Abstract: A process for manufacturing an optical element by hot-forming a glass sheet (2), comprising the steps of: positioning a mold (4) in a cavity (9) of a base element (7) of a muffle (6); positioning the sheet (2) on the mold (4) with a peripheral edge superimposed on a closure area (14, 15) of the base element (7) surrounding the cavity (9); closing a cover (8) of the muffle (6) onto the base element (7), so as to maintain the peripheral edge of the glass sheet (2) in contact with the closure area (14, 15) of the base element (7); and performing a controlled heating and cooling thermal cycle, by applying a pressure difference between the base element (7) and the cover (8) so as to press the glass sheet (2) onto the mold (4) after reaching the maximum temperature.