firing

A clay pot is soft and crumbly until you put it in a kiln. Firing is the high-temperature heat treatment that turns a fragile shaped powder, the green body, into a hard, dense, permanent ceramic. It is the step that gives ceramics their name, from the Greek keramos, meaning burnt stuff. Think of a snowman firming up on a cold morning: loose grains weld together at their contacts without ever melting into a puddle.

During firing the part is heated to a large fraction of its melting temperature (often 1000 to 1800 degrees C). Several things happen in sequence: leftover water and organic binders burn out; chemical reactions finish; and above all the powder sinters, meaning neighbouring particles bond at their contact necks, the pores between them shrink and close, and the part densifies. Atoms diffuse to fill the gaps, driven simply by the system lowering its surface energy. The part shrinks noticeably, typically 15 to 20 percent in each linear dimension, as its density climbs from roughly 60 percent of solid toward 98 percent or more. Firing does not usually mean melting: solid-state sintering densifies well below the melting point.

Firing is where the microstructure, the grains, the pores, and the thin glassy films between grains, is finally set, so it decides most properties. But hotter and longer is not always better: densification competes with grain coarsening, and if grains grow too fast they can trap pores inside themselves, where no further firing can remove them. Traditional wares are fired in kilns; advanced ceramics use tightly controlled schedules and sometimes pressure-assisted routes (hot pressing, hot isostatic pressing, spark-plasma sintering). The detailed science of this step lives in the sintering field; here it is the last link of the roadmap.