So I have a 20W T12 tube (Philips, made in Holland, 90's vintage, cathode guards) in semi regular service that's exhibiting some odd behaviour and wonder if those who know can shed some light.

So basically from almost new I ran the tube on an 18W electronic switch dim dimming ballast by Tridonic. In Europe/240V you can generally run T12 and T8 tubes of the same length on the same gear with no ill consequences. For several months the tube ran mostly non stop and accumulated, at a guess, 1500 hours, gaining a bright red off-cathode band typical of constant running at one end. I started and stopped it a few times with no perceptible ill effect at all.

One day I started it up from cold and while the tube started normally some black marks appeared at the ends of the tube, mostly at the end where the red off cathode band had formed. I didn't think much of it, particularly as the marks seemed to fade when run for a while, until it did a second time some weeks later. I concluded that perhaps I shouldn't be running it on this electronic control gear after all, as its programmed start may rely too heavily on "harder to start" krypton fill tubes and the now semi-ripened Philips was perhaps hard starting which was causing the black marks.

So I moved it to a traditional 18/20W magnetic ballast circuit using an electronic starter and the tube starts and runs as you'd expect it to. However, every time it starts, a black mark appears at the end which has the red band, always in the same place (right near the end cap). For about 5 minutes there is perceptible flicker coming off the tube if I look at it and the wall nearby (my eyes are naturally sensitive to this anyway) but after 5-10 minutes the flicker (which is not erratic but seems to be line frequency) mostly goes away and the black mark, while still slightly visible, has mostly worn off again almost like it's a mercury deposit. The tube otherwise runs fine, albeit with perhaps a tiny bit more flicker than I'd like, but it definitely doesn't seem to be rectifying. The arc seems perfectly stable.

What is going on here? Is this emissive sputter and if so what's causing it to happen every time I start the tube, and why does it then disappear again? if I had to guess I'd say this is what it is as it only happens at the end that has the red band, so the cathode has already started to deplete emitter. Could the tube be worn after only 1500 or so hours and does the prominent red band mean there is already significant emitter loss at this side of the tube?

Run this lamp for several hours continuously to get the mercury pressure reaching equilibrium, than you won't get this problem again.
This phenomenon of blackening at the ends of the lamp when turning on, is caused by mercury that evaporating from the electrodes. It is more common in T5 and CFLs.

Run this lamp for several hours continuously to get the mercury pressure reaching equilibrium, than you won't get this problem again.
This phenomenon of blackening at the ends of the lamp when turning on, is caused by mercury that evaporating from the electrodes. It is more common in T5 and CFLs.

Oh I've already done that. This isn't an issue that seems to be fixed by running the lamp for a long time. if I start it from cold again it always does the same thing. However, it doesn't seem to be getting any worse.

I just don't know whether the blackening is caused by emissive material or mercury. Does anyone know for sure if emissive material, once sputtered off the cathodes, ever "burns off" again or does it permanently stay? That would answer the question, because it mostly burns off the glass although it doesn't completely go, there is still some marking there. Given that the tube already has a prominent red band it seems seasoned already. Perhaps it is mercury, and some has condensed on the cathode guards causing this. However, I have also seen the same on a lamp without cathode guards, which was one reason I concluded the control gear may be bad for T12s.

If the band goes away, mainly when it repeats, it is definitely not the emission coating but really the mercury.
However tha does not mean anything about the state of the key components...

Does anyone know for sure if emissive material, once sputtered off the cathodes, ever "burns off" again or does it permanently stay? That would answer the question, because it mostly burns off the glass although it doesn't completely go, there is still some marking there.

The sputtering/blackening doesn't burn off, it's there forever.  This can be caused by normal wear, or the Cathode has a particular weak spot in it that get's a lot of attention and thus extra sputtering. 

I might as well just keep running it. I have a couple of these, the other one is NOS. The arc seems stable but my eyes are very perceptible to flicker and it is slightly more noticeable than the NOS lamp, which is what made me think it might be in trouble. Having said that, the flicker is no worse than some unshielded lamps I have, or 2D lamps which can be notorious for it. I almost prefer fluoro lamps on electronic gear for this reason, I love the tech but my eyes are so sensitive to the flicker, even from HID lamps too.