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What actually happens inside a neon tube during bombarding?
As an expert witness, I often analyze failed sign installations and report them in the courthouse. Because most judges possess a logical, but not necessarily technical, understanding, it‘s not always easy to explain the actual source of trouble.
But judges aren’t the only ones who lack a technical background about neon-tube production. The staff that manufactures neon lamps often lacks this knowledge, and, because workers need to know what they‘re doing, I’ll address tube-processing “FAQs.”
Background
To be honest, conditions vary so widely during the neon-tube bombarding process that we can’t definitively research or understand everything that occurs inside the tube during bombarding. This is also why neon-tube processing can’t be completely automated.
However, we can identify the primary physical and chemical interactions in the bombarding process. Bombarding serves several functions. Heating the glass vaporizes contaminants on the inside of the glass tube so they can be pumped away during evacuation. The electrodes’ metal shells are also heated to burn off impurities and activate the chemical emission coating on the shell’s interior.
Vacuum processing should completely remove “vacuum debris” (materials that outgas or decompose in the operating lamp’s plasma environment, inner surface and volume), which can impair lamp operation. The tubulation is the tube’s only exit point, but the vacuum pump can remove only gases, not liquids or solids. Thus, the bombarding process must release absorbed gases, evaporate liquids into gases and convert solid “vacuum contaminants” by chemical/thermal reaction into gaseous compounds the vacuum pump can expel from the tube. The bombarding process’ second goal is to properly turn the “emission coatings” into their active form so the electrodes can operate as designed.
The initial strike
Why is it so difficult to strike the discharge in a neon tube the first time? Moisture. When the lamp is bent, and the glass tube is still open after having been worked in an open fire, water vapor enters the tube and condenses on the powder layer in the glass’s cold parts. Also, by blowing into the tube, the bender introduces additional moisture (spit).