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Just as the microstructures in glass-ceramics encompass the range from nanocrystalline transparent materials to microcrystalline tough materials, so the paths of microstructural evolution in glass-ceramics vary widely. Evolution can proceed in numerous ways, their genesis being a perturbation of some type, including the surface nucleation used in glass frit processing, crystallization of the primary phase or phases upon distinct crystalline nuclei, and nucleation promoted by nano- or micro-scale amorphous phase separation in the parent glass. Examples of the crystallization history of several glass-ceramic materials are described, with emphasis on how their microstructural evolution influences their ultimate physical and optical properties.

Strong and tough glass-ceramic materials based on the complex chain silicate miserite (KCa5open square(Si2O7)(Si6O15)F) have been prepared using standard frit sintering and internal nucleation processes. The miserite may be accompanied by crystals of fluorite, cristobalite, xonotlite, a canasite-like phase, and fluorapatite. The highly crystalline glass-ceramics have a microstructure composed primarily of interlocked, lath- or log-shaped miserite crystals with pronounced cleavage planes. This microstructure provides abraded flexural strength values as high as 235 MPa (34 000 psi) and fracture toughness values >3.0 MPa·m1/2. These strength and toughness values are quite high for a glass-derived material. Miserite glass-ceramics may be useful for many applications in which such strength and toughness are desired.