Does the EL material deteriorate?
I'm not 100% sure, but I have some sabers>5yr old and there does not appear to be any deterioration in either the wire or the film.
Jedi Council Member
The substance in EL wire that gives off the light is phosphorus. When it is surrounded by a high freqency AC current, it glows, or, phosphoresces.
Phosphorus is a bit toxic, so make sure you wash your hands thoroughly after scraping the coating off the main wire when connecting EL wire.
As for EL wire deteriorating, I know that pure phosphorus reacts with oxygen, though I'm sure what's in the EL wire is mixed with other chemicals, especially the white EL wire, so it couldn't hurt to seal the cut end with a bit of glue or clear nail polish. This is also another good reason to put hot glue on the wires after you've soldered them inside the audio connector.
EL wire should glow for a few thousand hours before it starts to dim, it won't glow at the same brightness indefinitely. But unless you're using it as a night light, it should last longer than most people will be able to wear it out in a lightsaber blade, especially of you have more than one blade.
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