... from clay and pottery

Everyone of us knows clay, many of us have pottered something from this soft, earthy mass in our childhood or later, which can be plastically formed into objects of all kinds. Pottery" always sounds a bit stuffy and old-fashioned, but after all it is also old and baked. Fired clay is called ceramic and is due to mineralogical transformations of the minerals significantly more resilient than dried clay. Because of this great property this material is one of the oldest processed raw materials and has accompanied mankind around the globe for thousands of years.

The origins of pottery are as old as mankind and accompany us long before we became settled farmers. People all over the world have made use of this material - independently of each other! According to current knowledge, it is assumed that the firing of clay was a chance discovery - probably because the fireplace was built by chance on dried clay and the surrounding "earth" was fired in the course of the fire. The first purpose-made ceramics were formed by hand and fired in a campfire. They are kept rather simple and the focus is clearly on functionality. Vessels such as bowls and cups were made for daily use. However, art and idolatry also quickly found their way into pottery. The two oldest clay ladies are the Venus of Dolní Věstonice (made of clay) and the Venus of Willendorf (made of limestone). Both figurines are estimated to be 30,000 years old. The first flowering in the production of ceramics occurred about 15,000 years before Christ in China and Japan. Some finds are even considerably older. Mesopotamia, the land between the Euphrates and Tigris rivers, can be considered the cradle of the more modern Eurasian pottery. Around 4000 B.C., the fast-spinning potter's wheel was invented in the Near East and thus the mass processing of the material clay began. From the 3rd millennium BC onwards, clay was first fired on a large scale to make bricks. Along with wood and stone, clay was at all times one of the most important building materials of mankind. Because clay consists of a mixture of clay, silt and sand.

It is a material that occurs naturally in the earth's crust all over the world and consists mainly of clay mineral particles. Over time, it has evolved from the weathering of rocks, especially from feldspars. Wind, water and seasonal temperature differences gnawed away at whole mountain ranges and so even the hardest rock is eroded to gravel, sand or even clay. It is by no means a pure material. In most cases, it is a mixed product of layer silicates such as illite or kaolinite and other minerals that do not contribute to the plastic properties, such as quartz, calcite or dolomite. In addition, the material typically also contains organic, carbon-rich components. All clay particles have a size of no more than < 2 µm (= 0.000002 cm). This variety also explains the different colours that alumina can have. The rust-brown clay popular for terracotta is due to its high iron content. In contrast, chlorine inclusions cause the material to turn green. The popular black or brown clay indicates a mixture with the metal manganese. With a sufficient water content it is generally plastically deformable. If the water content is reduced by drying and/or during firing, it becomes brittle and breaks under load. As long as the dried clay is not fired, it is capable of swelling. Its volume increases with increasing water content and decreases with decreasing water content. It can be reworked as often as required with water to the familiar soft mass.

The iron-free porcelain clay (kaolin) is the purest clay found in nature. Pure clay deposits occur very rarely in nature. This white substance is - as its name suggests - used among other things for the production of porcelain. When fired, this type of clay becomes very hard and glassy. Due to the low proportion of foreign minerals and high quartz content, this material can be fired at a very high firing rate. Temperatures of 1500 °C are common for porcelain and lead to the desired translucent character. In Germany, naturally occurring clay engravings can be found in Brandenburg (clay landscape around Zehdenick) and near Mengerskirchen in the Westerwald.

In another of our worth knowing articles we will go into the differences between porcelain and ceramics in more detail.

Chamotte is clay that has already been fired once and is added to the raw clay mass. This fired clay is ground and sieved according to size. The undermixing makes the final ceramic more stable, which is an important aspect especially for build-up ceramics. The increased stability of the ceramic allows for larger objects, as the smaller layered silicate gets caught on the relatively coarse fireclay particles. Chamotte clay is available in various grain sizes, colours and mixing ratios. It is less suitable for turning on the turntable - unless you are quick to apply it. Turning chamotte clay is literally a strenuous activity.

Even if clay is basically to be regarded as an inorganic material, it is nevertheless to be attributed a certain life of its own. After it has swollen, it must first be processed. One has to imagine the layered silicate as scales or feathers. In order for it to regain its optimum properties, these must be aligned in the same way. There are various fulling techniques for this purpose, the closest comparison is with dough kneading. The individual platelets are brought into one direction and the mass is homogenized. When turning on the potter's wheel this usually happens automatically, but is not practical for a beginner. The processed clay is ready for processing when it no longer contains air bubbles and it is an evenly moist mass. Air bubbles are the death of every ceramic and a quality problem that should not be underestimated, because air expands when heated. When heating up in the kiln, this leads to explosive pressure discharges and downright pulverizations. As long as it is only your own ceramic that is broken, it is only annoying. If other ceramics or even the kiln is damaged by the flying debris, this is fatal.

In addition to the creative plastic processing possibilities in decorative ceramics, it is still used today in the following areas: Roofing and masonry bricks as well as for façade and paving clinker, cement, ion exchangers (for example, in the purification of drinking water and for decolourising solutions) and the sealing of sewers (unfired raw material). Large, naturally occurring clay pits are also under discussion as a storage site for (nuclear) waste. Due to its dense structure it would potentially be suitable for this purpose.