Pyrotechnic colorant


A pyrotechnic colorant is a chemical compound which causes a flame to burn with a particular color. These are used to create the colors in pyrotechnic compositions like fireworks and colored fires. The color-producing species are usually created from other chemicals during the reaction. Metal salts are commonly used; elemental metals are used rarely.
The color of the flame is dependent on the metal cation; the anion of the salt has very little direct influence. The anions however influence the flame temperature, both by increasing it and decreasing it, indirectly influencing the flame brightness and brilliancy. For temperature-decreasing additives, the limit of colorant may be about 10–20 wt.% of the composition.
Some common examples are:
ColorCompound nameChemical formulaNotes
RedStrontium nitrateSr2Common. Used with chlorine donors. Excellent red, especially with metal fuels. Used in many compositions including road flares.
RedStrontium carbonateSrCO3Common. Produces good red. Slows burning of compositions, decomposes yielding carbon dioxide. Fire retardant in gunpowders. Inexpensive, non-hygroscopic, neutralizes acids. Superior over strontium oxalate in absence of magnesium.
RedStrontium oxalateSrC2O4Decomposes yielding carbon dioxide and carbon monoxide. In presence of magnesium fuel, carbon monoxide reduces particles of magnesium oxide, yielding gaseous magnesium and eliminating the black-body radiation of the MgO particles, resulting in clearer color.
RedStrontium sulfateSrSO4Common. High-temperature oxidizer. Used in strobe mixtures and some metal-based red compositions.
RedStrontium chlorideSrCl2Common. Produces bright red flame.
OrangeCalcium carbonateCaCO3Produces orange flame. Yields carbon dioxide on decomposition. Often used in toy fireworks as a substitute for strontium.
OrangeCalcium chlorideCaCl2
OrangeCalcium sulfateCaSO4High-temperature oxidizer. Excellent orange source in strobe compositions.
OrangeHydrated calcium sulfateCaSO4x*
Gold/YellowCharcoal powder
YellowSodium bicarbonateNaHCO3Compatible with potassium chlorate. Less burning rate decrease than sodium carbonate. Incompatible with magnesium and aluminium, reacts evolving hydrogen gas.
YellowSodium carbonateNa2CO3Hygroscopic. Significantly decreases burning rate, decomposes evolving carbon dioxide. Strongly alkaline. Very effective colorant, can be used in small amounts. Corrodes magnesium and aluminium, incompatible with them.
YellowSodium chlorideNaClLoses hygroscopicity on heating. Corrodes metals.
YellowSodium oxalateNa2C2O4Non-hygroscopic. Slightly reacts with magnesium, no reaction with aluminium.
YellowSodium nitrateNaNO3Also acts as oxidizer. Bright flame, used for illumination.
YellowCryoliteNa3AlF6One of the few sodium salts that is nonhygroscopic and insoluble in water.
GreenBarium chlorideBaCl2
GreenBarium chlorateBa2Classic exhibition green with shellac fuel. Sensitive to shock and friction. Oxidizer.
GreenBarium carbonateBaCO3A pretty color when ammonium perchlorate is used as oxidizer.
GreenBarium nitrateBa2Not too strong effect. With chlorine donors yields green color, without chlorine burns white. In green compositions usually used with perchlorates.
GreenBarium oxalateBaC2O4
BlueCopper chlorideCuClThe Richest blue flame. Almost insoluble in water.
BlueCopper oxideCu2OLowest cost blue colorant.
BlueCopper oxideCuOUsed with chlorine donors. Excellent in composite stars.
BlueCopper carbonateCuCO3Best when used with ammonium perchlorate.
BlueBasic copper carbonateCuCO3·Cu2, 2 CuCO3·Cu2Occurs naturally as malachite and azurite. Good with ammonium perchlorate and for high-temperature flames with presence of hydrogen chloride. Not easily airborne, less poisonous than Paris Green.
BlueCopper oxychloride3CuO·CuCl2Good blue colorant with suitable chlorine donor.
BlueParis GreenCu2.3Cu2Copper acetoarsenite, Emerald Green. Toxic. With potassium perchlorate produces the best blue colors. Non-hygroscopic. Fine powder readily becomes airborne; toxic inhalation hazard.
BlueCopper arseniteCuHAsO3Almost non-hygroscopic. Almost as good colorant as copper acetoarsenite. Toxic. Can be used with chlorate oxidizers.
BlueCopper sulfateCuSO4·5 H2OCan be used with nitrates and perchlorates. Acidic, incompatible with chlorates. With red phosphorus in presence of moisture liberates heat, may spontaneously ignite. Less expensive than copper acetoarsenite. Anhydrous copper sulfate is hygroscopic, can be used as a desiccant. With ammonium perchlorate produces an almost as pretty a blue color as achievable with copper acetoarsenite.
BlueCopper metalCuRarely used, other compounds are easier to work with. Yields pretty blue color in ammonium perchlorate based compositions; but reacts with ammonium perchlorate and liberates ammonia in presence of moisture. The composition must be kept dry.
PurpleCombination of red and blue compoundsSr+Cu
PurpleRubidium compoundsRbrarely used
Silver/WhiteAluminium powderAl
Silver/WhiteMagnesium powderMg
Silver/WhiteTitanium powderTi
Silver/WhiteAntimony sulfideSb2S3
InfraredCaesium nitrateCsNO3two powerful spectral lines at 852.113 nm and 894.347 nm
InfraredRubidium nitrateRbNO3

