closed porosity
The other kind of pore is the one you cannot reach: a void sealed entirely inside the solid, cut off from the surface and from its fellow pores, like a bubble frozen deep inside a block of ice or a raisin buried in the middle of a bun. This is closed porosity. Dip the part in water and nothing gets into these pores, because there is no channel leading to them — they are private rooms with no doors. A part can therefore feel and test as watertight on the outside while still carrying void space hidden within.
Closed pores are the endgame of sintering. In the final stage, once a body passes roughly 90 to 95 percent of theoretical density, the once-continuous open channels between grains neck down and pinch off into strings of separate, rounded pockets, usually sitting at the corners where several grains meet. Now those pockets can only shrink by atoms diffusing across from the pore surface into the surrounding lattice, a slow process — and worse, any gas trapped inside (from the firing atmosphere or from binder burnout) builds up pressure as the pore shrinks and fights back, so the last one or two percent of porosity is the hardest to remove. If grains grow past their pores, the pore ends up marooned deep inside a single grain, where diffusion is slowest of all, and it may never come out.
This is exactly why firing hotter or longer does not always give you a denser ceramic, one of the field's great counter-intuitions. Aggressive firing coarsens the grains, and a runaway grain can trap pores inside itself, freezing in closed porosity permanently. The industrial cure is hot isostatic pressing (HIP): squeezing the part with high-pressure gas at temperature so the external pressure overwhelms the trapped-gas resistance and collapses the last closed pores. Closed porosity also has uses — deliberate closed pores make lightweight, sealed-cell thermal insulation that will not soak up water.
A batch of transparent alumina comes out of the furnace faintly milky instead of clear. Cross-sectioning reveals a scatter of closed pores just a micron across, each trapped inside a grain. A follow-up HIP cycle at 1200 bar collapses them, and the next batch is clear enough to read text through.
A single micron-sized closed pore per grain is enough to turn a transparent ceramic milky by scattering light.
Once a pore is trapped inside a grain, ordinary pressureless sintering can almost never remove it, because the bulk lattice is the slowest diffusion path. The lesson: keep grains small until the pores are gone, not the other way round.