Low Explosives
An explosive which utilizes chemical formulas which combust when a certain amount of initial energy is applied to them. Most fireworks, with
the exceptions of large salutes, fall into this category. The rate at which the pyrotechnic composition burns is mostly dependent upon the rate
at which the composition can transfer heat from one layer of itself to another. When a compositions rate of reaction is very slow it is known as
burning. Examples of a pyrotechnic composition which "burns" would be a star in an aerial shell, flares, or gerbes (fountains). Burn rate is
also highly dependent on pressure and temperature. Therefore, when a pyrotechnic composition is confined, its burn rate is accelerated. When the
reaction is sped up drastically due to increases in pressure and temperature, such as the case with the bursting charge in an aerial shell, it
becomes explosive, and is known as deflragration. In an aerial shell, the temperature and pressure build up while the composition inside of it
is burning. Once the pressure from the hot gases created during the reaction reaches a certain point, the casing will fail, rupturing outward,
giving an explosion.
High Explosives
A device in which the explosive composition will detonate once initiated. High explosives can be initiated in several manners. The one that
concerns us the most in the case of fireworks is deflagration to detonation. An example of this process is seen in large (or confined)
quantities of flash powder. One ingrediant in flash powder is aluminum. Aluminum is known for it's ability to transfer heat and it's high
heat of reaction (thermal energy produced when it combusts). Since the rate at which a pyrotechnic composition burns is dependent upon how
well it can transfer heat, this is a very important property. Once ignited, the flame front in flash powder reaches supersonic velocities,
thus creating a shock wave in the composition. Once a shock front is accomplished, the rate at which the composition burns is no longer
dependent on how well it can transfer heat (as in the case of a low explosive). The burn rate is now dependent upon how well the explosive
composition can trasfer the detonation wave (shock wave) through itself. A salute must be large enough to allow for a shock front to be formed
in order for a detonation to occur. This means a minimum amount (critical mass) of flash powder must be used to achieve detonation. Confinment
of the flash powder in a salute lowers the amount of flash powder needed. A detonation releases a great amount of energy in a very short period
of time, hence the reason salutes are much louder and have a "sharper"(caused by the shock wave) boom.
