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All gases that can be found in a mine, other than breathable air, were historically referred to as damps. This term is thought to have originated from the word dampf, which is German for vapor. Firedamp in particular refers to any number of different flammable gases that can be encountered in the mining process. These firedamp gases can gather in deposits, such as coal seams or other underlying rock formations, so any amount of open flame in an active coal mine can potentially lead to a fire or a destructive explosion. Specialized lamps that utilized very fine mesh screens were used to reduce the chance of igniting firedamp prior to the availability of safe electric light.
Any mixture of explosive gases found in a mine can make up firedamp, though methane is the most common component. These flammable gases can be found in a variety of different circumstances, though bituminous coal mines tend to have a large amount of associated methane. Modern mining techniques can extract these methane deposits by injecting carbon dioxide (CO2), but the presence of these flammable gases was historically just one of the many hazards associated with coal mining.
Since methane is often located in and among coal beds, early mining operations would often release it when striking into a seam. In some cases, a spark from a pickax or other ignition source would cause the released methane to immediately catch fire or explode. It was also possible for firedamp to seep into an existing tunnel or shaft where it could later be ignited by a lamp, airborne coal dust, or by other means.
Special lamps were constructed that could help reduce the likelihood of a firedamp explosion. This was accomplished by encasing the lamp flame in a fine mesh. The mesh material could allow methane to pass into the body of the lamp and be burned by the flame, but the holes were fine enough to contain the flame itself. Since the flame would tend to flare in the presence of flammable gases, these lamps were also used to determine if firedamp was present in a particular tunnel.
In the aftermath of a firedamp explosion, other dangerous gases were often released. These gases, which can consist of carbon monoxide (CO), carbon dioxide, and nitrogen, are referred to as afterdamp. Since they tend to be odorless, these gases can cause asphyxiation in miners, which led to the practice of bringing small rodents or birds down into mines. These small animals would typically react to the presence of the afterdamp in time for the human miners to retreat to an area with breathable air.