AlmaVerde’s traditional, working lime kiln

AlmaVerde is a traditional area for lime production
The remaining lime kiln on site was restored in 2001
Eating Montpellier snake not a good idea!
Using wood as a fuel makes production very labour-intensive

AlmaVerde's lime kiln

AlmaVerde’s lime kiln

The area around AlmaVerde lies towards the western end of the Barrocal, a hilly, geological sub-region running East-West across the Algarve, between the higher Serra to the North and the flatter, coastal Littoral to the South. The Barrocal  is characterised by the prevalence of calcaric rock at various depths.

Around AlmaVerde, the fractured limestone is exposed on slopes and is very close to the surface in level areas, covered only by a thin layer of sand or clay. For this reason, the area was a centre for the production of cal or lime for hundreds, if not thousands of years.

Lime has been used in construction from the earliest times, in mortar, as a stabiliser in mud floors and renders, and as an external protective and decorative coat in the form of whitewash.

Even today, if you travel by bus along the N125, and ask for the fornos de cal – the lime kilns – the driver will stop at AlmaVerde’s old entrance, close to the tennis courts and to the kiln that AlmaVerde rebuilt and fired in 2001, with its characteristic, pointy-headed , cupola or domed cover.

Continuing west past AlmaVerde on the N125 there is a pottery shop with the shape of a cupola in its roofline. This was incorporated into the design by local architect Guy Quintino in recognition of the historical importance of lime kilns to the area.  Next to the new roundabout between AlmaVerde and the pottery shop, is a structure that was formerly two lime kilns, now sadly without its cupolas.

In the wake of the 1974 revolution, lime production suffered a similar fate to other traditional, labour-intensive occupations such as sardine canning and small-scale agriculture, being replaced by tourism and construction as the main engines of the Algarve economy. The traditional kilns were abandoned.

 The restoration

By 1995, all that remained of the kilns were their bases. The idea for the restoration started after I met João, then a gardener at Luz Ocean Club. Already well into his 60’s, he grew up in Barão S. Miguel. His father was the man who operated the kilns on site, until their demise.

While cleaning out the base, João came across a nest of three large Montpellier snakes. He killed the snakes and took them home, saying what a good meal they would make, but he was wrong. Whether he ate the heads, where a low-toxicity venom is stored, or whether he simply ate too much, he was ill for a week.

The plan was to build a new lining inside the old one, so that the new structure could continue without a break, up into the cupola. The integrity of the lining at the base of the cupola is important because rectangular holes must be left at regular intervals all around it, to help the fire to breathe. The new lining must be built from a material that does not break up at the same temperature as the calcaric rock. The traditional material was a locally-occurring red sandstone called pedra ruiva.

João left much of the early work to another fellow who would generally be roaring drunk by lunchtime. His low productivity was inflating the budget and so he had to go.  João continued the rebuilding at his own expense, on the understanding that he would be able to retain the proceeds from the lime sales following the first firing.

The firing

Quite how labour-intensive lime production could be became clear when the restored lime kiln was fired.

The rocks were first piled up in spirals against the inner, circular wall, following the shape of the dome, and interspersed with plenty of branches and other flammable material. In the 1960’s, when the kiln was in regular use, fuel sources even included rubber off-cuts from a local shoe factory.

The central space at the base was left clear until all the rock was in position, and then filled with dry timber to get the fire going. The stokers needed plenty of fuel on hand close to the mouth or “eye” of the kiln, so that the fire could be continuously fed and its temperature driven up. The target temperature is 900oC, the critical level at which calcium carbonate (CaCO3) starts to separate into calcium oxide (CaO) and carbon dioxide (CO2).

In a traditional kiln of this type, and with more efficient fuels such as coal or even rubber, the firing process can take as little as three days but, with only timber as fuel, the 2001 firing actually took a marathon thirteen days and nights.

The firing developed an impressive rhythm, with each stoking causing an in-breath of air followed by a low-pitched “whoomp”, as tongues of flame shot out through all of the openings at the cupola’s base.

The work required a minimum of two men at all times, with three or more for all but the small hours. The total labour required was well in excess of a hundred man days. This made it clear why rubber would have been preferred as part of the fuel mix, as its higher burning temperature would have shortened the total firing time.

Once the limestone has been reduced to quicklime, the kiln must be left to cool for a couple of days. The quicklime can then be shovelled out and bagged up. It must be handled carefully, as it reacts violently with water to create slaked lime or calcium hydroxide. It is actually in the slaked form that lime is used in most building applications.

The single firing produced around forty metric tonnes of quicklime, which was enough for João and his team to pay themselves at the minimum wage, but not enough to properly reward all the overnight and weekend hours involved. By the end, João was completely exhausted and swore that he would never do it again.

Lime production and the environment

Carbon dioxide is a greenhouse gas, so the process by which limestone is heated to release carbon dioxide might be considered to contribute to global warming. However, common applications for the residual quicklime generally result in a two-stage conversion back to calcium carbonate. Firstly the CaO is slaked with water to produce Ca(OH)2 and then, left to “dry” in air, the slaked lime not only releases water vapour but absorbs back the carbon dioxide:

Ca(OH)2 + CO2 – H2O = CaCO3

Thus whitewash, when dried, becomes a wafer-thin layer of calcium carbonate.

It is not the lime cycle that generates the atmospheric carbon so much as the burning of the fuel used to heat the limestone. An environmentally responsible approach would be to ensure that the release of carbon through the burning of timber is offset by the planting of new trees that will grow to re-absorb the carbon, in other words that any forestry used to support lime production is sustainable.

John Tranmer

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