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Thermodynamic equilibrium modeling indicates that the introduction of H2O in oxidizing environments decreases Si stability due to formation of volatile hydroxide and oxy hydroxides. 3Al2O3·2SiO2 mullite bond offers only slight improvement over pure silica as the thermodynamic activity of silica in mullite is near unity. In reducing atmospheres Si stability is improved by the presence of H2O and Al2O3, transitioning from SiO and silane as the dominant volatile species to hydroxides, oxy hydroxides, and SiO with increasing water vapor partial pressure. Empirical studies reveal initial rapid releases of Si from stationary solid oxide fuel cell refractory materials followed by slower solid-state diffusion limited release.