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A flare, also sometimes called a fusee, is a type of pyrotechnic that produces a brilliant light or intense heat without an explosion. Flares are used for signalling, illumination, or defensive countermeasures in civilian and military applications. Flares may be ground pyrotechnics, projectile pyrotechnics, or parachute-suspended to provide maximum illumination time over a large area. Projectile pyrotechnics may be dropped from aircraft, fired from rocket or artillery, or deployed by flare guns or handheld percussive tubes.
The earliest recorded use of gunpowder for signalling purposes was the 'signal bomb' used by the Song Dynasty Chinese as the Mongol-led Yuan Dynasty besieged Yangzhou in 1276. These soft-shelled bombs, timed to explode in mid-air, were used to send messages to a detachment of troops far in the distance. Another mention of the signal bomb appears in a text dating from 1293 requesting their collection from those still stored in Zhejiang. A signal gun appears in Korea by 1600. The Wu I Thu Phu Thung Chih or Illustrated Military Encyclopedia written in 1791 depicts a signal gun in an illustration.
Flares produce their light through the combustion of a pyrotechnic composition. The ingredients are varied, but often based on strontium nitrate, potassium nitrate, or potassium perchlorate and mixed with a fuel such as charcoal, sulfur, sawdust, aluminium, magnesium, or a suitable polymeric resin. Flares may be colored by the inclusion of pyrotechnic colorants. Calcium flares are used underwater to illuminate submerged objects.
Many in-service colored signal flares and spectrally balanced decoy flares contain perchlorate oxidizers. Perchlorate, a type of salt in its solid form, dissolves and moves rapidly in groundwater and surface water. Even in low concentrations in drinking water supplies, perchlorate is known to inhibit the uptake of iodine by the thyroid gland. While there are currently no US federal drinking water standards for perchlorate, some states have established public health goals, or action levels, and some are in the process of establishing state maximum contaminant levels. For example, the US Environmental Protection Agency have studied the impacts of perchlorate on the environment as well as drinking water. California has also issued guidance regarding perchlorate use.
US courts have taken action regarding the use of perchlorate in manufacturing pyrotechnic devices such as flares. For example, in 2003, a federal district court in California found that Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) applied because perchlorate is ignitable and therefore a “characteristic” hazardous waste. (see Castaic Lake Water Agency v. Whittaker, 272 F. Supp. 2d 1053, 1059–61 (C.D. Cal. 2003)).
In the civilian world, flares are commonly used as signals, and may be ignited on the ground or fired as an aerial signal from a pistol-like flare gun, or launched from a self-contained tube. Flares are commonly found in marine survival kits.
Maritime distress signal
Red flares, either sent as a rocket or held in the hand, are widely recognized as a maritime distress signal. RMS Titanic however used Socket launched distress shells to attract assistance, emitting white stars to a great height and a "deafening" sound. 
Another type of flare is the fusee, which burns for 10–60 minutes with a bright red light. Fusees are commonly used to indicate obstacles or advise caution on roadways at night; in this usage they are also called highway flares, road flares, or ground flares. They are commonly found in roadside emergency kits.
Fusees are also known as railroad flares and are commonly used to perform hand signals in rail transport applications. Since they can be used only once, fusees nowadays are usually intended for emergency use (as opposed to the incandescent lanterns typically used during normal operating conditions). However, in the days before train radio communications, fusees were used to keep trains apart on un-signalled lines. A railroad fusee was timed to burn for ten minutes and quantities were dropped behind a train to ensure a safe spacing. If a following train encountered a burning fusee it was not to pass until the fusee burned out. Fusees made specifically for railroad use can be distinguished from highway fusees by a sharp steel spike at one end, used to embed the fusee upright in a wooden railroad tie.
In forestry and firefighting, fusees are sometimes used in wildland fire suppression and in the ignition of controlled burns. They ignite at 191 °C (376 °F) and burn as hot as 1,600 °C (2,900 °F). They are especially effective in igniting burnouts or backburns in very dry conditions, but not so effective when fuel conditions are moist. Since controlled burns are often done during relatively high humidity levels (on the grounds that they could not be safely contained during periods of very low humidity), the driptorch is more effective and more often used. Fusees are also commonly carried by wildland firefighters for emergency use, to ignite an escape fire in surrounding fuels in case of being overrun by a fire if no other escape routes are available.
Calcium phosphide is often used in naval flares, as in contact with water it liberates phosphine which self-ignites in contact with air; it is often used together with calcium carbide which releases acetylene.
In 1922, a "landing flare" was an aerial candle attached to a parachute and used for landing an airplane in the dark. The flare burned for less than 4 minutes and the candle power was about 40,000.
A special variety of flare is used in military aircraft as a defensive countermeasure against heat-seeking missiles. These flares are usually discharged individually or in salvoes by the pilot or automatically by tail-warning devices, and are accompanied by vigorous evasive maneuvering. Since they are intended to deceive infrared missiles, these flares burn at temperatures of thousands of degrees, incandescing in the visible spectrum as well. Soids are floating flares that are effective only in the terminal phase of missiles with infrared signature seeker heads.
Pyrotechnic flares are classified as class 1.4 explosives, and as such have been utilized in training exercises by explosive disposal units within police forces."
Several U.S. states, including California and Massachusetts, have begun regulating levels of potassium perchlorate, which can be unsafe at certain levels in drinking water. Contaminated drinking water can lead to such symptoms as gastric irritation, nausea, vomiting, fever, skin rashes, and even fatal aplastic anemia.
- Joseph Needham (1986). Science and Civilisation in China: The Gunpowder Epic. Cambridge University Press. p. 169. ISBN 978-0-521-30358-3.
- Needham (1986), p. 331.
- "What is in road flares?". Retrieved 2009-01-16.
- "The Merchant Shipping (Distress Signals and Prevention of Collisions) Regulations 1996" (PDF). UK Maritime and Coast Guard Agency. 1996. Retrieved 23 July 2013.
- The New Generation Fire Shelter (PDF). National Wildfire Coordinating Group. March 2003. Retrieved 2009-01-16.
- Hugh Chisholm (1922). The Encyclopædia Britannica: Abbe to English history ("The first of the new volumes"). The Encyclopædia Britannica, Company ltd. p. 86. Retrieved 2011-10-13.
Electric-Ignition Parachute Flare-Bombs...a landing flare to enable pilots to land in the dark...The candles burn for from 2½ to 3½ min....the candle power is about 40,000.
- "History of Flares". SiriusSignal.com. Sirius Signal. Retrieved 8 June 2015.
- Borowicz, et. al. (18 December 2014). "Disposal of Pyrotechnic Visual Distress Signals" (PDF).
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- Wallbank, Alister (2001). "Can anybody see me? (modified reprint from DIVER 2000; 45 (2) February: 72–74)". Journal of the South Pacific Underwater Medicine Society 31 (2): 116–119. Retrieved 2008-10-13.