The machine contains tens of small blow torches, which slowly heat the rotating tubes and eventually melt the glass to form a uniform seam. The excess lengths of the outer tubes drop off when the melting point is achieved. After a slow cool-down period, each tube is carefully inspected under a microscope for cracks or faults in glass seams. The tube is now very close to its final form, with only an extra glass pipe for evacuation protruding between the pins. The vacuum pumping station is a monster of a machine, where the evacuation takes place in several successive stages.
Evacuation takes quite a long time, sometimes several hours for large tubes.
Two or three dozen tubes are processed at the same time by the pumping machine. At the end of the pumping, the small glass pipe connecting the tube to the evacuation pumps is melted with a flame. The external air pressure automatically pinches the tube shut, sealing the entire tube. Quality Control Even in the most effective evacuation, gases may still be liberated from the internal parts while the tube is in service. To safeguard against this, all tubes contain getter material, such as barium, capable of absorbing gas.