This study investigates the crystallization behavior and microstructural evolution of lithium metasilicate (Li₂SiO₃) glass subjected to thermal histories designed to emulate the cooling stage of zirconia infiltration in dental restorations. Three thermal routes were examined: (i) a non-isothermal schedule to identify crystallization and melting events, (ii) a controlled isothermal schedule to obtain homogeneous glass–ceramic microstructures, and (iii) a quasi-isothermal natural cooling schedule from the molten state to mimic the thermal profile during infiltration without reproducing actual capillary flow or interfacial reactions. Phase identification and quantitative analysis were performed by X-ray diffraction with Rietveld refinement, and the resulting microstructures were characterized by field-emission scanning electron microscopy. Under isothermal conditions, lithium metasilicate (Li₂SiO₃), lithium disilicate (Li₂Si₂O₅), γ-spodumene (γ-LiAlSi₂O₆), β-lithium phosphate (β-Li₃PO₄), and quartz crystallized with an overall crystallized fraction of approximately 62±0.9 wt.%. In contrast, quasi-isothermal cooling produced a crystallized fraction exceeding 87±1 wt.%, dominated by lithium metasilicate (Li₂SiO₃), β-lithium phosphate, quartz, and cristobalite, with neither lithium disilicate (Li2Si2O5) nor γ-spodumene (γ-LiAlSi₂O₆) detected under the present XRD conditions. The quasi-isothermal route also generated significantly coarser morphologies: average crystal length and thickness were roughly 10-fold and 31-fold larger, respectively, than those in the isothermally treated sample. These results demonstrate that the thermal path strongly governs phase assemblage, crystallized volume fraction, and crystal morphology in lithium silicate glass–ceramics. By clarifying how controlled versus quasi-isothermal cooling histories shape the final microstructure, this work provides a structural basis for optimizing lithium silicate glasses used in zirconia infiltration technology and for guiding future studies on the mechanical and functional performance of these materials.
