Viscosity data of agZIF-62 as presented in Figure was obtained via a ball penetration technique . For comparison, previous estimates used calorimetric data to extrapolate viscosity from Tg and the liquid fragility index m; direct measurements in the supercooled range similar to the present data are currently not available. For instrumented penetration experiments, we used a sapphire ball with a radius of R = 0.75 mm, loaded with 10-20 mN, on which the rate of sink-in into the glass' surface was determined as a function of temperature (see Methods section for details). At the glass transition temperature Tg = 322 °C (determined using differential scanning calorimetry (DSC) ; log η(Tg) = 12 is assumed for normalizing the viscosity at Tg/T = 1, black pentagon in Fig. ). The dashed lines are tangents with slope m of the viscosity at Tg of various reference materials . For example, a kinetic fragility index of m = 16 indicates a strong glass such as silica. Larger m indicates more pronounced non-Arrhenian behavior. Values of m ≥ 100 are typical for polymer melts. The experimental data obtained by ball penetration further deviate from the non-Arrhenian prediction, in particular, approaching lower temperature (increasing relaxation time). For agZIF-62, we attribute this observation to three possible effects. For one, ongoing MOF pore collapse or mesoscale sintering contribute a constriction effect which manifests in an apparent decrease of viscosity. Secondly, for viscosities approaching 10 9 Pa . s, the timescale of observation used in the penetration experiment starts to deviate from the timescale of viscous equilibration . Hence, flow is observed in a non-equilibrium state, what leads to the typical deviation between observed (apparent) and stationary viscosity. Finally, an indentation size and strain-rate effect may contribute similarly when approaching higher viscosity (shallow penetration depth). Linear extrapolation of the data range of 10 7 -10 9 Pa . s to Tg gives m (agZIF-62) ≈ 35, which is in very good agreement with previous evaluations of DSC data. We revisited the optical properties, which were initially published by Qiao et al. on agZIF-62 made with a hot-pressing technique. Transmittance spectra of agZIF-62 and crystalline ZIF-62 in our work were obtained via three different techniques: direct UV/vis spectrophotometry (Figure ), diffuse UV/vis/NIR transmittance and reflectance spectroscopy (Figure ) and photothermal deflection spectroscopy (PDS, Figure ). The direct optical transmittance is strongly dependent on the surface quality and the presence of internal scattering centers (such as pores or grain boundaries) or striae (for example, caused by fluctuations in chemical composition or residual stress). Other than previously reported , we find a few distinct shoulders in agZIF-62 slightly above the UV absorption edge (Figure ), which we attribute to discrete, as-of-yet unspecified energy levels of the hybrid glass. We assume that this finding is related to material purity and processing parameters; the present agZIF-62 did not undergo high temperature/high pressure densification upon melting. From the peak position and the sharp band at wavelength 290-300 nm we can exclude that this band comes from a ZnO cluster, as these normally absorb in the regions above 350 nm .