Between 2001 and 2007, archaeological excavations undertaken along the path of the motorway A16 in the Ajoie region (Canton Jura, Switzerland) led to the discovery of thirteen lime kilns. Located within seven different sites, these structures date from the roman period to the 18th century. The most important site with seven kilns and segments of several paved roads is Boncourt-Grand'Combes (5 kilns date to the roman period, one to the early middle ages and one to the early modern period).

All of the kilns belong to the periodic type, also known as intermittent flare kilns. Although the basic technology remained identical, the architectural solutions evolved. The roman kilns were built according to the instructions given by Cato ; these buried structures were dug into a slope and were fired from below, which implied access via a stoking tunnel or a work trench. These are kilns with air entering at the base.

From the early middle ages onward, all lime kilns built in the Ajoie region seem to belong to a different type : kilns with air entering on the ground level. These only partially buried structures are stoked via an access located at ground level. The earliest known examples of this type of kiln date to the 2nd or 3rd century AD, but it becomes widespread only after the end of the roman period.

The success of this new model of lime kiln can be explained by referring to the fluid dynamics operating inside these structures. In the kilns with air entering at the base, the air flow rises vertically. In the partially buried kiln, the air enters laterally and circulates, a movement that grows more and more pronounced as the temperature rises. Before escaping through vents located in the calcination chamber, the air transfers its heat to the limestone charge, which further increases the airflow. This type of rapid combustion kiln attains the high temperatures necessary for calcination quickly but has a high fuel consumption.

The kilns were used repeatedly and were patched and repaired. This maintenance can be best seen in the roman structures. During a first phase of kiln use, the floor and walls of the kiln are formed by the unmodified surface of the pit housing the structure. During a second phase, these natural surfaces are reinforced with a layer of mixed clay and kiln waste (limestone fragments, burnt loam, charcoal). Repeated use damages this lining ; during the third phase, a stone wall lines the lower half of the kiln and reinforces the walls. In two cases this stonework extends to the stoking tunnel and the stoke hole. This remodelling to the original structure sometimes modified the shape of the hearth or of the vault separating the furnace from the limestone charge.

The evidence from the early modern period structures is more equivocal, but at least some of these kilns seem to have seen a first phase of use with "natural" side walls and were lined with stonework only during a second phase.

The location of the kilns was in part dictated by the topography, particularly so for the buried roman ovens : building these into an existing slope made the initial excavation of the pit much easier. But other criteria, such as the availability of the necessary raw materials, were at least equally important. In the the hills and the valleys of the Ajoie region, limestone layers lie often directly beneath the forest soil and can easily be split into movable blocks.

The necessary firewood was at first ubiquitous. The anthracological analysis of charcoal from the 1st century AD shows an almost exclusive use of beech. From the 2nd to the 4th century, beech and oak are used approximately in equal measure. A marked change in firewood use can be seen in the kiln F7 from Boncourt (17th to 18th century) : oak predominates but numerous other species are equally present, while the use of beech is very limited. This might reflect changes in the forest structure ; it seems that beech became generally less common due to the increased exploitation of the woodlands from the medieval period onwards.

No lime slaking pits were found at any of the excavated sites, implying that lime was transported unslaked from the kilns to the building sites. As slaked lime contains about 20 % water, this would have considerably reduced the transportation costs. The unslaked "quick" lime could be easily watered on the construction sites just before use or could alternatively be stored for several years in a watertight pit.

It took about 30 wagonloads to transport 22 tons of quicklime (estimated production of the kiln at Chevenez - Combe Ronde) to a client. Clearly, having passable roads close to the production site was important. At Boncourt, several road segments were built specifically to access this important production centre.

The earliest mention of lime production in the regional archives dates to the middle of the 16th century. Individuals or corporations ask the prince-bishop of Basel for permission to construct and operate kilns and have to submit to a tax. During the 18th century, a practice still rare in the preceding century becomes generalised : lime is burnt in the permanent kilns of local brickworks. This made the brickworks more profitable and at the same time transferred lime production to artisans particularly skilled in the use of fire. And skill was called for : from the 16th century onwards, the authorities increasingly insist on an economic use of firewood, a commodity in ever greater demand as the forests also had to supply the iron- and glassworks belonging to the prince-bishop.

A final shift in the local lime production occurs between 1780 and 1790, when itinerant lime-burners from the French Jura make their appearance in the Ajoie region. Their clients are usually village communities who themselves sell quicklime to individual customers. This arrival of foreign craftsmen might be linked to a reduced activity of the local brickworks. In any case, local masons would continue to produce small amounts of lime independently until the beginning of the 20th century.

Translation: Robert Fellner