[Boatanchors] Mercury Vapor Rectifier Lore Needed

[Wilson]

I’ve been resurrecting some gear with 866s in it, and Wonder about some of the things I’m seeing.
I’ve used 866s all my life, but never thought much about them, except good or bad.

Some have much more glow than others, and the more glow the whiter.   Is this because of air getting in?  
Some have what looks like shards of metallic stuff on the glass.  Is this a coating off the plate?
Last night, I had one that glowed pretty white, but also had a neat ring of fire dancing around the anode, with fingers pointing outward.  What was going on there?
Sometimes one will are over inside and blow fuses, etc.  How can that happen, if the plate and filament are not out of position?  
I recently had a big flash from the back of the rack, accompanied by a colossal bang, as loud as a .22 LR shot!
I looked all over the PS for evidence of arcing, but didn’t find any.  Later, I took hold of the 866 to take it out and the whole bulb came off in my hand!
It broke perfectly cleanly, right around where it went into the base.  This has now happened twice and I’ve never seen it before.
Both these tubes were loose in their bases, so I shot in some superglue to hold them.
Could it be that the superglue bound them so tightly to the bases that they couldn’t expand and broke from stress concentration?
Or could it have been that the superglue caused a thermal stress concentration in the class.
Anyone ever see one break this way?
Wilson
W4BOH