The easiest but most inefficient way to fire ceramics is an open bonfire.  A big part of the firepower escapes to the enviroment without any effect to the ceramics.  In times of global warming, deforestation and global climate change it should be an absolute no go. Unfortunately most of the pottery in LDC is still done in this way.
The green ware (unfired pottery) will be mixed with fuel (biomass) and stapled to a mound.  After that, this mound will be ignited to fire the ceramics.

Unfortunately this process is uncontrolled and so a lot of cracks will appear because of the heat stress to the clay. Beside the temperature is not homogeneous. That means, while some of the ceramics are burned (black) other parts are not durable (not strong/hard)
– a quality issue. The solution to avoid losses through heat stress and to improve the quality is a kiln.