Flux treatments with a molten glass slag are commonly used to promote deep liquid undercooling or even bulk glass formation. Although the technique has demonstrated frequently the capability to yield large undercooling, the underlying mechanisms remain uncertain. There are suggestions of flux-induced "nucleant removal" or "surface site deactivation", but the effectiveness and operations of these processes have not been identified clearly. As a model system, the undercooling response of pure gold encased in Pyrex glass was studied systematically to develop a consistent record of the undercooling behavior. Nucleation kinetics analyses of statistically significant sets of measurements performed under strictly controlled conditions reveal the presence of a new mechanism based on gas-solid interactions that trigger nucleation, through a nucleant precipitation as the first step of the interaction. A new model based on thermodynamic considerations and on the nucleation kinetics data are proposed, which accounts for the undercooling increase including initial conditioning, atmosphere effects and undercooling saturation in a self-consistent manner.
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