Complex sentence: The furnace includes a mechanism that rotates at constant speed around a vertical axis while glass, held in an open-topped container rotated by the mechanism, is heated until it is completely molten, and then allowed to cool until it has completely solidified.
Split into simpler sentences: The furnace includes a mechanism that rotates an open-topped container at constant speed around a vertical axis. Glass is held in the container and is heated until it is completely molten, and then allowed to cool while rotating until it has completely solidified.