The atmosphere created in the kiln is important to the firing result. Kiln atmospheres can be oxidizing, with a good flame and draught and plenty of oxygen present, or reducing. To achieve the latter the flow of air is restricted by closing the kiln doors, then the flames are damped down, perhaps by throwing wet wood onto the fire. This causes the oxygen in the atmosphere to be reduced significantly with the increase in carbon monoxide needed for the reduction process. Reduction was a very common technique the Chinese used to give a great diversity of colours and effects. As an example, in a reducing atmosphere any red ferric oxide (haematite) is converted to black ferrous oxide. The latter is also a stronger flux, thus lowering the melting temperature. The colour change can be reversed if cooled in an oxidising atmosphere. The final colour of an unglazed pot depends on its chemical composition and the firing regime. When fired in an oxidizing atmosphere, a clay high in iron will turn red or brown, while one low in iron will be white or pale buff. When fired in a reducing atmosphere, a clay high in iron will turn black or dark grey, while one low in iron will be pale grey. Organic matter within the clay will remain as carbon in a reducing atmosphere, as there is no oxygen to burn it out, and this will also darken the colour of the pot.

Stoneware usually contains high iron content, up to 8%, and after firing in a reducing atmosphere produces colours from purple to grey. An oxidising atmosphere produces reds, browns and yellows. Porcelain usually only contains 0.5% iron, but oxidation would result in a grey/yellow colour so reduction is used to achieve a bluish tint. Pottery kiln development went hand-in-hand with metal smelting technology, each benefiting from the other. Kilns were used quite early on in the Middle East, Egypt and India, but achieved a lower temperature (8-900 degrees C) than those in China, where higher temperatures were needed to produce stoneware and porcelain. Fewer ancient kiln remains have been discovered in these other countries compared with China, presumably because they were smaller, less significant structures.

Firing can cause expansion and/or shrinkage but to a lesser extent than drying, perhaps up to another percent or two. Shattering of pots through the generation of steam has already been noted, but this also happens if the clay has been poorly “wedged” so air pockets remain within the clay to expand and produce high pressure on heating. There are often stresses set up in the body during its forming that can cause deformation during drying and firing, and mechanical forming can introduce more stresses than hand throwing. Warping of the pots within the kiln often occurs if there is inadequate support during the stage in the firing when the body becomes soft. This can also be caused by uneven distribution of heat within the kiln, and there may be even more distortion if the pots are fired above their maturing point and so allowed partly to melt. Some pot forms are particularly liable to distortion and need special attention in the kiln, such as the “shoe” used to keep open the mouths of sauceboats. Round hollow wares (cups, bowls) can be inverted over a dome-shaped support so as it shrinks it rides up the support, remaining circular. Plates can be stacked if not glazed, with layers of sand or alumina between.