-- 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
coloured 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 production
The 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 concept
A 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:
Fig. 1. Black-body radiation.
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.
Table 1 gives a summary of visual temperature
phenomena of solid bodies - for instance, a glowing piece of charcoal, a good
approximation to the black body.
T, K oC Subjective colour
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)
Table 1. The perceived colour of heated solid
bodies.
The atomic and molecular emitters
As 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 fireworkers.) For other colours, we need
specific emitters of coloured 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 colours 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).
Table 2 summarises the sources of coloured light
used in today's fireworks.
Colour 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
Table 2. Sources of coloured light.
The chromaticity diagram and colour perception
The human eye may not be the best spectroscope
invented, but it is the best instrument for designing coloured fireworks. Although a
spectroscope can show the presence or absence of certain lines or bands in the flame
spectrum, it cannot decide whether the colour obtained looks pleasing to the human
observer. Pure, monochromatic colours a'la lasers are only a dream for pyrotechnists, but
well-designed impure colours do not lag much behind.
The chromaticity diagram shown below has been
designed with human colour vision system (three base colours) in mind. It is not necessary
to specify the intensities of all three base colours, 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 colours, 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 colour fractions,
the International Commission on Illumination published a standard chromaticity diagram in
1931 with three unreal primary colours. The above diagram and the colours are based on the
commission's recommendations.
The pure spectral colours can be found on the curved
line surrounding the tongue-shaped region of composite colours. The numbers along the
curve represent corresponding wavelengths (in nanometres).
Figure 2 shows the chromaticity diagram with a few
emission lines and bands of Table 2 drawn on the curve of spectral colours. The colours of
the diagram are only approximate.
Figure 2. Chromaticity diagram with some emission
bands..
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 colours, 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 colours of aerial fireworks come invariably from
stars, small pellets of firework composition which contain all the necessary ingredients
for generating coloured 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 colour 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" colour. Too much fuel would be a disadvantage, too, because the
glowing carbon particles quickly overwhelm the red colour.
Pure colours 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 colours. Potassium, with its weak atomic lines,
does not interfere with most colours, 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 aluminium 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 colour 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 colours.
Over the years, chemists, amateur pyrotechnists and
professional fireworkers have solved most of the problems of coloured 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 colour 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:
John A. Conkling: Pyrotechnics. Scientific American,
July 1990, 96.
Takeo Shimizu: Fireworks from a Physical Standpoint,
Part II. Pyrotechnica Publications, 1983.
John A. Conkling: The Chemistry of Pyrotechnics.
Marcel Dekker, Inc, 1985.
K.L. Kosanke: The Physics, Chemistry and Perception
of Colored Flames. Part I: Pyrotechnica VII, 5 (1981). Part II: Pyrotechnica IX, 42
(1984). Pyrotechnica is a serial published by Pyrotechnica Publications.
Takeo Shimizu: Fireworks - The Art, Science and
Technique. Second Edition. Pyrotechnica Publications, 1988.
For more information about the chromaticity diagrams
and colour science, see: What is CD by Eugene Vishnevsky.
