bloating
Bread rises because yeast makes gas bubbles that puff up the dough; overdo it, or trap the gas, and you get a bloated, blistered loaf. A ceramic can suffer the same fate for the same reason. Bloating is an unwanted swelling of a ceramic body — a puffing up, blistering, or ballooning that lowers its density — caused by gas trapped inside the body with no way to escape, expanding and pushing the material apart.
Bloating needs two things to coincide: gas being generated inside the body, and the body's pores having sealed shut so the gas cannot get out. Gas can come from binder or organics decomposing, from carbonates or sulfates breaking down, from over-firing a body until it starts to release gas, or from trapped air. The trap is closed porosity: early in firing the pores are open and interconnected, venting gas freely, but as sintering proceeds the pores pinch off into isolated closed bubbles. If gas is still being made after that seal, its pressure has nowhere to go and inflates the pores — the part swells, loses density, and may blister at the surface.
Bloating is why the pre-firing steps are done slowly and thoroughly: get all the gas-makers out during binder burnout and debinding, while the pores are still open, and there is nothing left to bloat the part once they close. Rush burnout, and leftover carbon or undecomposed binder waits like a buried charge to swell the body later. Bloating is not always a flaw, though — the same trapped-gas swelling is deliberately harnessed to make lightweight expanded-clay aggregate and foamed insulating ceramics, where the puffed, gas-filled structure is exactly the goal.
Two identical tiles are fired to the same peak temperature; one was debound slowly, the other rushed. The slow one comes out dense and flat; the rushed one comes out swollen and blistered, a good deal lighter — leftover carbon turned to gas after its pores had closed, and had nowhere to go but into puffing the tile up.
Bloating: gas made after the pores seal shut inflates the body, lowering density and blistering the surface; prevent it by removing gas-makers while pores are still open.
Bloating is a firing symptom with a pre-firing cause: the swelling shows up hot, but the fault was usually laid down earlier as incompletely removed binder, carbon, or carbonate. Fix it at burnout, not by tinkering with the peak firing.