Info taken from this page on Lamptech from @James of course. Here we go:

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1) Seriousness:
James writes "if measures are not taken to keep the sodium away from the seals, they will be attacked resulting in premature lamp failure". Alright, makes sense. But what kind of failure are we talking here? Will it just react and turn the glass black (I am assuming not), or will it crack and destroy the seal and make it leak? What actually happens when sodium attacks a seal?

2) Seal Design:
The seals are designed to remain hot while the lamp cools down so sodium doesn't condense on the seal. This was originally done (with some exceptions on question 3) with a magnesia bead fused to the end of the lead wires that were sheathed in inverted 2-ply glass tubing. That makes sense. But it also says that Philips later switched to a design that sheathed the wires in pure borate glass instead of 2-ply glass. Why wouldn't they do that from the start? That just seems like the simpler solution that is easy to think of. Also, is there any reason magnesia was chosen as opposed to alumina, or it it just convenience?

3) Glass types:
With the early SO dewar jacket lamps pictured on Lamptech, the ones made of soft glass had magnesia thermal mass beads and the ones using hard glass did not. Why does the type of glass used change whether or not it needs a thermal mass bead?

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Thanks!

I also know that a Kovar alloy was used for most hard glass SOX discharge tubes, were they ever made with the harder ~3.3e-6 borosilicate and tungsten?

1) The general failure mechanism is glass cracking.

2) Borate glass is known as a so-called “short glass”, i.e. its working temperature range is very limited.  When heating by flame, it transforms from solid to a low viscosity liquid very quickly, kind of like how ice suddenly melts into liquid water.  I understand this is due to reduced ionic bonding presence in the glass when hot.  It is predominately held together by Van der Waal’s forces when hot, which are weak, so it has low viscosity.  This makes is extremely difficult to work by traditional processes for making glass tubing, on Danner or Vello machines.  In the melting pot it is fully liquid, and when flowing onto the tube forming mandrel it is either so liquid that it drips to the ground, or so solid that it can no longer be drawn off the mandrel.

Philips developed a new technique of extruding borate glass rods in the 1960s.  After about 20 years of intensive efforts, they finally managed to extrude borate glass tubes.  From that moment onwards, a pure borate sleeving became feasible for SOX lamps, and in the late 1980s the so-called SR (sodium-resistant) seal was developed.

Magnesia is used rather than alumina due to its lower sintering temperature and hence cost.  In the later 1990s I did develop alumina beads for SOX lamps at GE, based on using scrap alumina powder that wasn’t quite good enough for our Stellox arc tube production.  It worked rather well.  But to scale up for production would have required expensive tooling.  The financial saving was too small, so never proceeded and we remaimed with magnesia beads.

3) For the hard glass sodium arc tubes, we BTH-Mazda developed the glass type C42.  It had excellent sodium resistance and also a pretty good working temperature range.  It was possible to draw into single-ply tubing.  So the seals were also sodium resistant, and no need for a ceramic insulator tube.

@James
Right cracking is definitely not a good thing, makes sense that you would want to avoid that lol.

Wow, I definitely am familiar with the fact that borate glass is hard to work with, but I had no idea it was so hard that it was impossible to draw into tubing for a while. That stuff must really suck.

So from what I am hearing it doesn't really matter what ceramic you use, as long as it has good thermal mass. Good to know.

I had no idea that the hard glass arc tubes were single ply! that must make them so much easier to make, though I suppose more expensive. Is C42 glass just a higher borate content version of plain borosilicate?

Thanks for your detailed response!

I am pretty sure the hardglass arc tubes were still 2-ply, for cost reasons.  I think C42 together with plain ordinary C9 borosilicate (modern name B37) or a slight variant.  I have all the old BTH-Mazda sodium lamp manufacturing specs, will look it up later.  Also the raw glass chemical compositions and properties.  It was of course higher borate content than regular borosilicate, but mainly because the silica was substantially removed.

The borate glasses were always extremely expensive.  Not so much for their materials, but because I remember the entire sidewalls of the furnace tanks were made from solid platinum, rather than the usual cheap ceramic walls.  Of course these days that would be monstrously expensive, but years ago it was just about tolerable.

@James
My bad, I misunderstood. That makes sense. I wonder why borate glass specifically had to use platinum furnaces, everything about this just seems like suck a pain in the butt for the manufacturers, I suppose that is why they weren't "perfected" for a long time.