Roman candles are a type of firework. A firework is a low explosive pyrotechnic device used for entertainment purposes. The most common use of a firework is for firework displays, like the ones seen on holidays, such as the Fourth of July in the US.
The Chinese are credited with the invention of the firework, and were creating complex displays centuries before others. Italy was also extremely influential in the development of fireworks for celebratory purposes, so it's not surprising that this popular firework's name reflects the name of the city Rome.
A Roman candle firework is a single-tube firework device constructed of alternating layers of a black powder lift charge, pyrotechnic stars, and a delay charge. The bottom of the tube, is plugged with a clay bulkhead. Roman candles burn slowly because of the delay charge. Once the topmost star ignites, the burning accelerates, igniting the lift charge. As the star is propelled out of the tube, the delay powder is also ignited, which restarts the whole process.
Roman candle fireworks come in a variety of sizes, and eject one or more stars or exploding shells. In other words, there are no definite results once Roman candles are ignited. The burst could be colorful comets with various effects, single commets, individual or multiple firework stars, or small aerial star shells.
The pyrotechnic stars found within a Roman candle are pellet-like pieces that may contain a mixture of compounds that, when ignited, exude a colorful spark. Stars are either pumped, or cylinder-shaped; cut, or cube-shaped; or rolled, or spherical-shaped. In order to make sure the stars ignite, they are often primed with black powder. For visual enjoyment, stars are also coated with a pyrotechnic colorant.
Roman Candles come in a variety of sizes, ranging from 0.2 inches (6 mm) in diameter to 3.1 inches (8 cm) diameter, which are used in fireworks displays. These larger Roman Candle fireworks will typically have more lift charge added to their top layers. This increases the acceleration and altitude of the stars.
Numerous injuries and accidental fires that have arisen due to fireworks, including Roman candles. In many locations, therefore, the ability for consumers to purchase fireworks is often limited. In the US, although The National Fire Protection Association (NFPA) has created the restriction and safety standards, it's the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) who regulates explosives, including all US firework displays.