The getter

We want a good, hard vacuum inside a tube, or it will not work properly. And we want that vacuum to last as long as possible. Sometimes, very small leaks can appear in a tube envelope (often around the electrical connections in the bottom). Or, the tube may not have been fully "degassed" on the vacuum pump at the factory, so there may be some stray air inside. The "getter" is designed to remove some stray gas.
The getter in most glass tubes is a small cup or holder, containing a bit of a metal that reacts with oxygen strongly and absorbs it. (In most modern glass tubes, the getter metal is barium, which oxidizes VERY easily when it is pure.) When the tube is pumped out and sealed, the last step in processing is to "fire" the getter, producing a "getter flash" inside the tube envelope. That is the silvery patch you see on the inside of a glass tube. It is a guarantee that the tube has good vacuum. If the seal on the tube fails, the getter flash will turn white (because it turns into barium oxide).
There have been rumors that dark spots on getters indicate a tube which is used. This is NOT TRUE. Sometimes, the getter flash is not perfectly uniform, and a discolored or clear spot can occur. The tube is still good and will give full lifetime. THE ONLY RELIABLE WAY TO DETERMINE THE HEALTH OF A TUBE IS TO TEST IT ELECTRICALLY.
Glass power tubes often do not have flashed getters. Instead, they use a metal getter device, usually coated with zirconium or titanium which has been purified to allow oxidation. These getters work best when the tube is very hot, which is how such tubes are designed to be used. The Svetlana 812A and SV811 use such getters.
The most powerful glass tubes have graphite plates. Graphite is heat-resistant (in fact, it can operate with a dull red glow for a long time without failing). Graphite is not prone to secondary emission, as noted above. And, the hot graphite plate will tend to react with, and absorb, any free oxygen in the tube. The Svetlana SV572 series and 572B use graphite plates coated with purified titanium, a combination which gives excellent gettering action. A graphite plate is much more expensive to make than a metal plate of the same size, so it is only used when maximum power capability is needed. Large ceramic tubes use zirconium getters. Since you can't see a "flash" with such tubes, the state of the tube's vacuum has to be determined by electrical means (sometimes by metering the grid current).

By Eric Barbour
Information from www.vacuumtubes.net