Prehistory's "connected" mass production

To find out how Mesopotamian kilns from the Chalcolithic (7000 to 5000 BC) were made and, above all, worked, Johnny Samuele Baldi and his colleagues from the ArchéOrient laboratory (CNRS / Université Lumière Lyon 2) decided to build a replica in the grounds of the Commandery of the Knights Templar at Jalès (Ardèche, southern France), a branch of the laboratory.

During their work, the archaeologists realised that several people would have been needed to make this type of kiln. The two chambers had to be built simultaneously, row of bricks by row of bricks, rather like 3D printing. This process requires close collaboration between the craftsmen. Moreover, even though the diameter of the experimental kiln (reconstructed on the basis of a medium-size model) did not exceed 1.5 m, the original kilns could be up to 8 m in diameter and were generally used as boilers for extensive networks of interconnected kilns.

It was in 2015, in Iraqi Kurdistan, that Baldi discovered for the first time this unique type of kiln consisting of two non-superimposed chambers (one elongated and the other circular), used for firing ceramics. Although they could operate independently of each other, the two chambers were linked by a masonry duct of varying length, usually underground. At first glance, this was nothing exceptional. Yet it was the first thing that puzzled the researchers, who were unable to explain what the duct was for, particularly as they realised that several kilns could be connected together so as to operate in series.

Claire Padovani, a postdoctoral researcher at the laboratory, shapes and then places the hearth (the platform that serves as a base for the ceramics during firing) in the circular chamber, which she divides into two parts. The hearth, which is made of mortar (a mixture of clay, straw and water), weighs several dozen kilograms. To prevent it from collapsing under its own weight while being laid, it is first left to harden in the open air.

Just a few more bricks and a dash of mortar, and the replica of the kiln will be complete. Ìn this image, the two chambers of the kiln can be clearly made out, as well as the hearth, thanks to the light placed just below it. Although there is no hearth in the first (elongated) chamber, it could, when necessary, also be used to fire ceramics. However, it was more frequently employed to burn combustible material (such as wood, reeds, dung and bitumen), and acted as an external furnace designed to generate heat. This heat then travelled along the duct and arrived in the lower circular chamber, whence it then ascended through the hearth and fired the ceramics.

It is time for the kiln, which is 1.5 m in diameter, to receive its first ceramics, numbering a few dozen. Once the pottery has been placed on the hearth, the kiln is sealed with mortar, while leaving a space for the fumes to escape, and, above all, to create a draught so that the fire continues to burn fiercely. It will take a few hours to fire the ceramics, which will have lost their residual humidity and up to 15% of their initial volume at the end of the process.

A fire is lit in the first (elongated) chamber in order to start the firing. The temperature there very rapidly rockets to exceed 900 °C, as compared with 300 °C in the ceramics chamber. However, after several hours, the pottery is still not fired, although it is now dry. For a ceramic to be considered fired, it must have been subjected to a temperature of at least 450 °C. Otherwise, it will be damaged when it comes into contact with a liquid. It was by lighting a fire under the hearth – and after sealing up the elongated chamber – that the scientists observed a rise in temperature to around 850 °C. Since the ceramics had already dried out, the firing process took only a short time and used very little fuel.

This reconstruction helped the archaeologists realise that the kilns were the centrepiece of an elaborate system, which was not used simply to fire ceramics, but rather to create a drying-firing sequence. In fact, the kilns were usually built in series and were therefore all interconnected by masonry ducts. This meant that the craftsmen could use the heat generated during the firing of kiln No.1 to dry out the ceramics in kiln No.2 and, once the firing in kiln No.1 was complete, move on to firing in kiln No.2 and drying in kiln No.3, and so on. This proto-industrial technique enabled them to produce ceramics in a continuous process.