A gerb is a type of
firework
Fireworks are a class of low explosive pyrotechnic devices used for aesthetic and entertainment purposes. They are most commonly used in fireworks displays (also called a fireworks show or pyrotechnics), combining a large number of devices in ...
which produces a jet of
sparks, usually lasting between 15 and 60 seconds. It is a thick-walled tube filled with
pyrotechnic
Pyrotechnics is the science and craft of creating such things as fireworks, safety matches, oxygen candles, explosive bolts and other fasteners, parts of automotive airbags, as well as gas-pressure blasting in mining, quarrying, and demolition. ...
composition and possessing a choke, which is a narrowing in the tube. Gerbs are often referred to as 'fountains' (
spark fountain).
Sometimes a small charge of
black powder
Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, carbon (in the form of charcoal) and potassium nitrate (saltpeter). Th ...
is added to the bottom of a gerb to make it finish with a bang: this charge is known as the 'bounce'. These are commonly used at sporting events.
Gerbs intended for use indoors near a proximate audience, such as at a rock concert, typically have shorter durations (from {{frac, 1, 8 to 30 seconds) and heights (4 to 50 feet).
Gerbs are usually measured in terms of time and height. For example, a 4×4 burns for 4 seconds at a height of 4 feet.
Eric Newby's book ''
The Last Grain Race'' (chapter two: "Mountstewart") contains a 1938 reference to what appears to be the original gerb—in effect a mortar bomb fired from a mortar.
Types of fireworks