Puzzler...

Most of you got the right answer to last week's puzzler: the orange and yellow flames in a wood fire are caused by particles (mainly carbon) heated to incandescance. The physics of burning wood is far more complex than you might think at first glance. The actual combustion that takes place happens between gases, including ordinarily solid materials that are heated to the point where they change state into a gas. If you see blue flames, that's actually a direct result of combustion – but much of the combustion takes place without emitting any light at all in the visible spectrum. Where things get really complicated is the conversion of the cold, solid wood into superheated gases ready to combust. The simplistic picture is a combination of radiant head and convective heat (from the gas combustion) heats the wood to the point where it decomposes to simpler compounds or elements, in gaseous form. The details are far more complex, and still not completely understood...

This week's puzzler is technological:

Up until the late 1960s, most electronic devices employed vacuum tube amplifiers as their fundamental component. These vacuum tubes were one of the first “high-tech” devices manufactured in very high volumes. There were several challenges in their manufacture, but one of the big ones was how to produce the extremely high-quality vacuum inside the tubes. How did they actually accomplish this?

As usual, no fair googling until after you've answered...