Saturday 6 October 2012

Intensive sustainable crofting: Lime burning

An old lime kiln at Swordle . Jon Haylett

The lime arrived this week in 1 tonne bags, its finely ground limestone the granules are about the size of sugar grains. The smaller the granules the greater the surface area and the more easily it is dissolved in the soil water.

Agricultural lime used to be even finer.  Historically ground limestone was burned in kilns to make it more readily available to plants. The lime kiln at Swordle (in the picture above) was used to do just this. These old Crofters knew a bit about chemistry.

The chemistry of lime burning

Stage 1
Limestone ( CaCO3) was burned, lets say 10kg and turned by heat into Calcium oxide or quicklime (CaO) and carbon dioxide (CO2) which went off into the atmosphere leaving about 6 - 7kg of "quicklime".

CaCO3 + heat = CaO + CO2

Stage 2
Calcium oxide or "quicklime" (CaO) is unstable as  it readily soaks up water  from the environment. In the process it becomes more stable as "slaked lime".

CaO + H2O = Ca(OH)2

Stage 3
Even slaked lime is not completely stable, it takes up carbon dioxide from the air over time and reverts to its original chemical form calcium carbonate. Our original sample of limestone would weigh 10kg again.

Ca(OH)2 + CO2 = CaCO3 + H20

So why did the people of Swordle go to all the trouble of burning limestone if it was going to revert to its original chemical form?  Well.... the texture was different. During the process lumps of limestone were converted to a powdery, floury form (without crushing and grinding) this dissolved more quickly in acid soils. Quicklime once "slaked" with water was mixed with sand and used as mortar in building before the advent of Portland cement.

When the oil runs out we may have to repair the old kilns as the energy cost of grinding limestone makes the process too expensive. Fraser Darling again!......" a crofting family could dig enough peats in a day to fire a kiln to produce enough lime for the croft".

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