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Fireworks makers fill the night sky with myriad effects in displays that are popular all over the world. Although the art dates back to ancient China, most of the effects you'll see in a typical display are inventions of this century. A typical example is the development of colored flames. Before the 19th century, only various yellows and oranges could be produced with steel and charcoal. Chlorates, an invention of the late 18th century and an industrial product of the 19th century, added basic reds and greens to the pyrotechnist's repertoire. Good blues and purples were not developed until this century, although it is not unusual to find unsafe display formulas for blue stars in earlier literature.
Basic principles of pyrotechnic light productionThe light emitters can be grouped into two main categories: solid state emitters (black body radiation) and gas phase emitters (molecules and atoms).
Black body radiation and the grey body conceptA black body is an ideal emitter which is capable of absorbing and emitting all frequencies of radiation uniformly. The excitance (M) of the black body, the power emitted per unit area, is defined as M = sT4 (1) where s is the Stefan-Boltzmann constant and T is the temperature. Thus, we could obtain a twofold increase in radiation by merely increasing the flame temperature from, say 2000 K to 2400 K. Furthermore, the radiation also shifts from infrared to visible light as the temperature increases. The calculated emission spectrum (the energy per unit volume per unit wavelength range) has the following shape:
In the real world, simplified models are not of much help. Many solids do emit light in the same relative proportions as a black body, but not in the same amounts. The emissivity of a solid substance is the factor relating observed and theoretical radiant energy. The emissivities of many refractory metals and metal oxides are higher in the short wavelength end of the visible spectrum - that is, they look bluer than expected when heated. T, K OC Subjective color 750 480 faint red glow 850 580 dark red 1000 730 bright red, slightly orange 1200 930 bright orange 1400 1100 pale yellowish orange 1600 1300 yellowish white > 1700 > 1400 white (yellowish if seen from a distance) The atomic and molecular emittersAs you can easily see from Table 1 (and very probably know from experience), it is not possible to produce anything but shades of orange and yellow with grey-body emitters. (In principle, we could generate blue light with a hypothetical black or grey body at 9000 K and up, which is the temperature of blue stars, but such temperatures are unattainable for pyrotechnicians.) For other color, we need specific emitters of colored light. Surprisingly few emitters are used in pyrotechnics, given the vast range of atomic and especially molecular spectra available. In fact, the production of some color is still a problem - next time you see a fireworks display, count all turquoises and ocean greens you saw. There are not many, because there are no commercially useful emitters available in the 490-520 nm region (blue-green to emerald green).
Color Emitters used Wavelength range
Yellow Sodium D-line atomic emission 589 nm
Orange CaCl, molecular bands several bands, 591-
599 nm, 603-608 nm
being the most intense
Red SrCl, molecular bands a: 617-623 nm
b: 627-635 nm
c: 640-646 nm
Red SrOH(?), molecular bands 600-613 nm
Green BaCl, molecular bands a: 511-515 nm
b: 524-528 nm
d: 530-533 nm
Blue CuCl, molecular bands 403-456 nm,
several intense
bands, less intense
bands between 460 nm
and 530 nm
The chromaticity diagram and color perceptionThe human eye may not be the best spectroscope invented, but it is the best instrument for designing colored fireworks. Although a spectroscope can show the presence or absence of certain lines or bands in the flame spectrum, it cannot decide whether the color obtained looks pleasing to the human observer. Pure, monochromatic color a'la lasers are only a dream for pyrotechnists, but well-designed impure color do not lag much behind. The chromaticity diagram shown below has been designed with human color vision system (three base color) in mind. It is not necessary to specify the intensities of all three base color, because the hue is not affected by the brightness of the light (the sum of all intensities). We can conveniently use the fractional intensities of two primary color, and this gives us a chart in two dimensions. The sum of all three intensities must equal one, so the third fraction can be easily calculated. In order to avoid negative primary color fractions, the International Commission on Illumination published a standard chromaticity diagram in 1931 with three unreal primary color. The above diagram and the color are based on the commission's recommendations. The pure spectral color can be found on the curved line surrounding the tongue-shaped region of composite color. The numbers along the curve represent corresponding wavelengths (in nanometers). Figure 2 shows the chromaticity diagram with a few emission lines and bands of Table 2 drawn on the curve of spectral color. The color of the diagram are only approximate.
