## 3 Results and Discussion  
The DSC spectra for the two 20 PbSe parent glass samples (lab-melt (PSPG) and commercial (DFPG) melt) were used as a preliminary indicator to assess the presence of phase separation. It has been previously reported, that the presence of more than one \(T_{g}\) is an indication of possible phase separation in a glass [50; 51]. The DSC spectra for the two base glasses heated at 10 \({}^{\circ}\)C/min are shown in Figure 2a. The traces show one endothermic inflection in the lab melt sample, followed by a broad exothermic crystallization peak, which has been attributed to Pb- and/or As- containing crystalline phases [45]. In prior studies varying the amount of PbSe content, the exotherm was seen to shift to lower temperature indicating that it is likely associated with the PbSe containing phase(s). In this work, it was also shown that Pb-rich droplets precipitate in the amorphous glass matrix of the 20 PbSe composition due to the PS [52]. However, the visible presence of only one \(T_{g}\) in the DSC data for this well-known phase-separated glass matrix may be attributed to the small batch size and higher cooling rate during quenching of the glass. Here, the second \(T_{g}\) may have been obscured by the small volume fraction of the secondary phase. The DSC traces of the commercial melt show two clearly resolved endothermic inflection points, suggesting the presence of two \(T_{g}\) followed by two exothermic crystallization peaks. The presence of the second endotherm in the commercial melt is likely due to the thermally-induced development of a secondary phase in the glass matrix; a curious and perhaps anomalous behavior. It may be similar to that seen in similar composition melts discussed elsewhere [52]. As seen in Figure 2a, both the \(T_{g}\) endotherm and the crystallization peaks shifted towards lower temperature in the commercial melt when compared to the lab melt. This slight shift is believed to be due to the vastly different thermal histories of the two glasses. The lab melt underwent very rapid quenching via constant air flow and eventually came to room temperature. Whereas, the commercial melt was rapidly brought from its initial, target temperature of 216 \({}^{\circ}\)C to room temperature, and subsequently, precision-annealed. The precision anneal involved raising the glass quickly to its annealing point temperature (185 \({}^{\circ}\)C) and soaking for 3 hr., cooling at 1 \({}^{\circ}\)C/hr. to 15 \({}^{\circ}\)C below its strain point temperature (160 \(\pm\) 15 \({}^{\circ}\)C), and finally allowing it to cool more rapidly to room temperature. The vastly different cooling protocol may result in the formation of a higher concentration of as-quenched nuclei. Therefore, this glass might be expected to crystallize at lower temperature. This is consistent with the lower temperature onset of the crystallization peak for the same composition glass. The variation in the thermal history did not result in any new features in the DSC traces, which would be an indicative of formation a new species in the glass matrix. While the \(T_{g}\)'s for the two melts were close, the variations in position of the first crystallization peak temperatures suggest that heat-treatment near this temperature could lead to the formation of varying crystallization phases. Thus, further analysis of the first crystallization peak in the nucleation- and growth-like curves helped in the selection of suitable heat-treatment protocol for these glasses. From the I-U curves, the evolution of different crystalline phases and their estimated volume fractions in the post heat-treated glass matrix could be predicted. As discussed earlier, the nucleation and growth-like curves are instrumental in understanding the relative rates of crystallization in the glass, based on the temperatures at which the glass matrix is soaked.