show

A show is a pale blue flame that appears above a candle flame in the presence of firedamp, methane gas. Underground miners used this visual signal to detect dangerous concentrations of explosive gas before instruments existed. The show appears as a distinct coloration, usually described as pale blue or greenish, sitting above the ordinary yellow candle flame. It indicates that methane is being oxidized in the thermal column of the flame, producing a cooler burn than typical wax combustion.

This detection method worked because firedamp ignites at lower temperatures than candle wax. When methane mixed with hot air near the flame, it would combust slightly, creating the characteristic colored flame. Miners would hold candles aloft in suspicious areas and watch for the appearance of a show. The more intense the blue color, the higher the gas concentration, though this was a crude scale at best.

The practical limits of flame testing

The show had obvious dangers. A miner holding a naked flame into a pocket of firedamp risked explosion rather than safe warning. If gas concentration reached the explosive range (5 to 15 percent in air), the show could become a detonation. Miners also had no way to distinguish between a light show in 1 percent gas and a heavier show in 8 percent gas until something went wrong. The method provided only the crudest binary information: gas present or absent, with no quantification.

By the mid-19th century, mining engineered alternatives like the Davy lamp replaced simple flame detection. The Davy's wire gauze cooled combustion products enough that firedamp above the lamp would not ignite, yet a miner could still see the flame change color in the presence of gas. Later, mechanical and chemical detectors made the show obsolete. The term persists in mining history and older technical literature, but the practice vanished once better tools became available.