'When in Rome'...build a Roman Candle Battery
The manufacture of roman candles is quite a technical challenge. There are several elements that must be brought together from numerous suppliers and each must function in harmony with the others. Good roman candle production is a process of trial and error to get the best results.
The tube must be of sufficient quality and thickness so that it will not catch fire and must be smooth on the inside otherwise powder will creep down and result in a machine gun effect when fired as the fire will flash down and ignite all the stars together. When the tubes are made on a ‘former’ they will shrink slightly as they dry.
The comet (star with a tail) or star must be made to precise tolerances too. The designer must allow for the shrinkage of the tube as it dries to a final dimension as well as consider lifting characteristics. If the comet is too snug in the tube it will be fired too fast and may extinguish itself. The fire is ‘pulled’ off the comet as it is ejected. If the comet is too loose in the tube the lifting gas will not push the comet high enough because the gas which provides the thrust escapes up the side of the comet. The initial burn-off of composition on ignition allows the comet to shrink just enough to balance these requirements. It is often referred to as ‘windage’.
In simple candles a delay fuse composition is made into a solid cylinder and is used to create a time separation between shots. It ignites from the previous lift charge, burning down, before igniting the next comet and lift charge. In larger calibre candles an engineered fuse is used which allows for precise timings. It is inserted into a felt wad which has a hole in the centre. The wad is slightly oversize which allows for a good seal in the tube and increases the compression of the lifting gas. Once the delay fuse has burned down the fire moves to gunpowder which is coarser and more angular than lift charge gunpowder. This is known as fire transfer powder and sits on top of the effect producing flame that not only lights the effect, but also travels down the sides lighting the lift charge powder under the effect. The coarse nature of the fire transfer powder means that it will stay in position and not slip down the side of the effect. Lift charge powder is much finer, designed to have a large surface area which allows it to ignite quickly producing the lifting gas very rapidly. The lift charge and effect light simultaneously, and the effect and the wad are ejected. Only the bright effect is seen during the display. A typical roman candle will have 8 or 10 shots. A section diagram of this style of candle is show below.
Roman candle batteries are a bundle of roman candles fused to fire together. In this way many shots can be fired together for a more impressive display. HEX sells several batteries, and these can be found here. These batteries are made in different way to that discussed above. The Chinese have developed a reliable cost effective alternative solution. They use a length of delay fuse which is like the delay fuse found on single ignition barrages. The fuse runs down the length of the tube. First, they load lifting charge, then a star (usually a spherical star) or whistle or even a very small Bombette then a charge of granulated clay or sawdust. Then the process is repeated. The clay acts as a barrier between the pyrotechnic components and therefore when the fuse burns down beside the clay you get a delay between shots.
Below is a drawing of a typical small calibre candle used in batteries. A typical tube will hold eight or ten shots. Here we have illustrated two.
A good quality roman candle can be easily identified when fired. It produces a good, clean, sharp sound as the effect lifts, the height reached by each shot is consistent and they burn out just before they turn over in the sky. The consistency of height is a technical challenge because the weight of lifting charge varies according to the position of the shot in the candle.
Reference source: Rev. R Lancaster MBE, Fireworks, Principles and Practice, 4th Edition, ISBN 0-8206-0407-0