open porosity

Not all pores are alike, and the first great divide is whether a pore can be reached from outside. Open porosity is the network of pores that connect to one another and to the external surface, forming continuous channels that run right through the part. Dip such a ceramic in water and the water wicks in, the way it soaks into a kitchen sponge or an unglazed clay flowerpot that sweats on the outside when you fill it. If a fluid can get in and out, that pore volume counts as open.

Open porosity dominates in the early and middle stages of sintering, and in any lightly-fired body. When a powder compact is still fresh, the void space between particles is a fully interconnected labyrinth. As sintering proceeds and density climbs, the channels narrow and eventually pinch shut; open porosity typically survives until the part reaches roughly 90 to 95 percent of theoretical density, after which the remaining pores become sealed and isolated (closed). Because a fluid can penetrate it, open porosity is exactly what the Archimedes water-immersion test measures — you saturate the part, and the water taken up gives the open pore volume directly.

Open porosity is the property you want when the job is to let something pass or to hold something. It gives filters their permeability, catalyst supports and membranes their throughput, kiln furniture and firebrick their gas-permeable insulation, and bone scaffolds the interconnected channels that living tissue and blood vessels must grow into. Conversely it is the property you must eliminate when the job is to keep something out or in: a porcelain dinner plate, a chemical crucible or a vacuum tube must have essentially zero open porosity, which is why such wares are either fired to near-full density or sealed with a glassy glaze.