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Flash powder is a pyrotechnic composition, a mixture of oxidizer and metallic fuel which burns quickly and if confined will produce a loud report. It is widely used in theatrical pyrotechnics and fireworks (namely salutes, eg. cherry bombs, M-80s, and firecrackers), and was once used for flashes in photography.
Different varieties of flash powder are made from different compositions; most common are Potassium Perchlorate and Aluminum powder. Sometimes, Sulfur is included in the mixture to increase the sensitivity. Early formulations used Potassium Permanganate instead of Potassium Perchlorate.
Flash powder compositions are also used in military pyrotechnics, when production of large amount of noise, light, or infrared radiation is required; eg. missile decoy flares and stun grenades.
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Safety and handling
Flash-powder mixtures pose serious risks of burn, amputation, blindness and death. They are shock, friction, and electrostatic discharge sensitive, and sensitive to some types of contaminates, such as strong acids and bases. They should not be handled by those who are unfamiliar with the properties of such compositions, or the handling techniques required to maintain safety. Flash powder and flash-powder devices pose exceptionally high risks to children, who typically cannot understand the danger, who may be less adept with safe handling techniques, and who tend to suffer more-severe injuries than adults.
Flash powders - especially those which utilize Chlorates - are often highly sensitive to friction, heat/flame and static electricity. Even as little as 0.1-10 millijoules spark can set off certain mixtures. Certain formulations that are prominent in the underground press contain both Sulfur and Potassium Chlorate. These mixtures become unstable over time and may spontaneously ignite with little or no warning, often with disastrous results. Modern pyrotechnic practices call for never using sulfur in a mix containing chlorate salts.
Some flash powder formulations (Those which use single digit micron flake aluminum powder or fine magnesium powder as their fuel) can self confine and explode in small quantities. This makes flash powder especially dangerous to handle, as it can cause severe hearing damage and amputation injury even when sitting in the open. Self confinement occurs when the mass of the pile provides sufficient inertia to allow high pressure to build within it as the mixture reacts. This is refered to as 'inertial confinement', and it is not to be confused with a detonation.
Flash powder of any formulation should not be mixed in large quantities by the amateur pyrotechnician. Beginners should start with sub-gram quantities, and should refrain from making large devices. Producing more than 0.05 grams (50 milligrams) for one device is considered a federal offense in the United States and is punishable by up to 6 years in prison. Flash powder should only be made on site and right before use.
Flash Powder Mixtures
Normally, flash powder mixtures are compounded to achieve a particular purpose. These mixtures range from extremely fast burning mixtures designed to produce a maximum audio report, to mixtures designed to burn slowly and provide large amounts of illumination, to mixtures that were formerly used in photography.
Aluminium and Chlorate
Because of the above mentioned instability, Aluminium powder and potassium chlorate is a poor choice for flash that is to be stored for more than a very short period of time. For that reason it has been largely replaced by the potassium perchlorate mixtures. Chlorate mixes are used when cost is the overriding concern, because potassium chlorate is less expensive than perchlorate. For this reason, such mixtures are often found in third world pyrotechnics, where cost is critical and safety concerns are secondary. By mass the composition is seven parts of KClO3 to three parts of pyro aluminum. It is critically important to exclude sulphur and any acidic components from these mixtures. Sometimes a few percent of bicarbonate buffer is added to the mixture to insure the absence of acidic impurities.
- KClO3 + 2Al → Al2O3 + KCl
Aluminum and Perchlorate
Aluminum powder and potassium perchlorate make up the only two components of the pyrotechnic industry standard flash powder. It provides a great balance of stability and power, and is the composition used in most commercial exploding fireworks.
The balanced equation for the reaction is:
- 3 KClO4 + 8 Al → 4 Al2O3 + 3 KCl
Although not stoichiometrically balanced, a ratio of seven parts Potassium Perchlorate to three parts Dark Pyro Aluminum is the composition used by most pyrotechnicians. However, a ratio of 2 mass units Potassium Perchlorate to 1 mass unit Dark Pyro Aluminum is closer to stoichiometric, and may produce a louder bang.
