In vacuum or gas-filled tubes, it is a small, ring or cup-shaped device containing a powdered metal that reacts strongly to oxygen. When the tube is sealed, the getter is fired (heated) to further evacuate a vacuum tube or to remove impurities from the gas. Firing causes the getter material to oxidize and absorb any free oxygen in the tube, which would otherwise oxidize the active electrodes and decrease the tube's life. Tubes with graphite-based electrodes do not use getters, because the graphite itself absorbs oxygen.In a vacuum tube, the location of the getter is sometimes identified by a shiny silver deposit behind the glass, which is residue from the firing. The metal used in the getter depends on tube manufacturer, type and composition. Barium is an excellent oxygen absorber and is used in tubes with a fired getter. Zirconium and titanium, also used as getter material, oxidize from the tube's self-generated heat and do not require firing.