Book: Ceramics - Art or Science? Author: Dr. Stan Jones

2. What are Ceramics & What is Clay?

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What is Clay?

Rock is formed beneath the surface of the earth by pressure and temperature. Sedimentary rocks are around 1km deep and are the result of sediment being solidified under pressure into sandstone, and carboniferous debris from oceans, for example coral and shells, into limestone, as mentioned earlier. Metamorphic rocks have suffered some heat from their depth (tens of km) and are harder, recrystallised marble and quartzite. Finally, igneous rocks have been hot enough to melt (tens to hundreds of km) and form very hard basalts and granites. Feldspar, found in Metamorphic and Igneous rocks, is the commonest rock constituent, making up 59% of the total earth’s crust. In turn compounds based on silicon, which are a constituent of most rocks, including feldspar, make up 26% of the earth’s crust. Silica or silicon dioxide (SiO2) is the most abundant oxide on earth and its combination with other oxides forms silicates. In nature, pure silica is found as quartz, and sand is sedimentary quartz.

When these rocks are pushed to the surface by, for example, volcanic activity, they are no longer in their stable habitat. They start to deteriorate and change chemically due to weathering and especially the acid in rain, for example carbonic acid from dissolved carbon dioxide (CO2). The soluble part of the rock, which contains sodium, potassium, calcium and magnesium salts, slowly dissolves and is carried by streams and rivers to the sea. Marine life takes in vast quantities of calcium and magnesium salts to form shells and bones, eventually forming thick layers at the bottom of the ocean. This leaves a predominance of sodium salts, such as sodium chloride, which is a major contributor to why the sea tastes salty. What remains at the original site of the rocks includes the insoluble silicates and aluminium compounds. The weathered rock is broken down eventually to become material varying in size from sand to fine clays. So, clay is formed from distressed rocks.

Because of the particle size difference, dispersal by rain water is also different, with larger particles remaining nearer the source, sand being carried some way, but clay, having very small particle size, staying in suspension in the water for considerable distances. This mechanism also explains why beaches are made of sand and rock, because the continuous wave action takes the finer particles further out to sea where they deposit in the deeper, calmer water. The clay in suspension in streams and rivers is often transported from its origin to secondary sites such as lakes, the sides of slow-moving rivers and flood plains. On the way the clay picks up impurities, particularly iron oxides, which produce the red/brown colour we expect of terracotta, for example as plant pots – even plastic ones are coloured to match! Because clay is created all over the world and transported to other sites, there is an abundance of it throughout the world. Soil itself is a mixture of silt, sand and clay, along with decaying vegetable matter.

Clay used for pottery has to have certain characteristics for success. It has to be made up of constituents that, when properly prepared, are mouldable into the required shape. It must hold its shape during the firing process and, when heated or “fired” at a temperature and for a duration that can be achieved by the potter, produces the required quality product. The earliest method of firing pots was to place them on a bed of fuel, usually grasses, twigs and dung. Dung was often used as it burnt rapidly and evenly, holding its shape as an ember, protecting the pottery from too rapid cooling.

Clay mineral constituents have a particulate size which is extremely small (one hundredth the size of a grain of sand). This makes it very plastic when wet; so much so that it is often too sticky to be workable on its own and would collapse under its own weight. Also most “pure“ clays require very high temperatures for long periods to fire properly. Many early clays could be used “as dug”, since by good fortune they had a useable mix of constituents, with larger particles as “impurities”. However, from the earliest time it was realised that some clays had to be mixed or “tempered” with larger grained non-plastic material to make successful pots. The clay and temper material used by the early potters depended on what was available in the locality, as it would not be sensible to transport them over any distance. All sorts of material were added as temper, including sand, crushed shells and animal dung.

Feldspar is the source of the “best” or “China” clay. When it breaks down it leaves silica (SiO2) and alumina (Al2O3) that together form pure white clay.  Pure white or China clay was quite elusive in the past. Although it would have been discovered in the Bronze Age during mining activities, and was used for example in producing Cyprus White Slipware, its true significance was not recognised in the West. It was first discovered as an important ingredient for pottery in China. It is called Kaolin after the Chinese name Kao-Ling for a High Ridge or mountain where it was first discovered. It was the key to the production of the best quality pottery – porcelain. To produce the very best clay, potters in China left large mounds of dug clay to further weather for up to 40 years! No wonder pottery was a family craft passed from father to son.

 

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