Making glass-to-metal seals for home­made vacuum tubes. (Maurycy's blog)

Making glass-to-metal seals for home­made vacuum tubes.

2026-06-13 (Electronics)

This page discusses sealing metal through borosilicate/lab glass:<br>other chemistries behave quite differently.

When making vacuum tubes, the glass is actually the easy part:<br>premade tube stock of almost any size is easily available.

Heating the end of such a tube softens the glass and allows surface tension to close it off.

I used a rotary vane pump to remove all the air from the tube and heated the middle,<br>which the atmosphere crushed to create a sealed-off ampule.

Because glass is practically impermeable, it will retain that vacuum for a very long time,<br>which can be shown by bringing it close to high-voltage AC (like a tesla coil):

This glow is due to residual air being ionized,<br>but the fuzzy appearance indicates that the vacuum is good enough to work in a triode or similar device.

For those, the capacitive coupling trick won't work:<br>I'll need to make electrodes that pass through the glass without letting air in.

This is a lot harder than it might appear.

Copper's red oxide bonds very well to glass.<br>In fact, the bond is stronger than the bulk glass:<br>when it breaks, there's always a thin layer of glass left stuck to the metal.<br>Along with it's excellent electrical properties, it's seems like an ideal electrode material.

I tried sealing off the end of the tube like before, but this time with a .75mm wire inside:

The red color indicates a good contact

... and it leaks.

Look under a microscope, the glass around the joint cracked as it cooled.

The culprit is thermal expansion:<br>After the glass solidifies at below around 800 °C, it contracts by around 3 &mu;m/m for each degree.<br>During that same degree of cooling, the copper contracts by 17 &mu;m/m.

Once it's down to room temperature, the metal is around 1% smaller than the glass around it.<br>Since both the metal and glass are incompressible, the resulting stress builds up until something breaks.

There are some metals that are well matched to borosilicate glass,<br>like tungsten (4.5) or molybdenum (5), but they are all rather exotic.

Steel wire is common, and while it's not really matched (CTE is around 11 &mu;m/[m*K]), it's an improvement over copper.<br>However, the carbon content of the metal produces carbon monoxide on contact with hot glass:

... but there's no reason the bulk metal has to be in contact with the glass.<br>I had no luck plating the steel out of a copper sulfate solution:<br>because the reaction is spontaneous, it always happens very fast and creates a fine metal power:

Fe (s) + CuSO4 (Aq) → Cu (s) + FeSO4 (Aq)

However, electroplating copper works fine in the presence of ammonia.<br>The copper can dissolve as a tetra-amine complex, but the iron is completely insoluble under these conditions.

To create a plating, the copper has to be forced with electricity:<br>I connected the negative lead of my power supply to the iron and the positive to a piece of sacrificial copper.

At 20 mA, this produced a nice coating in a few seconds:

The wire should be sanded clean before plating

Sealing this in glass created a bubble free seal (if it was done quickly), but it still failed during cooling:

This photo was taken through two layers of glass

Steel differs by ~7 &mu;m/[m*K], and that's enough to break the glass.

However, this plated wire can work in soda lime glass, which has a CTE of around 10.<br>This is the most common (and cheapest) type of glass, but I haven't been using it because of it's tendency to crack while cooling:

Large pieces need to annealed in a furnace over several hours.

... but I did adding a bead around the wire:

Instead of the wire breaking away from the glass, the two glass types broke apart.<br>This actually made the problem worse because the bead is a lot bigger than the wire, so it expands and contracts more.

Ok, I lied about tungsten wire being exotic.<br>Filament wire is quite common, and I happen to have some.

The snag is that it's 10 &mu;m thick.

I'd say it's hair thin, but that would be an understatement by almost an order of magnitude<br>(most of my hair is around 70 &mu;m)

That's a standard 2.45 mm header.

For the seal, this is a good thing: less metal means less expansion...<br>but this size is nearly impossible to handle.<br>I kept loosing bits of it until I started attaching bright-orange tape to the ends.

Like many metals, tungsten is flammable.<br>At this size, my oxy-propane torch is able to burn through it in under a second.<br>This made glassworking a rather frustrating experience.

I initially attempted to make something similar to a neon indicator by passing two wires through a single pinch...<br>but invisible wire leads to invisible short circuits.

Sealing a single wire in each end worked fine:

... but I had to add glass tee-joint to attach the vacuum.

While the operating voltage is well above a thousand volts for a tube this size (filled with air), it does glow...