River banks, pot holes and powder

mud piesPlaying mud pies was a near universal childhood experience of my generation. I used my seaside spade to dig a hole in the hidden place behind the trees in my back garden. Excavation was followed by pouring water into the pit and making a slurry. As a seven year old, I also formed little objects out of the brown, pliable soil.  The first ceramic objects were probably the result of similar experiments. Investigating the properties of the clay they found alongside river banks, Palaeolithic humans found they could mould objects out of the earth.

Clay from near London Bridge station, Alison Cooke
Clay from near London Bridge station, Alison Cooke

Today, almost all potters buy their clay in bags from their supplier. There are a few purists who purchase clay in its powdered form then reconstitute it from dry material. Some ceramic artists such as Alison Cooke use hand-dug clays as part of their work. But most of us take our clay from 12.5kg blocks that are delivered on a pallet.

It has been a long journey from the Stone Age to the present, from digging out river mud to precisely weighed slabs of clay encased in polythene bags.

In Britain, the best earthenware and stoneware clays were laid down as a secondary deposits from the Carboniferous period onwards, about 350 million years ago.  Transported by rivers to the flat Midland plains, high quality pottery clay lies less than a metre below the ground’s surface in many parts of central England. A geological map of north Staffordshire shows a seam yellow Keuper marl running north to south through Burslem and Hanley. Below this yellow clay lies red Etruria marl laid down side-by-side with Carboniferous coal seams.

Geological map of north Staffordshire.
Geological map of north Staffordshire.

Streams and rivers exposed the clay and Britain’s first potters gathered their raw material from where it lay bare and visible. If more clay was needed, village potters simply dug back into the clay seam. Anywhere that clay deposits were exposed, village potters would dig. Horse-drawn carts and coaches often left ruts in dirt roads that could uncover a seam of clay. If the raw material looking promising, potters would dig back, leaving a ‘pot hole’ in the road. Eventually, lanes and highways became so dangerous that the practices was outlawed.

We can still do this, although it is better to avoid digging up the road. Find a river bank or take a spade and hollow out a hole in the garden until you reach the clay that lies below the soil. Take a small sample and rub it between your fingers, if it feels sticky, the soil has a high clay content. Roll the clay into a sausage then loop it round your finger. This is a test for plasticity – it the clay forms a ring without breaking, it can be used as pottery clay.

Potters have been undertaking the same test for thousands of years. When they found a good seam they dug, discarding as many stones and roots as possible. The clay was then transported it back to the homestead, where it was crushed before mixing it with water into a slurry-like slip. It was then sieved before being left to dry and mature, prior to a final treading and kneading.

Clay is still extracted and prepared using the same method in many parts of the world. In climates where water is in short supply, clay is left to dry before being ground into a powder, sieved and then reconstituted as clay. Watch the video from of a woman potter in western Iran, using techniques that have not changed over thousands of years.

Mechanisation has made the preparation of clay less of a backbreaking process in Britain, although the method remain essentially the same. Here is another video, this time of Isaac Button, a potter who worked near Halifax, until he retired in 1965. Although spades have now been replaced by huge hydraulic excavators, most earthenware and stoneware clay used in ceramics is prepared in the same way today, albeit on a larger scale.

The clay is quarried from opencast sites – in Staffordshire these pits are still known as Marl holes. It is then transported back to the slip house, ground down and the waste material removed. The clay is then mixed with water to form a slip inside a big mixer called a blunger. (Ceramics has a vocabulary of fabulous words: blunger, jogglenatch, twiffler). The liquid clay is then sieved, before being left to dry. It is then mixed up and kneaded in a pugmill before being extruded into blocks, stacked up and left to ‘sour’ or mature. Finally, after a few months, the clay is re-pugged, extruded again and is bagged up, ready for use. Let the transformation of mud begin!