Figure 2. Chromaticity diagram with some emission bands. Click on the picture to see the true-color version (44K jpg). All would be well if we could just pick up the light from the above emitters. However, the emitting molecules, especially SrCl and BaCl, are so reactive that they cannot be packed directly into a firework. To generate them, we need pyrotechnic compositions designed to generate the above molecules, to evaporate them into the flame and to keep them at as high temperature as possible to achieve maximum light output. To get good color, there must be substantial amounts of emitters present in the flame. The emitters are not alone: in order to achieve the high temperature, a fuel - oxidiser system is also needed, as well as some additional ingredients. The color of aerial fireworks come invariably from stars, small pellets of firework composition which contain all the necessary ingredients for generating colored light or other special effects. They may be as tiny as peas or as large as strawberries. A typical red star might contain Potassium perchlorate, 67% by weight Strontium carbonate 13.5% Pine root pitch (fuel) 13.5% Rice starch (binder) 6% Care must be exercised in selecting the ingredients. The composition must be safe and stable in storage. In addition, it must work as expected and burn with a red color once lit. For a deep red we need only SrCl and SrOH emission - and nothing else. To generate the emitting molecules at a sufficiently high temperature, a fuel-oxidiser system (pine root pitch - potassium perchlorate) is used. Strontium carbonate is used as the Sr source, and chlorine comes from potassium perchlorate (KClO4 --> K+ +Cl- + 2 O2). An excess of fuel is used to prevent the formation of SrO, which would solidify in the flame and emit grey body radiation. This will result in a "washed-out" color. Too much fuel would be a disadvantage, too, because the glowing carbon particles quickly overwhelm the red color. Pure color also require pure ingredients. Sodium D-line atomic emission is so strong and so easily excited that even minute amounts of sodium impurities will quickly ruin the color. Potassium, with its weak atomic lines, does not interfere with most color, and potassium salts can usually be used. Organic fuels, such as pine root pitch, various gums and rosins and synthetic resins, cannot generate as high temperatures as metallic fuels. The pyrotechnist is tempted to use powdered magnesium and aluminum for his/her brilliant stars, because they provide an easy method of raising the flame temperature and increasing the brightness. Unfortunately, the molecular emitters are quickly destroyed if the flame is too hot. CuCl is probably the most fragile color emitter. It can be used with metallic fuels only with difficulty. Consequently, blue stars are never very bright. Another problem with metals are their oxidation products, metal oxides, which are powerful grey body radiators due to their refractory nature. Their incandescent glow can easily wash out all color. Over the years, chemists, amateur pyrotechnists and professional fire workers have solved most of the problems of colored flame production. Excellent formulations exist for yellow, orange, red, blue and green stars. The problem I've been working on is the production of deep forest green or ocean green. As you can see in Figure 3., there are no bands in that region (490 nm - 500 nm). A composite color made of BaCl and CuCl emissions is an obvious choice, but unfortunately BaCl emission is seldom - if ever - free from interfering BaOH and BaO emissions, which fall in the yellow and yellowish-green region of the visible spectrum. It seems that it is easier to generate greenish blue and turquoise than the long sought after bluish green and forest green.
Further Reading:
This article deals with the physics of aerial shell fireworks. These are the type of fireworks that are used at most Fourth of July or sporting event shows. Aerial shells contain the chemicals that when ignited, produce the brilliant flash of colored light. These shells are loaded into mortars, which are basically just small cannons , and are fired into the sky. To the right you can see a table that lists all commonly used shell sizes and their corresponding initial mortar velocities. These velocities are the speeds that the shells are traveling as they are fired out of the mortar. The 2" through 6" shells are used at almost all fireworks shows and are used almost exclusively at small shows. The 8", 10", and 12" shell sizes are usually used at only large fireworks shows as they are more expensive than the smaller sizes. The 24" and 36" shell sizes are even more expensive because they produce extremely large burst patterns. These monstrous aerial shells are only used at the largest shows and during special circumstances. As you can see in the table, larger shell sizes produce greater initial mortar velocities. This happens because the larger mortars used to fire larger shells have the capacity to house greater amounts of blackpowder/pyrex used to propel the shells out of the mortar. Greater amounts of blackpowder/pyrex, when burned, produce more excess gases than do smaller amounts. These larger amounts of excess gases cause the shell to be pushed or propelled out of the mortar faster, resulting in greater initial velocities. The greater initial velocities produced by larger shells result in the shell attaining a greater height before it explodes and emits its bright flash of light. Shells usually travel about 100 feet vertically for every inch they are in diameter; depending on the angle they are fired from. The relationships between the initial velocities and the distances traveled by the shells can be understood and manipulated by using the following formulas and mathematical methods:
2"- 12" Shell Trajectories Fired at 75 Degrees
Pyrotechnicians must also consider shell burst sizes when planning shows. They must know how big certain bursts are when compared to others so that the choreographing of the show is in sync and so they don't exceed their safe zone requirements. As with initial mortar velocities, the bigger the shell size the larger the effect. It follows the same principle in that larger shells contain greater amounts of chemicals that when ignited produce greater explosion forces than do smaller shells. This results in varied burst sizes. Shell burst sizes are usually about 45 feet in diameter for every inch in shell size, depending on how tightly the shell is packed. As you can see in the diagram on the right, the differences in burst sizes can be extremely huge. It is just one more thing that pyrotechnicians must take into account to produce entertaining and attractive fireworks shows.
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The first two formulas you see are primarily used to chart
trajectories like in the graph on the left that shows the flight
paths of 2" through 12" shells fired at 75 degrees. These graphs are
very useful tools that allow pyrotechnicians to visualize how high
and how far their shells will travel during a show. This information
can be used to aid the process of choreographing the show to music,
and determining if some shells will exceed the safe zone for that
particular site. The Pythagorean Theorem is used to find a certain
initial velocity value if the other two are known. This is helpful
in determining information needed for the other formulas. The
Trigonometric Functions are also used to find initial velocity
values, but are used to find vertical heights, horizontal distances,
and firing angles as well. Pyrotechnicians use these mathematical
methods along with charts, graphs, and computer programs derived
from them to plan their impressive displays.
