porosity
No fired ceramic is ever completely solid unless you work hard to make it so. Scattered among its grains are voids — pockets of empty space, or trapped gas — left over from the gaps between the original powder particles that the fire never fully closed. Porosity is simply the fraction of the total volume that is void rather than solid. Think of a loaf of bread or a block of Swiss cheese: the same weight of material can be packed dense or riddled with holes, and porosity is the number that tells you how holey it is.
Numerically, porosity P and relative density D are two sides of one coin: D + P = 1 (or in percent, D% + P% = 100). Relative density means the fraction of the material's full theoretical density (the density it would have with zero voids) that the real part actually reaches. A green (unfired) powder compact might be 55 to 60 percent dense, so 40 to 45 percent porous. Good sintering drives that above 98 or 99 percent dense — under 2 percent porosity. Porosity is measured by weighing the part dry, saturated with water, and suspended in water (the Archimedes method), or by comparing measured density to theoretical density.
Porosity is usually the enemy of mechanical and functional performance: pores are stress concentrators that lower strength, they scatter light so a porous ceramic looks white and opaque, they cut stiffness and thermal and electrical conductivity, and they let fluids leak through. A common rule is that stiffness falls roughly as E = E0 times exp(-b times P), with b around 3 to 4, so even a few percent porosity noticeably softens a part. Yet porosity is not always bad: it is engineered on purpose into filters, thermal-insulation firebrick, catalyst supports, membranes and porous bone-replacement scaffolds, where the holes are the whole point.
A dense alumina wear tile is sintered to 99.7 percent of theoretical density (0.3 percent porosity) so that no pore can act as a fracture origin. A diesel particulate filter made of cordierite is deliberately held at 45 to 50 percent porosity, an open sponge of channels that traps soot while letting exhaust gas flow through.
Whether porosity is a defect or a feature depends entirely on the job the part must do.
Porosity by itself does not tell the whole story. Two ceramics with the same total porosity can have very different strength if one has a few large pores and the other many tiny ones — the size and shape of the pores, not just their volume, set the damage.