Coal is created from the remains of dead plants that have formed layers of peat. Over millions of years, through high pressure and high temperature of deposited clay and sand, these layers of peat are compressed into lignite or coal. During this process, called carbonization, mine gas (methane gas or CH4) is created.
The mine gas is trapped in rock layers can be partly released during the extraction of lignite and coal. This is particularly dangerous in coal mining because it is mined underground (unlike lignite, a brown coal, which is mainly dug out from the surface). Mine gas is odorless, colorless and tasteless, making it difficult for human senses to detect. Before there were technical solutions for measuring mine gas, miners took canaries underground to detect the mine gas. The canaries are more sensitive to poisonous gases than humans. The bird died even at a minimal concentration of mine gas. Due to the canary falling off the pole, the miners were warned and were able to flee in time. Mine gas is lighter than air and therefore rises upwards, so the miners hung the canary's cage from the mine tunnel ceiling where they were working.
Petrol lamps were utilized in the Limburg mines at the beginning of the 20th century and were consequently also used as a measuring instrument. If a miner used the petrol lamp to measure mine gas, he turned the flame of the lamp down to a minimum until only a blue flame remained. He then held the petrol lamp to the ceiling of the mine tunnel to investigate. If mine gas was present, it elicited a reaction from the flame, the flame grew larger. At 1% mine gas, the flame was 1cm high, at 2% 2cm high (see image below).
Every petrol lamp could be used to measure mine gas. The measuring was only done by instructed miners, such as drillers / shooters, the ventilation service and supervisors.
Mine gas when mixed with air it is flammable from 0 up to 4.5%. Mine gas when mixed with air it is explosive from 4,5 up to 14%. Mine gas above 14% is no longer flammable or explosive due to lack of oxygen.