The Ice Tube Clock, available as a kit, uses a lovely IV-18 vacuum-fluorescent display (VFD) tube.
Unlike Nixie tubes, which are current-operated gas-discharge displays, VFDs are thermionic voltage-controlled devices, just like the vacuum tubes used in old radios and amplifiers (as well as some modern high-voltage equipment). Specifically, VFDs are a special kind of directly-headed triode.
VFDs have three basic parts: the filament (cathode), the grid, and the phosphorescent anode(s). In the case of the IV-18 device used in the Ice Tube, the entire tube has two filament wires stretching the length of the tube (wired in parallel). Each separate digit has it’s own grid, and each of the 7 segments of the number display are ganged together is an anode. By quickly multiplexing the grids you can make the device appear to display all the digits at once.
In this photo you can see all three electrodes. Those two dark lines running horizontally across the photo are the filament wires.
Here you can see a couple of illuminated digits. The nebulous nature of the illumination across each segment is caused by variations in the electric field, and the grids casting an electron shadow onto the segments.