Pottery ceramics are made by forming clay, along with some fluxes and impurities, and firing at high temperature. They are divided into earthenware, stoneware and porcelain. Classification depends on ingredients, firing temperature and water absorption. The raw materials are inexpensive – natural geological processes have taken care of most of the work. Thus, the price of finished ceramic ware is largely dependent on the handwork involved, firing cycles and glaze.

Since it is impractical to remove impurities from the clay, the quality and colour of ceramics were, in the past, largely dependent on local supply. Regions became renowned for the quality of their ceramics, and this was largely determined by the kaolinite (hydrated aluminosilicate) content of the local clay. First discovered in China – the history and development of ceramics is a fascinating one – it is found in abundance throughout Europe, the Americas and Asia, although material from different mines will have markedly different properties. The commercial value of kaoline is determined by purity, fineness (grain size) and whiteness. It is used in applications other than ceramics, most significantly paper and textiles.

Water in the clay mix creates plasticity, which allows it to be formed by pressing, moulding and casting. As the water evaporates the clay body shrinks until the particles are in contact. At this point it will be quite brittle and not strong at all. At high temperature, over 800 degC, the water that is chemically bonded to the clay minerals is released and the particles fuse (sinter) together. This process is a one-way reaction and irreversible. At this point the ceramic is porous, and the surface is suitable for glazing. Firing at higher temperature, over 1,250 degC, melts the minerals together to produce a vitreous material, which is much harder, stronger and impervious to liquid. Porcelain and some types of stoneware are able to tolerate such high temperatures are reach their optimum strength. Other types, including earthenware and mid-fire stoneware, would melt and collapse at this temperature.

The colour of fired clay depends on the ingredients and firing technique. Traditional red pottery, such as terracotta, is the result of iron in the clay oxidising during firing. Reduction firing (without oxygen) results in dark colours, from grey through black. Other metal oxides produce different colours. So called “through-body” ceramic, is unglazed and the colour of the clay body gives the finished appearance. It may be speckled as a result of intentional inclusions, such as shards of stone and flint, for example. Likewise, improper mixing of different coloured clays produces swirls and blends, which can be used to create decorate surfaces.

In many cases, the same product can be made in all types of ceramic. Earthenware is the least expensive, fired at the lowest temperature, lightest, easiest to cut and shape, and can be glazed to create a durable and impervious surface. The porosity of earthenware makes it prone to water absorption and so vulnerable to cracking in freezing conditions. Stoneware bridges the gap between earthenware and porcelain and provides a good balance of properties utilised in diverse applications, from durable tiles and bricks to furniture and tableware. Porcelain is the most expensive (although still pretty cheap compared to many other material types), difficult to work, hardwearing and impervious. It is used in the finest tableware, sanitary ware and construction materials. Screen and digital printing techniques are used to produce surface patterns, and effects that mimic other materials like stone, with the glaze becoming integral to the ceramic after firing.