What Happens in the Fire? Turning Clay Into Something Permanent

Clay is a mineral of fine platy particles that hold water between them, which is what makes it plastic and workable, and drying removes the water added for shaping while leaving the material still clay, so a dried unfired pot dissolves in water. Firing changes that permanently. Around five hundred degrees the water chemically bound within the mineral structure is driven off, which destroys the clay mineral and cannot be reversed, so from that point the material is ceramic rather than clay. Higher temperatures cause particles to fuse at their contacts, which is called sintering and which gives strength, and higher still causes some components to melt and fill the spaces between particles, producing a dense body that absorbs no water at all.

A firing passes through distinct phases and each has its own hazards:

• •Drying out, where remaining physical water leaves and where heating too fast turns it to steam and shatters the pot
• •Burning out, where organic material in the clay oxidises and must be given oxygen and time
• •The point around five hundred and seventy degrees where quartz in the clay changes form and expands suddenly
• •Sintering, where the body gains strength
• •Vitrification, where partial melting closes the pores
• •Cooling, which must also pass the quartz change slowly or the ware cracks

The technical history is a sequence of solutions to the problem of reaching higher temperatures and holding them. Firing in an open bonfire reaches perhaps seven hundred degrees unevenly and is how the earliest pottery was made. Enclosing the fire in a pit and then in a built chamber retains heat and raises the achievable temperature. Separating the firebox from the chamber holding the ware protects the pots from direct flame and from ash. Adding a chimney creates a draught that pulls air through and burns fuel faster and hotter. Building the whole thing on a slope lets the chamber act as its own chimney and allows a series of chambers each preheated by the one below, which is how east Asian kilns reached the temperatures porcelain requires long before Europe managed it.

Knowing how hot a kiln is running is a problem with several traditional and modern answers. Pyrometric cones are slender pieces of ceramic formulated to soften and bend at particular combinations of temperature and time, and a row of them visible through a spyhole tells the potter what the ware has actually experienced, which is more useful than temperature alone because the effect of heat depends on duration as well. Watching the colour of the interior gives an experienced eye a good estimate, running from dull red through orange to white as the temperature rises. Thermocouples give a direct electrical reading and are standard in modern kilns, and they measure the air rather than the ware. Draw trials, small test pieces pulled out during firing, show the glaze directly.

Whether the kiln has surplus oxygen or a shortage of it changes the chemistry and the colour substantially. An oxidising firing supplies more air than the fuel needs, and iron in the clay and glaze ends up in its oxidised form, giving the reds, oranges and buffs of ordinary earthenware. A reducing firing starves the fire, so combustion pulls oxygen out of the oxides in the clay and glaze, turning iron to its reduced form and shifting colours towards grey, green and blue, which is the mechanism behind celadon glazes and behind the grey bodies of much stoneware. Copper behaves dramatically, giving green when oxidised and deep red when reduced. Controlling atmosphere is straightforward in a fuel-burning kiln and is essentially impossible in an electric one, which is why electric firing is limited to oxidised results.

Around five hundred degrees the chemically bound water leaves the clay mineral irreversibly, after which the material is ceramic, and higher temperatures sinter and then partly melt the body. Quartz changes form near five hundred and seventy degrees on both heating and cooling, which is where ware cracks if rushed. Starving the fire of oxygen shifts iron and copper colours dramatically.