Limekilns are Loveable!

Limekilns are Loveable!

Limekilns are largely coastal or riverside dwellers due to the transport issues of raw materials and finished goods, or they are clustered around limestone quarries and inland waterways. The need for lime mortars, putties and washes for building purposes saw them spring up temporarily on significant building sites, such as a castle, church or Roman villa and they were often removed on completion of the works. Permanent kilns however were prolific in some regions, such as the North Devon Coast because of the local acidic geology. Lime was needed to reduce the acidity in the soils to make them more productive for farming and to mix with clays to make a better loamy substrate for crop growing, so the scale of agriculture predominantly created the demand in the South West. The knowledge of lime for mortars and agricultural usage is ancient and the trend in building was cemented in British culture by the Romans. Records indicate that the use of lime on the land was widespread by the 1500’s (pardon the pun). The industrial era created further demands for burnt lime as it was found to be essential in loads of chemical processes and particularly in the iron and steel industry as a flux.

You cannot just import limestone and pulverise it for a field-feed, you need to burn the limestone chunks (at a very particular size) in between layers of coal. The coal of course was delivered by sea from the mighty collieries in South Wales. So North Devon, like many coastal regions, was a no-brainer for the lime industry!

The limestone is chemically converted to calcium oxide (also called quicklime or burnt lime) during the heating process, (called calcining) at a temperature of between 900° and 1100°C. The limestone breaks down yielding burnt lime and carbon dioxide gas.

limestone + heat = burnt lime + carbon dioxide gas

CaCO3 = CaO + CO2

This alkaline inorganic compound of calcium, or lump lime as it was called, may be used in its primary state or crushed to a fine powder (ground burnt lime), but was quite alive and needed swift and careful attention lest it went-up in flames or went-off and became useless, fallen lime. By the addition of water (slaking), you can make slaked lime, (hydrated lime, calcium hydroxide). By adding even more water, lime putty can be produced and even more and you have limewash! The quality of the lime had a noticeable effect on the qualities of the products produced. High quality limestone, such as that in Dove Holes Dale, could produce a very rich or fat lime product, up around the 99% pure mark!

Prior to the affordability and availability of coal, wood or peat was used to burn the limestone, but coal was better. However, half a ton of black lumps were required per ton of lime yielded, so we’re talking 15 tons per average kiln-load! Not cheap!

There were two well-practiced methods and therefore kilns; the flare kiln for the single-batch burn and the draw kiln (introduced circa 1750) for a continuous mass-production burning.

In either case, the size of the limestone chunks was essential to the quality of the burn because the correct size allowed air to circulate well throughout the charge; we’re talking 1 to 2.5 inch lumps, and the physics limited the overall size of the kilns and dictated their shapes!

For the flare kilns, the charge took a day to load. The loading was crucial to the speed and quality of the burn; too fast and it could collapse and extinguish itself, too slow and it would create too much dead (over-cooked) lime, which is useless. A 25-30 ton lime yield required three days to burn and two further days to cool before the kiln was raked out, so a lime manufacturing plant often had multiple kilns to create a successive production. Only trial, error and experience passed on through generations prevented enormous losses, because if the kiln-load went wrong it often couldn’t be salvaged, and losses of this magnitude were costly in time and money.

In order to contain the outward pressure of the loaded kiln and to provide sufficient insulation for a three-day burn and two-day cooling, the kilns were substantial structures to say the least. I liken them to mini castles or strongholds because even after all these years, those that weren’t robbed of stone remain particularly chunky. They were technical buildings and had draw holes, poking holes, charging ramps, were lined and had access arches, grates and throats!

Despite specific requirements for these structures, they varied remarkably over the years and over the country. A landowner may commission a kiln or several kilns on his land and they were built according to the local architect and materials available. The beach kilns were often made of large beach pebbles, like the one at Heddon’s Mouth in North Devon. Others were crenellated, given a castle-like feature, around the top, like those built by Lord Rolle of Barnstaple along the river Taw and Torrington (Hallsannery and Town Mills), purely for aesthetics! A complete extreme are the unique style designed by Samuel Oldknow. His limekilns at the heart of his industrial complex on the embankment of the Peak Forest Canal were specifically designed to not spoil his view and look more like a medieval castle with gothic revival windows – I kid you not! GooseyGoo (with a little help from friends of industrial history) hopes to map and visually recognise the different kilns across the country – this is modern-day, collaborative, grassroots industrial archaeology at its best!

Their biggest threats today are neglect and overburden or growth. As the roots take hold, the masonry is damaged and coupled with weathering, the structures inevitably decline. Sadly by some, they are considered to be ugly, or useless or allowed to become dangerous. I obviously think they are superb pieces of industrial heritage and must be preserved at (nearly) any cost!

Make it your mission to find a limekiln, photograph it and pop it on GooseyGoo.

20th June 2015No comments, , Built Environment | limekilns | Relics

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