## 4 Conclusions  
Homogeneous and phase-separated glasses of the same chemical compositions were evaluated for their crystallization behavior in order to investigate the role of parent glass morphology on resulting mechanical properties. The presence of a known crystallization-inducing species (Pb) was shown to vary in these two glasses, either existing as Pb-rich droplets in the lab-scale (PSPG) rapidly quenched melts, or as homogeneously dispersed Pb when melted in large plates of DFPG prepared by a proprietary large melt processing protocol. The difference in the nucleation and growth-like curves for the two glasses provided the first indication of the presence of different base glass morphology. This starting glass morphology difference confirmed via DSC and TEM was shown to dramatically impact the type, size, and size distribution of GCs formed when the two media were subjected to identical heat-treatment protocols. Specifically, the predominant secondary crystalline phase formed following the same heat-treatment protocol. To quantify the crystallite sizes and distribution of crystals responsible for the mechanical property evolution in the GCs, SEM with image analysis was carried out to quantify the particle sizes and their distribution. The average size (\(267.89\pm 20.3\)) and size distribution (\(120.76\pm 6.98\)) of the PbSe crystallites was greater in the lab melt, whereas the average size (\(157.93\pm 24.5\)) size distribution (\(104.02\pm 12.1\)) of the As\({}_{2}\)Se\({}_{3}\) was greater in the commercial melt. The volume fraction of the crystal phases formed at each phase was quantified via XRD. The cubic PbSe and the needle-shaped As\({}_{2}\)Se\({}_{3}\) crystallites were found to be the preferred secondary crystalline phases (maximum crystal volume fraction) in the Pb-rich lab melt and the commercial melt, respectively. This variation in observed secondary phase, as well as in the size and the size distribution of the crystallites in the heat-treated samples, can be explained by the fact that the activation energy barrier difference in the homogenous commercial plate was lower than that in the lab-melt, resulting in crystallization of the As\({}_{2}\)Se\({}_{3}\) phase (lower \(\Delta H\!\!/\)) before the PbSe phase (higher \(\Delta H\!\!/\)). The mechanical properties (Vickers hardness and Young's modulus) of the heat-treated Pb-rich lab melt and the homogenous commercial melt supports this variation in the volume fraction, size and size distribution of the secondary phase. In the commercial melt, the presence of needle-shaped As\({}_{2}\)Se\({}_{3}\) crystals created an interlocking pattern that was lacking in the lab melt. Therefore, the predominant presence of As\({}_{2}\)Se\({}_{3}\) crystallites in the heat-treated commercial melt resulted in a \(1.11\times\) increase in hardness, as compared to only a \(1.05\times\) increase in the lab melt, following the same heat-treatment protocol. The Young's modulus of both the lab melt and the commercial melt GCs also showed a \(1.05\times\) and \(1.11\times\) increase respectively, by following the same heat-treatment protocol. The consistent increase in the Young's modulus during the heat-treatment protocol was attributed to the increase in volume fraction of the crystallites. The post heat-treated % transmission was not quantified in this study but would be critical to any application where a GC with an enhanced mechanical and optical property was required. Thus, while providing higher post heat-treated mechanical properties, the homogeneous dispersion of glass constituents enabled by the novel commercial processing route, reduces the ability to precisely control the formation of certain phase-specific crystallites in the GCs.