Spencer Woodburn uses two tube types in his clocks: Vacuum Flourescent Display (VFD) and Nixie.
The VFDs display numbers in an aqua color and are typically made in Russia as more of their equipment still uses this older technology.
The nixies display numbers in an orangish-hue and have metal screens incorporated into their tubing with one screen being dedicated to each digit. Nixie tubes are typically New Old Stock (NOS) that are now being re-purposed for Spencer’s clocks as well as for other mechanical items. These tubes initially were used in displays relating to yesteryears’s main frame computers, for instance.
Spencer has found that Nixie tubes can last for over 15 years. VFD tubes last for a shorter period, maybe 10 or 12 years based on his experience. You are more likely to drop or package your clock for shipment and have its tubes break rather than burn-out. When in use, these tubes do not generate significant heat levels.