Andreas Heege, 2019
The main ingredient in the production of high-quality pottery is clay.
Clay: mineralogical and geological term denoting an (unfired!) mineral raw material, a fine-grained sedimentary rock with various grain sizes (< 2µ). Clay results from the weathering of rocks that contain feldspar (e.g. granite, gneiss, quartz porphyry). Most clays are made up of fine-grained minerals (aluminium silicates <2 µ, which rarely account for more than 50% of the total volume) and non-plastic (and sometimes staining) impurities (remnants of the bedrock, lime, iron, manganese, silt, sand, gravel, organic components) that were picked up during their transportation. Because of its platelike structure, clay is plastic and can be shaped without tearing as long as it retains water. This is one of its most important properties.
“Fat” clay, or clay with a high plasticity will “shrink” more when it dries out, while “lean” clay with a high silt or sand content is less plastic and will exhibit a lower degree of shrinkage.
The definition of “loam”, another term that is often used in the literature as a raw material in pottery production, is not the same as that of clay. It is, rather, a mineralogical and geological term denoting clay that contains quartz and mica sand and sometimes fine-grained lime. As a “lean” ceramic raw material, loam is more suited to tile and brick making. Using a loam deposit for pottery production requires special treatment to prepare it, i.e. its coarser components have to be removed. Many of the local clay deposits in Switzerland were formed towards the end of the Last Ice Age and contain such impure loams that they have to be prepared before they can be used to make pottery.
Clay deposits can be primary or secondary deposition sites of clay. They result from the transportation of material from the weathering site to the deposition site and this process can take various forms: aeolian (carried by the wind), aquatic (by water), glacial (by ice) or fluvioglacial (by glacial meltwater). Further processes of redeposition create so-called secondary clay deposits. Because several consecutive processes of deposition can occur, clay deposits often consist of (horizontal) layers. The composition of these layers can vary (lime or iron deficient, fat or lean), resulting in the clays having different properties when they are worked. Despite these differences, it is possible to distinguish between individual clay deposits (and thus production centres) based on the elemental concentrations which can be identified by means of geochemical analyses (e.g. neutron activation analysis – NAA, X-ray fluorescence analysis – XRF).
The properties of clay and its colour after firing depend on the clay mineral composition and the metal/mineral admixtures. The latter differ depending on the age and type of the deposit, quaternary or tertiary. Many of the clay deposits are suitable only for the production of earthenware (fired at temperatures of up to 1100°C) and few contain clays that vitrify before they soften and are therefore suitable for stoneware production (wide vitrification range, temperatures above 1200°C). Such deposits usually date from the Tertiary and none are found in Switzerland, which is why stoneware or porcelain can only be produced here if the raw materials are imported. Switzerland typically produces earthenware. Classic stoneware clay deposits, on the other hand, can be found in the Siegburg region near Bonn, at Frechen, Langerwehe and Raeren, in the Westerwald region, in southern Lower Saxony, northern Hessen, in Saxony-Anhalt and Saxony. The clays from around Betschdorf and Saverne in the Alsace region are just about suitable for making stoneware.
Only clays that are lacking in iron turn light-coloured or white when they are fired. Used in the production of slip (for slip-coating or slip-trailing), such types of clay no longer exist in Switzerland and have been imported, mainly from France, for some time now. Clays that do contain iron turn red or reddish in colour when fired in an oxidising atmosphere; depending on the lime content and the firing temperature, they can also turn a light red or pink.
Clay was and still is extracted by open-pit mining. Claypits were usually easy to establish and did not require much maintenance. They were therefore rather widespread. The exploitation of the clay deposits was relatively unsystematic, and the pits were often only a few metres deep. From the 19th century onwards, open-pit mining became more and more mechanised and today it involves the use of excavators and trucks. Shafts and tunnels, which often required elaborate reinforcements, were also sometimes dug to exploit clay deposits located at greater depths. The biggest problem was water accumulating in the pit or on the surface. Deep mining in shafts and tunnels is believed to have commenced in the early 18th century, but there are very few written sources and almost no archaeological evidence to support this theory. A special way of deep mining involved digging so-called bell pits, which formed self-supporting cavities in the clay deposit. The bell-shaped hollows measured 8-12 m in diameter and 12-15 m in height.
