viscous sintering
Crystalline ceramics sinter the hard way, shuffling atoms one at a time along diffusion paths. A glass has no crystal lattice to obey, and it densifies by a completely different and much simpler route: the whole material just flows, like extremely stiff honey, and the flow lets pores close. Viscous sintering is the densification of a glassy (amorphous) powder, or a compact containing enough glass, by this bulk viscous flow. Warm a heap of glass powder and the grains slump together, the necks fill in, and the pores are squeezed shut — not by diffusion, but by the material oozing under the pull of its own surface tension.
The driving force is the same surface energy as always, but here it acts as a capillary pressure that the soft glass simply flows to relieve. Frenkel's classic model for the first stage gives the neck between two glass spheres growing as x^2 / a = (3 times gamma / (2 times eta)) times t, where a is the sphere radius, gamma the surface tension, and eta the viscosity. Everything hangs on that viscosity. A glass has no sharp melting point; instead its viscosity falls smoothly and enormously as temperature rises — often by a factor of ten for every 30 or 40 degrees near the softening range — so a glass powder that is rock-solid at one temperature flows freely a little higher up. Raise the temperature into the working range and eta drops low enough that surface tension can push the glass to fill the pores in minutes. Because the flow is bulk motion, not surface shuffling, viscous sintering genuinely densifies, and glass powders are famously easy to sinter to full density for exactly this reason.
Viscous sintering underlies a whole family of processes: firing glass frits and glazes so they flow into a smooth continuous coating, sintering porous glass and fused-silica bodies, densifying glass-bonded whitewares, and the first step of making many glass-ceramics, where a glass powder is viscously sintered dense and only then heat-treated to crystallize. The main pitfall is a rival reaction: if the glass crystallizes (devitrifies) before it has finished flowing, the newly formed crystals shoot the viscosity up and the flow — and the densification — stops dead, leaving trapped porosity. Controlling the race between viscous densification and crystallization is the central craft of sinter-crystallized glass-ceramics.
A glaze is powdered glass painted on a pot: in the kiln it viscously sinters, the grains flowing together and the pores closing, until it forms one smooth glassy skin — the same viscous flow that lets glass powder densify far more easily than any crystalline ceramic of the same temperature.
A glass densifies by flowing, not diffusing — surface tension squeezes the pores shut as viscosity falls with temperature.
Viscous sintering is fast and effective only while the material stays glassy. If it crystallizes first, the viscosity soars and flow stops — which is why an over-crystallizing glass-ceramic can be left stubbornly porous.