The workability of the clay depends on its plasticity and strength. It must take a shape fairly easily, stand up to the stress of stretching, and support its own weight, when shaped, without collapsing. Many natural clays fire satisfactorily with washing alone, but overly plastic or “sticky” clays need more attention to avoid problems including cracking during drying. For example, before the clay is worked, non-plastic materials may be added to those already present in the clay. These are usually known as “temper” or “fillers”. They improve the strength of the clay by reducing plasticity, and speed up the drying process because their larger grain size enables any water to migrate to the surface to evaporate, as well as letting other gases escape. They also reduce shrinkage and warping both before and during firing and can also usefully adjust the silica content of the clay. Among these materials that have been used as temper are quartz sand, ground shell and bone, rock fragments, grog (fired clay or pottery ground into small particles), chaff, grass and dung. Eraclius, writing in the late 12th century AD, describes the production of grog from strong clay when none is available, and Theophilus (1070-1125) a Benedictine monk, describes the use of grog as a temper in the manufacture of crucibles for metal smelting. The amount of temper added depends on the plasticity of the basic clay and the purpose for which it is required. Clay used for making tiles, for example, is often coarse with a high proportion of sand or grog. The tile-making process does not require plasticity, and the high shrinkage rate that plasticity entails would be a disadvantage. However, in 15th century London, tiles were made so poorly that they lasted only 3 to 4 years, so the potter’s guild had to set new quality standards in 1468. Temper is usually added as the potter does the final preparation of the clay prior to forming vessels, which entails further wedging to compress the clay and expel any remaining air that would otherwise expand and could cause the pot to shatter during firing.

High quality clays such as kaolin are refractory materials and on their own are likely to vitrify (melt) at above 1,600 degrees C. If one is mixed with a material having a lower melting temperature (the flux), the refractory clay successfully retains the shape of the object while the lower melting temperature material vitrifies and the two coalesce to form the ceramic. A flux is a substance that promotes fusion between two or more materials. For example, potassium and sodium feldspars have a lower melting point than quartz, so they can be used to reduce the firing temperature of a high quartz potter’s clay. All feldspathic materials act as fluxes in pottery. They do not have “abrupt” melting temperatures but soften as they heat up until they become a viscous mass that can cool into a glass. The most common feldspars are based on potash (K2O) or soda (Na2O). Potash feldspar “melts” at 1,290 degrees C (starts at 1,160) and soda “melts” at 1,160. However, if the two are mixed in the proportions soda 35% and potash 65%, the composite melts completely at 1,070 degrees C. Other common temperature lowering fluxes are calcite (calcium carbonate, becoming CaO or lime when heated), which when added at a level of 2% to pure quartz (melting temperature around 1,800 degrees C) lowers the melting temperature to 1,200 degrees C, and borax (sodium borate) which is particularly used as a flux in ceramic enamels (decorative ceramic paints) to enable their melting temperatures to be significantly reduced. Oxides of iron are powerful fluxes, so the usual contaminant of sedimentary clays (up to 10% in red clays) is most helpful in reducing the firing temperature (to around 1,000 degrees C with silica).

So, in any of the ceramic bodies, each component part of the clay mixture usually serves a specific purpose. For example, in porcelain, kaolin or China clay is the pure white powdery refractory mineral that gives some plasticity to the mixture and allows moulded pieces to retain their shape without collapsing in the kiln. China stone is a non-plastic, hard feldspathic material that, when finely powdered, is the flux that melts during firing to form a glassy mass binding together the clay particles. It is transparent in thin layers and milky white in bulk. Quartz may also be added, often as relatively pure fine sand. It has a high melting temperature and also provides the structure and strength to prevent distortion during firing. Of course early potters did not understand the chemistry, but by trail and error they were able to select clay constituents that had the components to allow them to successfully produce pots using the firing technology of the time.