. Toft

Industrial Archaeology Review Vol X1, no1, Autumn 1988

Luke A Toft, 2011

A.B. Searle, a furnace technologist, wrote in 1935:

The production of lime is an art which depends, largely on the skill of the lime burner, and most of the statements which imply that lime burning is simple and easy to control are misleading, by those who have little or no knowledge of the subject. The main principles involved are simple and capable of accurate scientific explanation, but the limitations imposed by existing (vertical) kilns are such that there is necessarily much more ‘art’ than ‘science’ employed in producing lime.

A man with a good scientific training would probably not make a good practical Burner, and although he would be able to point out what was wrong and the probable cause and remedy, he would probably not be able to carry out his own instructions!

Lime Kiln situated on Limekiln Road, Oystermouth

Lime Kiln, Oystermouth

The ‘draw’ tunnel of the same Lime Kiln

Limekiln Road, Oystermouth is marked

Lime kilns were usually operated by a two man team, the 'quarryman' and 'burner'. The division of work between them is obvious from their titles and in each case required skill. Although an apparently simple process ‘burning’ in a static kiln was like most processing, something of an instinctive art. Significant 'overburning' could make the lime unsuitable for subsequent processing. Overburning produces a glazed surface on the lumps which hinders or even prevents 'slaking' with water. Unless the lime reacted with water it was useless for producing mortar for buildings or 'slaked lime' (Calcium Hydroxide).

Successful production of 'quick lime' requires achieving the correct temperature, which is a function of the amount of carbon dioxide present in the gas leaving the kiln. Carbon dioxide is released during the conversion of limestone into lime and is also produced when fuel containing carbon is used (coal or wood). Although allowing extra air to enter would reduce the concentration of carbon dioxide present and thus the necessary temperature it would at the same time increase the temperature of the fuel and add to the risk of 'overburning'. (It may be that this is the background to Cato's admonition 'Do not let the wind come to your kiln door'). Although it is very unlikely the 'burners' understood the scientific background to their work they were obviously aware of the advantages of adding a diluent gas, because it was a common practice to feed the kilns wet limestone. An advantage of using wood instead of coal is that it gives off more inert gas than coal, thus reducing the carbon dioxide concentration.

In order to achieve economic operation it was necessary to ensure that the gas flow through the kiln was evenly distributed across it, something which required great skill when placing and selecting the kiln 'fill'. The size of the stones was important, when too large they increase the burning time whilst if too small they impede gas flow. It was a practice in North Gower to add a layer of shells (to distribute the flow of gas across the kiln?) and maybe this practice can be explained by the following extract from Walter Davies' study published in 1814:

Formerly the stones were broken too small for burning well; they lay too close, admitting no free vent for either air or heat; in the other extreme, they are not now broken small enough. The Burners, however, impute the badness of the lime to this or that coal, to the nature of the stone, weather, kiln, and etc. anything but the truth; and the farmers too easily give them credit. — E W

Limekiln at Kilvrough, Gower by Bryan Taylor

Lime Kils operative in 1878, by Luke Toft

Whilst the quarryman worked a day shift (in the 1920s 7.30am to 5.00pm) the burner worked when required, even at night if 'topping up,' adding coal and limestone, was necessary. The burner's job also included removing the newly burnt 'quick lime' and general attention to kiln operation. Work on the lime kilns was arduous, hot and dusty. The gases leaving the top of a continuously operating kiln must have made manual charging an exhausting and unpleasant operation, although the top layer was cool. Gas leaving a coal fired kiln was not toxic but could induce nausea, a characteristic put to good use earlier this century by the children of at least one Gower village (Llanrhidian) when they wanted a day off from school. Removing the newly burnt lime from the base of the kiln had an element of danger, because it was both hot and caustic and in those days protective clothing was very primitive. The work was often seasonal when not supplying industrial undertakings requiring a continuous supply of lime.

Until very recent times the operators of processes such as 'lime burning' were usually unaware of the scientific background to their work. Providing sufficient time is available, however, an absence of scientific knowledge does not prevent the development of an adequate operating technique by 'trial and error' methods.

Even when the background chemistry of a process is available, an element of 'art' is often present in operating the process on a commercial scale, but being an ancient process, lime burning techniques developed without the aid of science. As with all such techniques those who had the knowledge would safeguard their own position by keeping the secret of the process to themselves and for this reason written accounts do not give the whole picture.

Sources and technical data are to be found in

Industrial Archaeology Review, Vol. XI, no. 1, Autumn, 1988

Intensive work on maps and in the field revealed twice the number of kilns expected, and on dolomite in preference to other limestones. Work continues for the Sites and Monuments Record and some kilns may be excavated or renovated. Medieval onwards: building and agricultural uses, but all survivors are 19th century. Combustion studies are reviewed. See also author's gazetteer of 150 lime kiln sites in Gower, 39, 1988, 64-79.

Abstract

As part of the Glamorgan-Gwent Archaeological Trust's survey of the Gower peninsula, a study of disused lime kilns on the peninsula's limestone belt was initiated in 1984. At its commencement the project appeared to be straightforward, but it soon expanded into an extensive study as both calculation and documentary research revealed previously unexpected aspects. An intensive map search doubled the anticipated number of kiln sites and revealed the fact that the early nineteenth century kilns were located in the portion of the limestone belt where the greatest amount of Dolomite was present, thus suggesting that the early lime burners preferred Dolomite to other limestones. Then by analysing the lime burning process and using contemporary data, the process of lime kiln development during the nineteenth century became apparent, also the possible background to statements in contemporary accounts. In addition, analytical studies also revealed why the use of Dolomite for lime production could have been an attractive alternative to using other forms of limestone. With the completion of the historical research phase, the Friends of the Trust have set up a group to visit each known site and compile the formal Sites and Monuments Record. It is anticipated that, in due course, some kilns will be excavated or renovated as appropriate. In order to maintain knowledge of kiln site locations now that reference to them will no longer be present on as maps, a list of them was published in the 1988 issue of the journal Gower together with an article placing them in their local context.