Limestone kilns

Calchere are the kilns which were used for the production of quicklime. Limestone rocks were collected, piled together and then cooked in order to obtain the prized end-product which was used in a variety of sectors. Although no longer in use, about forty of these facilities can still be found in the Ledro area.

The traditional calchera was a circular vat-like structure made of stone and ranging in size between 3-5 m in diameter and 4-6 m in height, normally built inside an embankment. The surrounding earthwork had a double function: on the one hand it made the vertical walls easier to build since they could avail of partial support, and on the other hand an access slope was provided at the top end of the kiln.
The materials used to build these calchere can be divided into two groups: calcareous dolomite rock and glacially deposited rocks (granites, porphyries, metamorphic rocks). Kilns built with the former group of materials had the obvious disadvantage that they soon showed signs of wear: part of the inside of the structure would also be cooked during the production process, with consequent reduction in the thickness of its walls. This caused structural problems and the kiln would need to be rebuilt or abandoned after a certain amount of use. In order to reduce wear the walls of the kilns were coated with a layer of lime.
The calchera was built in an area where there was a plentiful supply of cookable rock which was also accessible by cart. Then prior to starting the ‘cooking' process, a large amount of firewood had to be collected in 30-40 cm diameter bundles weighing 8-10 Kg. For an average-capacity kiln (with an output of about 25 tonnes of quicklime) more or less 3000 bundles were needed. An expert woodcutter could manage to prepare this amount of fuel in about a month. And so a good portion of the costs involved in the production of lime obviously derived from the need to procure these amounts of firewood. The best fuel came in the form of bundles of fir or pine sticks, but often twigs and leftovers from forestry work were used as well as a range of bushes including hazel, ash or willow which were not deemed suitable for domestic fires.
Calcareous rocks which could be found almost anywhere in the Ledro area were used as raw material for production. The rocks were normally collected near the kilns, along the banks of the rivers at the bottom of mountain slopes, and of course the ability to tell ‘good limestone' from ‘bad limestone' obviously depended on the years of experience put down by each ‘calcherot'.
Before lighting the kiln, the bundles collected were deposited nearby. Generally the weight of the quicklime produced was equal to the weight of the firewood consumed. Once the bundles were piled, the kiln was prepared. A first phase saw the construction of the ‘mouths' of the furnace and of the ‘vault'. Suitably hewn blocks of stone framed an opening through which would be fed the bundles of fuel. A vent was left under the opening, which was also useful for extracting any build-up of ash from combustion. The building of the vault, situated like a diaphragm between the kiln and the material to be loaded, was a delicate operation. The blocks were put in place using a sort of wall plate, using the classical technique for construction of barrel vaults. The interlocking stones in this drystone vault were then held in place by mutually exerted force, a large wedge-shaped keystone being fitted into the top of the vault for added stability. The kiln was then ready to be loaded with the rock collected. The material was loaded in declining order of size, with the largest rocks nearest the flame. Once loading was completed, the door was closed.
Then everything was ready for the lighting of the furnace: a pile of fuel was placed in the furnace and lighted with a torch. From this point on the feeding of bundles into the fire had to proceed without interruption, one new bundle added every three minutes for 90 unbroken hours. In the first phase of cooking, red flames rose from the top of the kiln, while in the last phase of cooking the flames had a bluish colour. The kiln was unloaded a few days later, when the cooled quicklime was more manageable. Generally the quicklime produced was weighed on the spot and then sold to locals.
The lime produced was used in a number of sectors for a variety of functions. It was an essential material in traditional building, used for cement, plaster and in dyeing, as well as for disinfection of premises. It was so widely used in agriculture that in farmyards there was generally a ‘busa della calce', or pit containing a certain amount of cooled lime always ready for use. It was also used as a preservative.
40 kilns have been officially registered in the Valle di Ledro, scattered throughout its six municipalities, especially in Tiarno di Sopra, in Val d'Ampola, in Concei and in Molina di Ledro.