The estimated activation energy for conduction during flash sintering is compatible with ionic diffusion in silicate melt. Moreover, evidence of magnesium diffusion toward the cathode is recorded. The estimated sample temperature is in almost all cases lower than 1355 °C, which is the temperature at which the first liquid is formed in the ternary system MgO-SiO2-Al2O3. Finally, it is shown that the application of an E-Field accounts for efficient liquid phase sintering at temperatures at which it cannot be reproduced conventionally.