A method is described whereby &bgr;‐magnesia windows may be sealed into heavy steel tubes, thus making them suitable for vacuum furnaces. These seals, utilizing copper or aluminum bands around the edges of the windows, have been made and tested up to 587°C without leaking. A needle valve is designed which permits the opening or closing of an evacuation port in the side of a vacuum furnace at will. This valve operates at the temperature of the furnace and does not introduce a cold spot into the absorption chamber. It has been tested up to 587°C and holds well against pressure differences of one atmosphere. Being made of stainless steel, it is relatively inert and is free from oxidation scale.