Abstract: An apparatus for vitrifying a soot preform from which an optical fiber is produced by drawing, is composed of a main vacuum chamber 20 in which a muffle tube 70 for vitrifying the soot preform PF is located, and an auxiliary vacuum chamber 30 mounted on the main vacuum chamber 20 and communicated therewith through a passage, and a gate valve 60 for opening and closing the passage. The muffle tube 70 has a inner surface covered with silicon carbide layer 70a.

Abstract: An oven for heat treating, curing, stress relieving or otherwise treating objects within the oven has side walls that can be moved into contact and out of contact with side edges of a movable floor. All interior surfaces of the oven are insulated and, when the floor is located in the oven, the side walls can be moved so that the insulation on the side walls forms a sealed relationship with the insulation along the adjacent side edges of the floor. A door of the oven can then be closed. When the door is opened, the side walls can be moved out of contact with the floor and the floor can be removed. Preferably, the side walls are moved by the operation of pneumatic cylinders.

Abstract: The oven includes a baking compartment and a lower compartment located beneath the baking compartment for receiving baked articles therefrom; a horizontal shelf separates the baking compartment from the lower compartment and is formed of a series of pivotable plates; means are provided for pivoting the plates at predetermined time to cause the baked articles in the baking compartment to fall under the action of gravity in the lower compartment.