Several times I have (just for fun) been able to put glass beads on copper wire and achieve seals with the proper color associated with a copper-to-glass bond. But, I hear that copper wire isn't that good for use with soda-lime glass because of the mismatched COE. I have the following questions:

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1) Failed Seal Appearance:
Since copper's COE is higher than that of soda-lime glass, the copper wire inside the glass should be trying to shrink away from the glass surface in a completed seal. Obviously thin annealed copper wire is able to just deform and accommodate that, but with thicker wires I hear this isn't the case. Would the red color disappear, or would there be some other indication of a failed seal?

2) Making More Reliable Seals:
The copper wire I use is thin, but I think I can do better. What if I smash the middle of the wire really thin (like molybdenum foil seals in quartz) and then make the seal? Would that be better / more reliable?

3) Pre-Beaded vs Pinched:
Most mass-produced pinch seals are made by just pinching the glass tubing over the Dumet wire. But, I feel like making the glass-to-metal bond in a more controlled and finely-tuneable setting would be better for at-home glassworking. Is there any reason not to put a bead on the wire first and then pinch that beaded wire into the glass stem for a completed seal?

4) Nickel-Iron Alloys For Borosilicate:
 • There is Dumet for soft glass, Kovar for "hard glass" (COE 5), but what is the name of the alloy used for borosilicate? I doubt they just use tungsten, there has to be an alloy out there somewhere.

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Thanks so much!

Does anyone have a lamp that has a borosilicate envelope that they can look at the pinch seal on? If it isn’t yellow then we can confirm that they don’t use tungsten wire.

I'm very interested in vacuum tube audio when I'm not messing with lamps, and I've seen vacuum tubes being made live. I was told that all pass-through wires must be made from tungsten or else the seal will fail over time. I'm pretty sure it has something to do with the different expansion rates of materials vs the glass pinch itself. I'm sure using copper would work for a short while, but would probably fail quite soon. smashing the lead wire flat doesn't really make any difference, the molten glass flows all around it anyways, or at least it should. You need to have really good and even temperature to get a good seal I may have a lamp somewhere that we can look at, I'll have a look!!

You can find out more information at https://en.wikipedia.org/wiki/Glass-to-metal_seal