a nucleating agent
Crystals do not usually spring up out of nowhere; they need somewhere to start, a seed to build on. A nucleating agent is a small ingredient added to a glass to supply those seeds on demand, so that when the glass is crystallized it grows an enormous number of tiny, evenly spread crystals rather than a few big scattered ones. It is the same principle as seeding clouds to make rain, or dropping a crystal into supersaturated syrup to start rock candy: give the crystallization a launch pad and you control where and how finely it happens.
The physics is that starting a brand-new crystal from scratch, homogeneous nucleation, is hard, because a tiny embryo crystal has a lot of costly surface for very little volume, so most embryos redissolve before they can grow. A nucleating agent sidesteps this by providing a ready-made surface to grow on, heterogeneous nucleation, which lowers the energy barrier dramatically. Common nucleants for glass-ceramics are titania TiO2, zirconia ZrO2, phosphorus pentoxide P2O5, and fine dispersions of noble metals like platinum or gold. Added at just a few percent, they typically phase-separate first into millions of nanometre-scale droplets or particles per cubic millimetre, each of which then seeds one crystal.
That is precisely why a nucleating agent is the heart of making a glass-ceramic. The finished microstructure, and hence the strength, toughness, and thermal-expansion behaviour, is set by how many crystals grow and how small they stay, and that in turn is set by how many nuclei the nucleant provided. Flood the glass with nuclei and every growing crystal quickly bumps into its neighbours and stops, giving a fine, uniform, strong microstructure. Leave the nucleant out and you get the opposite, a few coarse crystals, which is just ordinary uncontrolled devitrification, cloudy and weak. Controlling nucleation is what separates a prized glass-ceramic from a spoiled glass.
Add about 2 to 4 percent titania to a lithium-aluminosilicate glass and, on the first heat-treatment hold, it separates into a fog of titania-rich droplets, billions per cubic millimetre. On the second, hotter hold, a lithium crystal grows on each droplet until the crystals meet, filling the glass with grains under a micrometre across. Leave the titania out and the same glass, given the same schedule, grows only a sparse scatter of coarse crystals and stays weak and hazy.
A nucleating agent seeds vast numbers of nuclei so crystals stay small and uniform, the key step in making a glass-ceramic.
A nucleating agent controls how many crystals form, not merely whether they form. Its whole value is producing many fine grains instead of a few coarse ones; without it, crystallization is just uncontrolled devitrification that ruins the glass.