Making glass-to-metal seals for home­made vacuum tubes

Post by adrian_b » Mon, Jun 15, 2026, 4:59 PM UTC

CamperBob2 wrote: post #7

alister wrote: post #4

What was the large-scale commercial procedure for making electrodes that pass through the glass without letting air in? I assume that electronics manufacturers must have been making millions of such vacuum tubes in the past. Is the knowledge lost (or not practical for hobby use)?

The article pretty much tells you: "Copper's red oxide bonds very well to glass. In fact, the bond is stronger than the bulk glass: when it breaks, there's always a thin layer of glass left stuck to the metal. Along with its excellent electrical properties, it seems like an ideal electrode material." If you look at how vacuum tubes are constructed that's essentially what you see. Tubes are evacuated through a hole cr…

Post by CamperBob2 » Mon, Jun 15, 2026, 5:04 PM UTC

adrian_b wrote: post #11

CamperBob2 wrote: post #7

Earlier quoted context omitted.

The article pretty much tells you: "Copper's red oxide bonds very well to glass. In fact, the bond is stronger than the bulk glass: when it breaks, there's always a thin layer of glass left stuck to the metal. Along with its excellent electrical properties, it seems like an ideal electrode material." If you look at how vacuum tubes are constructed that's essentially what you see. Tubes are evacuated through a hole cr…

Nope, as also mentioned in TFA, copper has a too great thermal expansion coefficient in comparison with glass. If vacuum tubes had pins of copper, the glass-metal joining would have cracked very soon during normal usage cycles, and there would have been no vacuum left in the tube. Real vacuum tubes and gas tubes had pins made of kovar, which is a Fe-Ni-Co alloy with a TCE matched to a certain composition of borosilic…

Post by drum55 » Mon, Jun 15, 2026, 5:12 PM UTC

NoMoreNicksLeft wrote: post #14

SoftTalker wrote: post #5

Earlier quoted context omitted.

Vacuum tubes are still made today, so I'm sure the knowledge is not lost. I'm curious about the answer as well.

I was under the impression that they were only made in eastern Europe at this point, former bloc nations. Even then, the demand must be microscopic at this point.

Post by adrian_b » Mon, Jun 15, 2026, 5:12 PM UTC

SoftTalker wrote: post #10

adrian_b wrote: post #9

Earlier quoted context omitted.

As mentioned in TFA, the most important factor for successfully joining a metal and a glass is to match their thermal expansion coefficients. Most pure metals have a much greater thermal expansion than any glass, which will cause cracks. In the nineteenth century, the first successful joinings of metal with glass were done using platinum, but that is obviously too expensive for normal applications. Eventually a speci…

Ordinary incandescent bulbs must have similar sealing requirements, but they probably mostly rely on using a thin conductor that doesn't contract much when it cools. Also IIRC modern incandescent bulbs do not use a high vacuum but contain a low pressure inert gas so leakage would be slower if it occurs at all.

Post by ssl-3 » Mon, Jun 15, 2026, 5:30 PM UTC

SoftTalker wrote: post #10

adrian_b wrote: post #9

Earlier quoted context omitted.

As mentioned in TFA, the most important factor for successfully joining a metal and a glass is to match their thermal expansion coefficients. Most pure metals have a much greater thermal expansion than any glass, which will cause cracks. In the nineteenth century, the first successful joinings of metal with glass were done using platinum, but that is obviously too expensive for normal applications. Eventually a speci…

Ordinary incandescent bulbs must have similar sealing requirements, but they probably mostly rely on using a thin conductor that doesn't contract much when it cools. Also IIRC modern incandescent bulbs do not use a high vacuum but contain a low pressure inert gas so leakage would be slower if it occurs at all.