A method comprises flowing an oxidant and a fuel into a submerged combustion burner in a glass tank furnace, the glass tank furnace receiving a feed of glass forming material and producing molten glass, the burner and furnace comprising a melting system. The melting system has a variable system vibration and/or oscillation due to the nature of submerged combustion. One method includes predicting a value of at least one property, such as viscosity, of the molten glass using the variable system vibration and/or oscillation.