The * indicates that the compound will burn orange where x=0,2,3,5.

Radiating species

Despite the wide numbers of metal ion donors, they serve to form only a few atomic and molecular species that are useful as light emitters.
In many cases, chlorine donors have to be added in order to achieve sufficiently deep colors, as the desired emitting molecules have to be generated.
Some color emitters are of atomic nature. Presence of chlorine, and the reaction to monochlorides, may actually impair their color purity or intensity.
At high temperatures, the atoms will ionize. The emission spectra of ions are different than of neutral atoms; the ions may emit in undesired spectral ranges. For example, Ba+ emits in blue wavelengths. Ionization can be suppressed by addition of an easier-to-ionize metal with weak visible emission of its own, e.g. potassium; the potassium atoms then act as electron donors, neutralizing the barium ions.
The color blue is notoriously difficult to produce in fireworks, as the copper compounds need to be heated at a specific temperature for the optimal shade of blue to be produced. Thus, a deep, rich blue is usually viewed as the mark of an experienced fireworks maker.
Care should be taken to avoid formation of solid particles in the flame zone, whether metal oxides or carbon; incandescent solid particles emit black-body radiation that causes "washing out" of the colors. Addition of aluminium raises the flame temperature but also leads to formation of solid incandescent particles of aluminium oxide and molten aluminium. Magnesium has less such effect and is therefore more suitable for colored flames; it is more volatile than aluminium and more likely to be present as vapors than as particulates. Formation of solid particles of magnesium oxide can further be inhibited by presence of carbon monoxide, either by negative oxygen balance of the composition in presence of organic fuels, or by addition of the colorant in the form of an oxalate, which decomposes to carbon dioxide and carbon monoxide; the carbon monoxide reacts with the magnesium oxide particles to gaseous magnesium and gaseous carbon dioxide.
ColorEmitterWavelengthsNotes
YellowSodium 589 nmVery strong, overpowers other colors, avoid contamination
OrangeCaCl most intense: 591–599 nm and 603–608 nm, and others
RedSrCl a: 617–623 nm
b: 627–635 nm
c: 640–646 nm		The SrCl species tends to be oxidized to less desirable SrO; strontium-containing compositions are therefore usually formulated to be oxygen-deficient.
RedSrOH 600–613 nm
RedLi
GreenBaCl a: 511–515 nm
b: 524–528 nm
d: 530–533 nm		Lines of BaOH and BaO are also present, emitting in yellow and yellowish-green. The BaOH lines are much stronger than the BaO lines. In absence of chlorine, the BaCl lines are not present and only the BaOH and BaO lines are visible.

The BaCl species tends to be oxidized to less desirable BaO; barium-containing compositions are therefore usually formulated to be oxygen-deficient.

Presence of Ba+ is undesired, as it emits in a blue region at 455.4 nm. Potassium may be added to suppress barium ionization, as it ionizes easier and acts as an electron donor for the barium ions.
BlueCuCl several intense bands between 403–456 nm, less intense at 460–530 nmLow dissociation energy of copper compounds causes presence of free copper atoms in the flame, weakly emitting in green. In presence of chlorine, CuCl is formed, emitting strongly in blue. At higher temperatures CuCl dissociates and lines of atomic copper are present in the spectrum; CuO and CuOH are also formed, emitting molecular bands at green-yellow for CuOH and at orange-red for CuOH. Adequate control of temperature is therefore required for blue-burning compositions.
InfraredCarbon particlesblack-body radiationFor good broadband infrared output, compositions producing lots of heat and carbon particles are required. The burning temperature should be lower than of visible-illuminating compounds. The intensity of the emitted radiation depends on the burn rate. Temperature can be increased by addition of magnesium. A magnesium/Teflon/Viton composition is common for missile decoy flares.
InfraredCO2 mostly 4300 nmProduced by carbon-containing fuels.
InfraredCs two powerful spectral lines at 852.113 nm and 894.347 nmUsed in infrared illumination compositions. Metal is avoided in the compositions to prevent formation of bright, visible-radiating particles.
InfraredRb spectral lines in near-infraredUsed in infrared illumination compositions, less commonly than cesium.