For best results, the aluminum powder should be "Dark Pyro" grade, with a flake particle shape, and a particle size of less than 10 microns. 5413H-S "German Dark" aluminum manufactured by Eckart Germany is reputed to be the best performing aluminum in existence. The KClO4 should be in powder form, free from clumps. It can be sieved through a screen if necessary to remove any clumps prior to use. The particle size of the Perchlorate is not as critical as that of the aluminum component, as much less energy is required to decompose the KClO4 than is needed to melt the aluminum into the liquid state required for the reaction.
Although this composition is fairly insensitive, it should be treated with care and respect. Hobbyist pyrotechnicians usually use a method called diapering, in which the materials are poured separately onto a large piece of paper, which is then alternately lifted at each corner to roll the composition over itself and mix the components. Some amateur Pyrotechnicians chose to mix the composition by shaking in a closed paper container, as this is much quicker and more effective than diapering. Paper/cardboard is chosen over other materials such as plastic as a result of its favorable triboelectric properties.
Large quantities should never be mixed in a single batch. Large quantities are not only more difficult to handle safely, but they place innocent bystanders within the area at risk. In the event of accidental ignition, debris from a multiple pound flash powder explosion can be thrown hundreds of feet with sufficient force to kill or injure.
No matter the quantity, care must always be taken to prevent any electrostatic discharge or friction during mixing or handling, as these may cause accidental ignition.
Magnesium and Nitrate
Another flash composition common among amateurs consists of Magnesium powder and Potassium Nitrate. Other metal nitrates have been used, including Barium and Strontium nitrates. Compositions using nitrate and magnesium metal have been used as photographic flash powders almost since the invention of photography.
- 2 KNO3 + 5 Mg → K2O + N2 + 5 MgO
- Ba(NO3)2 + 5 Mg → BaO + N2 + 5 MgO
Mixtures designed to make reports are substantially different than mixtures designed for illumination. A stoichiometric ratio of three parts KNO3 to two parts Mg is close to ideal, and provides the most rapid burn. The magnesium powder should be smaller than 200 mesh, though up to 100 mesh will work. The potassium nitrate should be impalpable dust. This mixture is popular in amateur pyrotechnics because it is insensitive and relatively safe as such things go.
For photographic use, mixtures containing magnesium and nitrates are made much more fuel rich. The excess magnesium is volatilized by the reaction and burns in air providing additional light. In addition, the higher concentration of fuel results in a slower burn, providing more of a "poof" and less of a "bang" when ignited. A formula from 1917 specifies 5 parts of magnesium to 6 parts of barium nitrate for a stoichiometry of nine parts fuel to one part oxidizer.[1] Modern recreations of photographic flash powders may avoid the use of barium salts because of their toxic nature. A mixture of five parts 80 mesh magnesium to one part of potassium nitrate provides a good white flash without being too violent. Fuel rich flash powders are also used in theatrical flash pots.
Magnesium based compositions degrade over long periods of time, as Magnesium does not form a passivating oxide coating, meaning the metallic Mg will slowly react with atmospheric oxygen and moisture. In military pyrotechnics involving magnesium fuels, external oxygen can be excluded by using hermetically sealed canisters. Commercial photographic flash powders are sold as two part mixtures, to be combined immediately before use.
Magnesium and PTFE
A flash composition designed specifically to generate flares that are exceptionally bright in the infrared portion of the spectrum use a mixture of pyro grade magnesium and powdered Polytetrafluoroethylene. These flares are used as decoys from aircraft that might be subject to heat-seeking missile fire.
- 2n Mg + (C2F4))n → 2n MgF2 (s) + 2n C (s)
Antimony Trisulfide and Chlorate
This mixture, and similar mixtures sometimes containing pyro aluminum have been used since the early 1900's for small "Black Cat" style paper firecrackers. Its extremely low cost makes it popular among manufacturers of low-grade fireworks in China. Like all mixtures containing Chlorates, it is extremely sensitive to friction, impact and ESD, and is considered unsafe in pyrotechnic devices that contain more than a few tens of milligrams of the mixture.
- 3 KClO3 + Sb2S3 → Sb2O3 + 3 SO2 + 3KCl
This mixture is not highly energetic, and in at least some parts of the United States, firecrackers containing 50 mg or less of this mixture are legal as consumer fireworks.
References
- ^ Watkins, The Photographic Journal of America, Vol 54, 1917, Philadelphia, pp 384
External links
- Flash Powder Information and Instructions
- Flash Powder Three different types of Flash powder
See also
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