I'm thinking about buying some IN-3 nixie tubes for projects I have in mind.
Based on pictures available, some of them look like normal, clear glass tubes, some look like the glass is dark brown.
Is that some kind of coating that can be removed (if yes, how?), or is the glass just dark?
Is that some kind of coating that can be removed
Every coating can be removed with enough money put into the effort. Is it practical? Not at all.
The tubes you show are just worn out. If you get a brand new neon tube without the metal deposit on the inside of the glass, and drive it with high current for 5-10 minutes, you'll get the same coating, just perhaps less of it. Driving these tubes really hot - so that the glass is at say 200-300C - will cause this metal to deposit even faster.
Why would someone be selling them like it? Well, if there are buyers, there are usually sellers.
The picture shows brand new lamp. They look like this out of the box.
When I'm selling a used smartphone on eBay, I usually put it back into its original box. It doesn't mean it's brand new. But it will sell for a better price even though it's listed truthfully as used and so on.
Same here. Used neon bulbs are for sale in a nice box where the replacements probably originally came from. With a пасспорт, bring nostalgia and retro vibes. The box ended up on the secondary market and now there will be potential buyers (ahem) who think that's how fresh neon bulbs look... and may even try to argue that point.
The sellers are laughing all the way to the bank probably?
it's a coating on the inside, as far as I can see on your lamp there (it's not a Nixie tube, these have multiple cathodes, not at most one).
And it does look like these are lamps with a getter coating, i.e. some metallic film that is designed to absorb any non-inert gases that leak into the tube. But: the area seems too large to be proper, as they block the important part of the lamp. Or, this is a neon lamp after the end of its life, when the AC applied to its electrodes has finally managed to sputter the inner surface of the glass with electrode metal.
So, this might be a production defect, or pulled material from a large panel. I'd bet on the latter, because these kinds of things became obsolete all over the world when LEDs became available for control panel indicators; and that would very much fit my quality expectation for ebay neon lamps!
UPDATE: after seeing the ebay offer this is most probably from, yes, these are end-of-life indicators pulled from an old soviet control panel or something. See how variable their degree of metal surface contamination is!
And the picture shows brand new lamp.
That seems extremely unlikely, seeing the decades-old, dirty cardboard box they came from.
