Science of Fireworks - Roman Candles
"Roman candle" is the traditional name for a firework that has existed for many centuries. In its simplest form, it's just a tube with a shell inside. A fuse runs into the tube and ignites a lifting charge, forcing the shell out of the open end and into the air where it explodes, giving the effect.
The lifting charge is made of black powder which when ignited produces a lot of hot gas which in turn pushes the shell upwards.
Roman candles can have multiple shells inside, stacked one on top of the other, separated by lifting charges. The fuse runs down the inside of the tube, igniting each charge in turn. The shells are launched one after the other, with a pause between each one. This is known as a barrage, repeater or multi shot candle.