There are several techniques for ceramic firing and different techniques that require various types of ceramic kilns. The first type of shot, probably accidentally discovered by ancient humans, is the pit shot.
This involves placing accidental or bisque pottery in a large hole in the ground, covering pottery above and below with burning materials such as wood or charcoal, and lighting it up and letting it burn for hours or overnight. You can navigate various online sources if you want to know more about Beginners Guide to Kiln-Formed Glass.
Towards the end of combustion, the hole can be covered with sand or earth to cut off the oxygen and cause the atmosphere to contract in the hole. Modern oven wood fires are generally made of brick and sometimes contain some space for combustion and a port to feed fuel.
When wood is burned, the ash, which naturally contains silica, calcium, potassium, and other minerals, is deposited on the ceramic in the kiln, creating a pleasant ash glaze effect. The individual combustion quality and the different mineral content of the wood produce very different effects and firing can take several days or weeks.
Another traditional combustion technique uses the Anagama kiln, which consists of a long cooking chamber that has a combustion chamber at one end and a chimney at the other. The side of the oven contains small ports for stacking.