Hollow glass tubes used to make neon lamps come in 4, 5 and 8 ft lengths. To shape the tubes, the glass is heated by lit gas and forced air. Several compositions of glass are used depending on the country and supplier. What is called 'Soft' glass has compositions including lead glass, soda-lime glass, and barium glass. "Hard" glass in the borosilicate family is also used. Depending on the glass composition, the working range of glass is from 1600' F to over 2200'F. The temperature of the air-gas flame depending on the fuel and ratio, is approximately 3000'F using propane gas.
The tubes are scored while cold with a file and then snapped apart while hot. Then the artisan creates the angle and curve combinations. When the tubing is finished, the tube most be processed. This process varies depending on country; the procedure is called "bombarding" in the US. The tube is partial evacuated of air. Next, it is short circuited with high voltage current until the tube reaches a temperature of 550 F. Then the tube is evacuated again until it reaches a vacuum of 10-3 torr. Argon or neon is back filled to a specific pressure depending on the diameter of the tube and sealed off. In the case of an argon-filled tube, additional steps are taken for the injection of mercury; typically, 10-40ul depending on tube length and climate it is to operate in.
Red is the color neon gas produces, neon gas glows with its characteristic red light even at atmospheric pressure. There are now more than 150 colors possible; almost every color other than red is produced using argon, mercury and phosphor. Neon tubes actually refer to all positive-column discharge lamps, regardless of the gas filling. The colors in order of discovery were blue, white, gold, red, and then different colors from phosphor-coated tubes. The mercury spectrum is rich in ultraviolet light which in turn excites a phosphor coating on the inside of the tube to glow. Phosphors are available in most any pastel colors.
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