After applying a high voltage between the metal front grid (anode) and a number (cathode) applied neon gas becomes ionized at the cathode. This leads to the lights of the number with the characteristic orange .
Although the voltage is quite high, the current flowing amount is very small. Therefore, the clocks use little power and generate almost no heat. The tubes are not hot and just feel warm. It is a shine not a glow.
The most common tube has ten cathodes representing the numbers 0 through 9. Observe Nixie tubes and you see numbers made of metal sheet arranged behind each other.
Only the number with applied voltage lights. The remaining numbers remain barely invisible.
Because it is a shine and not a glow Nixie tubes do not wear out. Over the life time of Nixie tubes there is no reliable information. It should be a few decades.
Nixie tubes have no gaskets. The connection wires are fused into the glass and accordingly tight. The contained Neon can not escape.
Today built-in Nixie clocks tubes are found in old warehouses or removed from discarded equipment. Usually the built-in Nixie clocks tubes are approximately about 40 years old. Stocks of Nixie tubes are difficult to obtain and are expensive. These are rare.
Nixieclocks.ch installs right now unused tubes.