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

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




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

2026-06-13 (Electronics)

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

When making vacuum tubes, the glass is actually the easy part:
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,
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,
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,
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:
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.
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 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

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

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

Once it's down to room temperature, the metal is around 1% smaller than the glass around it.
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,
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 μm/[m*K]), it's an improvement over copper.
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.
I had no luck plating the steel out of a copper sulfate solution:
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.
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:
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 μm/[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.
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.
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.
Filament wire is quite common, and I happen to have some.

The snag is that it's 10 μm thick.

I'd say it's hair thin, but that would be an understatement by almost an order of magnitude
(most of my hair is around 70 μm)

That's a standard 2.45 mm header.

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

Like many metals, tungsten is flammable.
At this size, my oxy-propane torch is able to burn through it in under a second.
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...
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 nicely:

Neon-free neon sign.

In addition to the plasma, the leads are glowing white hot.
They don't have any air to cool them, are very thin and have poor thermal conductivity.
Tungsten is one of the few metals that can handle this, so I accidentally got a 2-in-1 lamp.

While it is an option, but I'd really rather avoid using this.

Thermal expansion is a factor of size, so the smaller the conductor the less of of a problem it will be.
10 μm wire is rare, but 10 μm foil is common: you probably have some in your kitchen.

I rolled out some wire into some thin (30 μm-ish) foil and tried sealing it glass:

The seal looks excellent, but it leaked horribly.

This technique supposedly works in soda-lime glass, where the CTE difference is smaller and the softening temperature is lower,
but it's no good for borosilicate.

(... interestingly,
the crack formed around the edge of the ribbon, not along the surface.
I'll come back to this later.)

One of the weirder glass-to-metal seals is the houskeeper seal:
attaching an thin walled copper tube inside a glass tube:

A tube seal used on a high-voltage capacitor

The hollow metal can easily stretch to release any stress from thermal expansion.
However, manufacturing such a tube is difficult without a precision lathe.

A thin copper disk sealed to the end of a tube should also work because it's thickness is unconstrained:
the disk can increase it's radius by stretching thinner.

Both are rotationally symmetrical

For a long wire sealed inside a pinch, the metal's only options are to decrease it's density (very hard)
or for it to pull more in from outside the glass (also very hard)...
so stress builds up until the seal breaks.

Producing such foil is easy with a small rolling mill although a hammer would also work.
It's important to periodically heat the copper to a red heat for a few seconds.
This reforms the metal crystals and allows it be worked without cracking.

Looks ugly, but it's vacuum tight!

The copper should be cut to size after bonding it to the glass.
If the glass reaches the end of the foil, it will crack around that point.

Once sealed, a hole can be punched in the foil, and a wire soldered through it.
Because there's no limit on the size of that wire, such a feed-through could be made to handle thousands of amps.

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