## 2 Experimental
Further information about the theory and findings and the creation of nucleation-like and growth-like curves in ChGs and Tellurite glasses is outlined in previous works [45; 46; 47]. To assess the impact of as-formed base glass morphology on post heat-treatment mechanical properties, coupons of both lab-melt and commercial melt glass were subjected to the same, two step heat-treatment protocol for a fixed set of growth temperatures. Specifically, all coupons saw a nucleation heat-treatment of 220 \({}^{\circ}\)C for 2 hr., followed by a growth step at temperatures of either 250 \({}^{\circ}\)C, 260 \({}^{\circ}\)C, or 270 \({}^{\circ}\)C for varying time durations. These values were chosen based on rates defined by the glass' I-U curves. To evaluate the point at which growth proceeded to a point that reduced transmission to an unusable level (defined as 50 % of the initial transmission at 4.5 \(\upmu\)m), the maximum time and temperature duration of heat-treatment was determined to be 270 \({}^{\circ}\)C for a total growth time of 150 min. All samples were polished (no annealing) after heat-treatment to remove any residue that may have developed on the coupon's surface during the process. TEM was used to study the morphology and chemical composition of phases within the parent base glass and post heat-treated glass samples. The glass was ground into powder and placed on a carbon grid. Bright field (BF) images were collected using a FEI Tecnai F30 TEM, with an electron beam acceleration voltage of 200 kV. A Zeiss Ultra 55 FEG SEM was used to image crystallite shape and their dispersion throughout the surrounding glass matrix. ImageJ software was utilized to detect crystallites and extract their sizes from the SEM images. The crystallization behavior of the base glass and that of the heat-treated samples was studied using a PAN analytical Empyrean X-ray diffraction system employing Cu K- alpha radiation source (\(\lambda\) = 154 nm). This system was used to probe samples across a 20 range of 10 to 80\({}^{\circ}\). Lab-prepared crystalline Selenium (Se), lead selenide (PbSe), and Germanium lead selenide (Ge\({}_{0.1}\)Pb\({}_{0.9}\))Se were used as standards to quantify the volume fraction of crystallites in the heat- treated samples. The density and transmission spectra of the pre and post heat-treated glass coupons is shown in Figures S1 and S2 of the Supplementary Information. The Vickers hardness of pre- and post-heat-treated glass coupons was measured using an indentation technique [48; 49] on a Shimadzu DUH-211S Hardness Tester, with a load of 100 mN (0.01 kgf). This load value was chosen as it resulted in clean indents without any crack initiation on the corners of the indents. The measurement of the resulting indent's diameter, \(d\) = 2\(a\) (where \(a\) is the average length of the diagonal left by the indenter) with the corresponding load \(P\), allows the calculation of the Vickers hardness according to Equation (1) (kgf/mm\({}^{2}\)). A polished coupon of commercial glass of similar composition (Ge\({}_{10}\)As\({}_{40}\)Se\({}_{50}\)) (SCHOTT IRG 24) with a known hardness value was used as a comparative reference. Sources of error for this measurement can be due to non-parallel sample surfaces, pre-existing surface damage (cracks or scratches), the specific annealing protocol applied to the glass sample, and from the manual measurement of the indent size on the display screen. \[H_{V}=\frac{0.4636P}{a^{2}} \tag{1}\]  
The Young's modulus \(E\) was measured using the Sonic Modulus method. Here, \(E\) was calculated by using Equation (2), where \(v\) is the Poisson's ratio, \(C\) is the ultrasonic wave velocity and \(\rho\) is the density of the sample. The Poisson's ratio was measured by using Equation (3), where \(C_{L}\), and \(C_{T}\) are measured ultrasonic wave velocities in the longitudinal and transverse directions, respectively.