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So, what happens after we weld a tube onto the manifold, hook up the bombarding wires then open the vacuum stopcock? Well, vacuum develops within the tube. That’s obvious. But…

What is vacuum?
Vacuum is a relative term that defines a void. Vacuum means less pressure. In this case, there are fewer gaseous particles in the tube after vacuuming than before. We measure this vacuum relative to normal atmospheric pressure, which supports a column of mercury 760 mm or nearly 30 inches high. And we all know how heavy the liquid metal mercury is, right? But why create a vacuum? Why reduce the number of gaseous particles?

The answer: To lessen electrical resistance. Otherwise, electrical bombarding would require far greater voltage to move electric current through the tube. We would need more powerful bombarding equipment and, no doubt, additional safety measures. We would also need a stronger glass tube that could withstand higher stresses.

At around 2 mm or 3 mm of pressure (vacuum), it becomes easy to light a tube; force electricity through it. But there is another reason to begin bombarding the tube at this pressure. At this pressure there are enough particles remaining inside the tube to impart sufficient momentum upon others. This causes heat. But why create heat?

Tube fever
We elevate a tube’s temperature to increase turbulence. We use fast moving, molecule-sized particles to drive out stubborn, trapped impurities. These impurities often consist of water molecules and other gases snared within the glass wall during tube manufacturing. But impurities also are introduced during glass working: particles like specks of cork, bacteria, and more.

Electrical bombarding at this pressure generates heat and lots of it. During this ever more turbulent state, pressure inside the tube rises. It rises due to increased temperature and because vaporized impurities add to the total number of particles.

Remember why, as the first step in tube processing, we reduce pressure in a tube? We do this to lower electrical resistance. As the pressure raises so does electrical resistance. So, this is when the tube usually begins to flicker. And from experience we already know if we don’t reduce pressure inside the tube by opening the vacuum stopcock, the tube will go out. The electrical resistance grows too high to overcome.

Yet, even before the tube goes out, electricity is seeking an alternate, easier path to flow. Hopefully, it finds this route via the Jacob’s Ladder or other safe pathway we’ve installed on our equipment.

But let’s say we kept the tube lit and its exterior has reached proper processing temperature, about 450 degrees Fahrenheit. The interior of the tube is far hotter, enough to vaporize most all organic materials.

Now one might think this is the best time to stop bombarding and evacuate these high energy, impure particles. But it isn’t. Why? Because there is some chemistry to finish.

Electrode chemistry
Electrodes are electrical current carrying devices used to transfer electricity from the transformer to gases inside the tube. That’s the simple definition. But electrodes are much more than this.

Electrode shells are a blend of metals designed to survive the hostile environment created during tube processing. Most electrode shells are coated with a chemical blend; an emissive coating that emits electrons during bombardment, a type of shield.

These free electrons protect the electrode shell by neutralizing heavy particles called CATIONS (cat~ions). A cation is an atom missing one or more electrons. During electrical bombarding, they number probably in the billions. They are drawn to the shell by strong electromagnetic forces, sufficient force to knock away the metal atoms that make up the shell.

The destruction of the shell by cation pulverization is called sputtering. See Figure 1 for an over-simplified graphical description of how sputtering during tube processing is kept in check by the emission of electrons.

Conversion is accomplished by briefly raising the electrode shell temperature to about 1600 degrees Fahrenheit. Increasing both electric current and vacuum raises the shell temperature to this level. Once the electrodes are converted and heated their entire length a cherry red color for several seconds, the current is stopped while the vacuum pump goes about its job removing hot, vaporous impurities.

It is important that the tube ultimately be evacuated to a pressure (vacuum) of about 1 to 5 microns. This assures the tube is clean. A clean environment contributes to a long-life tube, one that requires less voltage to light, and discourages bad behaviors like color changes or mercury stains too.

Fill it up please
Most gas fill charts are developed for room temperature gassing. So, it is important to let the tube cool to near room temperature before filling. If we fill the tube too warm, the gas expands giving a false impression of final pressure because when the tube cools, the pressure lessens. And too low of pressure contributes to shorter tube life and increased electrical resistance; the tube requires a larger transformer to operate.

Once the tube is properly processed and gassed then comes the tip-off. We heat a small section of the tubulation tube connecting the tube to the manifold. Once molten, the tubulation tube shrinks until it closes and seals. But why does it shrink then seal?

Because the molten tube is under vacuum pulling it closed. Actually, a more realistic view is the molten glass collapses because atmospheric pressure, the weight of the air surrounding the tube, presses it closed. Once pressed together, the tube remains sealed because glass molecules have an affinity for one another, a molecular attraction that binds them together.

During the final phase of tube processing, burn-in, the tube electro-chemically stabilizes. Any residual impurities are absorbed by the emissive coating’s chemical “getter” action. It is this final cleaning that assures the tube’s proper color and stable operation.

I’ve always been entertained by the science behind processes and found the knowledge useful. I hope this basic introduction into the science of tube processing proved entertaining and useful to you.