Depending on the structure of the enterprise (cottage industry/craft, early industrial manufacture or sale of clay), clay mining was done in the autumn and winter, in the spring, or all year round. Some potters mined their own clay, others had it mined for them or exchanged it for finished pottery through pottery traders. Clay mining rights were usually held by the landowner, be it the sovereign, a monastery, a village community or an individual owner. The so-called Bergregal, a historic right of ownership of untapped mineral resources, did not usually apply to clay deposits. Potters had to acquire clay mining rights, either by making payments or offering services, e.g. by producing and supplying stove tiles or by fitting and repairing tiled stoves in the estates and houses of the landowners.
Once extracted, clay must be prepared before it can be used for pottery making. The following steps are required:
After extraction, the clay is loosened by exposing it to frost. This is known as weathering. It can take place right next to the clay pit, in a “claybank”, or at the workshop.
The second step, ageing or maturing, involves adding water, which is why a good water supply is important for any potter’s workshop. It allows the clay minerals to settle and the organic components to decompose by going through various processes of decay and fermentation, resulting in an increase in plasticity. This step usually takes place at or near the workshop. Tempering agents are sometimes added at this point.
Depending on the quality of the raw clay or loam, it is sometimes soaked, mixed up into a slurry and then poured through a screen to remove coarse or organic impurities. The clay then has to rest until the water content is reduced evenly and the slurry thickens. Before the introduction of filter presses in the late 19th century, this was a rather lengthy process.
The aim of the next phase is to homogenise the mass. Depending on the workshop, different methods are used. Before mechanisation was introduced, the clay was kneaded using the feet. In this way, impurities could be detected and removed, while the clay was mixed and tempering agents added. The second method was to beat or wedge the clay, throw it down, tear or cut it into pieces and knead it again. This process was repeated several times. This step gave potters a further opportunity to find and remove impurities and mix the clay. Before the introduction of mechanised pugmills, mixers and extruders in the second half of the 19th century, homogenising the clay was hard manual labour. An idealised picture of a workshop in Bonfol in the Jura region from the late 19th century shows the partial mechanisation of the steps and techniques used in all workshops throughout the region and beyond. It shows a pugmill as well as the cutting of the clay. The aim was to produce an homogenous, plastic and suitably tempered mass without air bubbles that could be used to throw pots on a fast wheel.
Usually, natural clay cannot be used directly to create ceramic vessels or objects. It tends to be too fat and must be tempered to achieve the plasticity that is required for throwing and the desired degree of shrinkage when it is fired. The main reason why tempering is added to fat clay is to reduce the shrinkage, prevent the clay from tearing or cracking and, in cookware, to make it more resistant to fluctuating temperatures. Suitable tempering agents include: quartz sand, ground quartz, sandstone, granite, grus, mica, graphite, lime, seashells, grog (ground pottery) and organic materials such as straw or threshing waste, flax boon or dung. The creation of temper involved thermic or mechanical crushing, pounding, grinding and sieving to achieve the desired particle size. Besides the raw clay and its firing properties, archaeologists use the type, amount, size and dispersal of the temper as criteria to classify wares. The amount and size of the temper have an impact on the visual appearance of the surface of the pottery after firing.
After preparation, the clay is kept moist and usually stored in a pit or cellar at the workshop. Before it is shaped into vessels or objects, it is once again wedged, kneaded and beaten to further homogenise the mass. It is then divided into portions or balls whose size and weight vary depending on the kind of vessel to be thrown. There are many old depictions of potters with balls of clay waiting to be used.
Translation Sandy Haemmerle
German: Ton
French: argile
References:
Blondel 2001
Nicole Blondel, Céramique: vocabulaire technique, Paris 2001, bes. 30-35.
Mämpel 1985
Uwe Mämpel, Keramik. Von der Handform zum Industrieguss, Hamburg 1985.
Mämpel 2003
Uwe Mämpel, Keramik. Kultur- und Technikgeschichte eines gebrannten Werkstoffes. Ein Beitrag zur Geschichte der Porzellanindustrie Nordost-Bayerns (Beiträge zur Wirtschafts-, Sozial- und Technikgeschichte 6), Hoheneger 2003.
