## 2 Experimental  
Coupons of PSPG, the lab-melted parent 20 PbSe glass, were compared to commercially melted (DFPG) coupons to assess the role of varying melt protocols and the resulting as-melted glass morphology. A 100 g 'lab-scale' batch of 20 GeSe\({}_{2}\)-60 As\({}_{2}\)Se\({}_{3}\)-20 PbSe [20 PbSe] was prepared using the conventional melt-quenching technique in the Glass Processing and Characterization Lab (GPCL) at University of Central Florida (UCF). The glass was prepared using high purity elements from Alfa Aesar: Selenium (99.999%), Germanium (99.999%), Lead (99.999%), and Arsenic (99.999%). The elements were weighed in a nitrogen purged MBraun Labmaster 130 glove box and loaded in a 30 mm (in diameter) quartz tube. The quartz tube was sealed under vacuum using a methane-oxygen torch. The tube was melted in a rocking furnace overnight (16 hr.) at melting temperature of \(T_{m}\) = 850 \({}^{\circ}\)C and quenched at \(T_{Q}\) = 650 \({}^{\circ}\)C using compressed air flowing over the tube. The glass was annealed at \(T_{a}\) = 177 \({}^{\circ}\)C for 2 hr. to prevent any quench-related stress in the glass. The prepared glass rod was cut into 2.5-mm-thin slices using a slow speed saw. The slices were polished on both sides using a SiC paper with a final 0.05 \(\upmu\)m Al\({}_{2}\)O\({}_{3}\) slurry polishing. Employing their specialized method designed to eliminate phase separation upon cooling, collaborators at Amorphous Materials Inc. (AMI) produced the same 20 PbSe composition using 5 Ns or higher purity chemicals and the proprietary melt processing protocol previously described. The 2.4 kg commercial melt was made as a \(\sim\)200 mm diameter plate. An IR camera was used to inspect the plate visually in the NIR and identify regions with excessive levels of striae. Based on this information, low striae regions of the plate were selected, core drilled, and sliced into several 2.5 cm diameter (\(\sim\)3 mm thick) coupons. The coupons were polished on both sides and used for the measurement of physical properties prior to and following heat-treatment protocols. Both sets of coupons were subjected to side-by-side heat-treatments and re-polished prior to measurement of post-heat-treatment properties. Thermal properties (ramp rate derived \(T_{g}\), \(T_{x}\)) of the parent glasses were measured on a Netzsch DSC 2014 F1 Phoenix differential scanning calorimeter (DSC). An empty aluminum crucible was used as a reference, and a constant sample size (20 \(\pm\) 2 mg) of glass (sieved to be \(<\)150 \(\upmu\)m particle size) was used to measure the thermal profile of the glass. The DSC was operated at a rate of 10 \({}^{\circ}\)C/min, with the relative glass transition temperature (\(T_{g}\)) and crystallization temperature (\(T_{p}\)) determined by applying the first derivative technique to the generated DSC curves. Nucleation-like (I) and growth-like (U) curves were measured to compare the tendencies of crystals to form under identical thermal protocols. Such I-U curves, as discussed in [43; 44], complement traditional DSC data that only measures \(T_{g}\) and \(T_{p}\), as well as provides information on the relative rates for nucleation and growth of crystals as a function of temperature. However, the measurements do not give absolute values of crystallization rates, as the precise quantity of as-formed nuclei is non-zero and not accurately detectable. These I-U rate curves were used to define suitable temperatures and times for heat-treatments. An ideal material for completely controlled nucleation has an I-U curve consisting of two, well-separated nucleation and growth areas. The nucleation and growth-like curves in this study were quantified using methods described in detail elsewhere [45]. An initial DSC program was run from 25 \({}^{\circ}\)C to 350 \({}^{\circ}\)C at a rate of 10 \({}^{\circ}\)C/min in order to determine the \(T_{g}\) and \(T_{p}\).