Vitrification is the process by which a clay body becomes glassy and non-porous under heat, as its particles partially melt and fuse together, closing the body's internal pore structure. It is a matter of degree rather than a single sharp event: a clay body vitrifies progressively as firing temperature rises, and the practical measurement of how far that process has gone at a given point is the body's absorption rate.
Full vitrification is what makes a fired stoneware or porcelain body watertight without needing a glaze, unlike earthenware, which is fired well below its own vitrification point and stays porous unless glazed. Pushing a clay body past its intended vitrification point, by firing it hotter than its recommended range, risks overfiring: the body can warp, bloat, or slump as it approaches the point where it starts to melt outright rather than just fuse.
Because different clay bodies vitrify at different temperatures, matching a body's actual firing schedule to its intended maturity point (see cone charts) is central to getting predictable, durable results from a given clay.
In practice, a potter judges whether a firing actually reached a body's maturity point with a witness cone: a pyrometric cone of the target number, placed inside the kiln next to the ware (visible through a peephole, or read afterward) so its bend confirms the kiln actually delivered the intended heat-work, rather than trusting a thermocouple reading alone. See cone charts for why a cone measures heat-work rather than peak temperature, and why that distinction matters here.
Vitrification is driven by flux-bearing materials in the body, most commonly feldspar, which begin to melt well before the body's other components do. That melt spreads through the gaps between the remaining solid particles and, on cooling, resolidifies as glass that bonds the whole matrix together; it is this glass phase filling interparticle space, not the clay particles fusing directly to one another, that closes off open pore space as a body vitrifies.
Firing a body past its intended range risks two distinct failures, not one generic "overfiring." Bloating is a gas-pressure problem: gases released as remaining materials decompose get trapped once the glass phase has sealed the body's surface, and visibly swell or blister the piece as they expand. Warping (or slumping) is a viscosity problem instead: as the glass phase becomes fluid enough to flow, the piece's own weight deforms it under gravity, the way an over-fired handle can sag out of shape. A body pushed far enough past maturity can suffer either failure, or both, and telling them apart matters for diagnosing which side of the firing actually went wrong.