How Firing Transforms Clay into Ceramic

Clay contains very small mineral particles that slide past one another when mixed with water. This gives the material plasticity, allowing it to hold a shape while remaining workable. Artists can build forms by pinching, coiling, slab construction, moulding or using a wheel.

Air bubbles and uneven moisture can create weak points. Wedging helps make the clay more uniform and removes some trapped air. Walls also need reasonably even thickness so different areas dry and heat at similar rates.

As a piece dries, water between the particles evaporates and the clay shrinks. The leather-hard stage is firm enough for trimming or attaching handles. When the piece becomes bone dry, nearly all removable water has gone, but it remains fragile and can still return to mud if soaked. Handles, rims and joined pieces are often attached at the leather-hard stage because the surfaces still contain enough moisture to bond.

During firing, the kiln temperature rises gradually. Remaining physical water leaves first. At higher temperatures, water chemically bound within clay minerals is driven off, and the original clay structure changes permanently.

The particles begin to fuse through a process called sintering. Depending on the clay and temperature, some materials may partly melt and form a glass-like phase between particles. This process, called vitrification, increases strength and reduces porosity.

Heating too quickly can turn trapped moisture into steam and crack the piece. Cooling too quickly can also create stress. Kiln schedules are therefore planned in stages, giving heat time to spread through the work evenly. Different clay bodies mature at different temperatures, so a firing suitable for earthenware may be unsuitable for stoneware or porcelain.

Glaze is a mixture of minerals applied to bisque-fired or sometimes unfired clay. In the kiln, it melts and bonds to the ceramic surface. It can add colour, texture, shine and a less porous finish.

Glaze results depend on ingredients, thickness, firing temperature and kiln atmosphere. The colour before firing may look very different from the final surface. Artists use test tiles because memory and optimism are not reliable glaze measurements.

A careful ceramic process includes:

• •Prepare clay with even moisture.
• •Build walls with suitable thickness.
• •Dry the piece slowly and completely.
• •Use a firing schedule suited to the clay.
• •Test glaze combinations before important work.

Unfired clay softens and washes away, but fired ceramic resists rot and decay. A broken pot does not vanish. It splits into fragments called sherds, which can survive in the ground for thousands of years. Because pottery styles, shapes, decorations and clay recipes changed over time and differed between regions, archaeologists use sherds to help date the layers they excavate and to trace trade, since a pot made from one area's clay may turn up far away.

Firing also resets a natural clock. Minerals such as quartz in clay slowly store energy from tiny amounts of natural radiation in their surroundings, as electrons become trapped in their crystal structure. Heating in a kiln releases this stored energy and starts the count again from zero. In the laboratory, scientists can heat a small sample and measure the light it gives off. This method, called thermoluminescence dating, estimates how long ago the ceramic was last strongly heated.

The air inside a kiln affects colour as much as the temperature does. In an oxidising firing, there is plenty of oxygen, so metals in the clay and glaze stay combined with oxygen. Electric kilns usually fire this way. In a reduction firing, a potter burning gas, wood or oil restricts the air supply. The fuel cannot burn completely and produces carbon monoxide, which pulls oxygen away from metal oxides in the clay and glaze, changing their chemical form and therefore their colour.

The same ingredient can give strikingly different results. Copper oxide usually produces greens in an oxidising firing, but in reduction it can create deep reds, famous in Chinese glazes sometimes called oxblood or sang de boeuf. Small amounts of iron give warm yellows and browns with oxygen, yet soft blue and green celadons in reduction, a type of glaze prized in China and Korea for centuries. Reduction also darkens and speckles many stoneware clays. Potters often keep careful records of the atmosphere as well as the temperature for every firing.

A thermometer tells you how hot a kiln is at one moment, but clay and glaze respond to both temperature and time. A piece held a little below its target temperature for longer can mature much as it would if heated slightly higher for a shorter time. Potters call this combined effect heatwork. To measure it, they use pyrometric cones, small slender pyramids made from carefully blended ceramic materials that soften and bend once they have received a particular amount of heatwork. The German ceramic chemist Hermann Seger developed an influential early set in the late nineteenth century.

Cones carry numbers on a scale. In the system common in North America, cone 06 bends at roughly 1000 degrees Celsius, a typical range for earthenware, while cone 10 bends at around 1300 degrees, a common target for stoneware and porcelain. Potters often stand three cones in a row where they can be seen through a spy hole: a guide cone that bends first, the firing cone for the target and a guard cone that should stay upright. When the firing cone bends until its tip is level with its base, the firing has reached its goal, and a bent guard cone warns of overfiring.

Clay shrinks twice on its way to becoming ceramic. It shrinks first as water leaves the spaces between particles during drying, and again in the kiln as the particles draw together and fuse. Total shrinkage for many clays falls somewhere around 10 to 15 per cent, but it varies with the clay body and the firing temperature. Fine porcelain usually shrinks more than coarse clays containing grog, which is ground-up clay that has already been fired. Grog has done its shrinking, so it stays the same size in the kiln and helps hold the rest of the body steady.

You can measure shrinkage yourself. Roll a slab of fresh clay and mark a line exactly 10 centimetres long. After drying and firing, measure it again. If it now reads 8.8 centimetres, the clay has shrunk by 12 per cent and keeps 88 per cent of its size. To make a mug that ends up 8 centimetres across, divide 8 by 0.88, so you should throw it about 9.1 centimetres wide. The same test explains why lids are made from the same clay as their pots, and why joining two different clays can lead to cracks, since each one shrinks by its own amount.

Firing transforms soft clay into ceramic by removing water, changing minerals and fusing particles. Glaze can then create a bonded glass-like surface. Understanding drying, temperature and material compatibility helps you see a ceramic object as the result of controlled changes rather than simply hardened mud.