Monday, July 28, 2008

Firework Types, Explosives & How They Work Part 2

Set Pieces

These fireworks are, as their name suggests, used to create a static effect. They can be in the form of waterfalls strung horizontally on string or poles, wheels, flying pigs, fountains or lancework e.g. “Good Night”. These are mini flares linked together with fuse and burn for about 60 secs.

Careful attention needs to be paid to setting these up in order to gain the maximum effect from them, especially with respect to their height. Care must be taken to ensure that the moving fireworks set off in the right direction initially.

Most set pieces are not dangerous if properly assembled, however keep these out of the firing line to avoid premature ignition. Especially lance-work which must be set up-wind of the rest.

Lancework is a set piece of a special type. They are made up from small fireworks set on a frame which burn simultaneously as individual points of light to present a continuing two-dimensional picture. The picture may bes symbolic, such as a badge or crest, or in word form, such as “Good Night”.

More complex effects can be produced by creating different pictures in sequence on the same frame, to produce an impression of motion.

Explosives – How they work

Explosives can be classified under three headings:-

Deflagatory explosives (sometimes called low explosives), of which black powder is a typical example, are a mixture of a fuel and an oxidising agent.

On ignition, rapid combustion takes place producing quantities of hot gas. It is the expansive properties of this hot gas that, if suitably contained, gives rise to any explosive force produced. Deflagatory explosives form the vast majority of pyrotechnics mixtures.

By adjusting grain size and degree of compression of the powder before it is burnt, a considerable range of burn rates can be achieved. By the degree of containment of the hot gases after combustion has started, a considerable range of explosives force can be attained.

Hence the same quantity of black powder can be used to make a Gerb lasting 25 seconds or a lift charge for a shell fired from a mortar that burns for 6 micro seconds.

Because deflagatory explosives contain both fuel and oxidising agent, once alight they cannot be extinguished by blanketing with foam, Co2 etc. The best solution is to try to get sufficient water onto the fire to reduce the temperature to below the ignition point.

Caution! If however the fuel is metallic e.g. Magnesium, the application of water will have the same effect as squirting a bonfire with liquid oxygen! Magnesium is a very powerful reducing agent and can easily extract oxygen from substances commonly used in fire extinguishers e.g. water, carbon dioxide and even sand.

Fires involving pyrotechnic mixtures containing metallic fuels, are best covered with DRY sand in vast quantities in the hope that the thermal mass of the cold sand will cool the fire to below ignition temperature and before the metal fuel has had a chance to extract the oxygen from the sand.

It is also worth noting that pyrotechnic mixtures containing Aluminium and a nitrate oxidiser (e.g. Potassium Nitrate) will spontaneously ignite and then explode if they get wet.

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