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This study proposes a new strategy for achieving rapid volumetric heating of moldable low-dielectric-loss glass over a wide temperature range using only microwave sources. As a proof of concept, both multi-physics modeling and experimental demonstration of microwave heating (MWH) of Pyrex 7740 glass are performed. The results reveal that there is an activation threshold of 400 °C for this glass, i.e., below this threshold the glass is mainly heated by SiC molds through interfacial heat transfer, while above it the glass can be directly heated by 2.45-GHz MWs through volumetric dielectric loss. As a result, the glass preform can attain an internal temperature of 727 °C (well above its transition point) within a short MWH process (130 s), at a low level of specific energy consumption (6.43 kW h/kg). The average volumetric heating rate of the glass is estimated as 5.44 °C/s, which outperforms the reported rates (2.00−3.60 °C/s) in previous precision glass molding (PGM) studies. Furthermore, the temperature difference in the entire glass can be minimized through MWH combined with post-annealing. With the demonstrated capabilities of instantaneous, volumetric, and selective heating, the proposed MWH strategy holds great promise in PGM and many other glass thermoforming sectors.