Chemistry of Pyrotechnics
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.
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
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
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.
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
850 580 dark
1000 730 bright
red, slightly orange
1200 930 bright
1400 1100 pale
1600 1300 yellowish
> 1700 > 1400 white (yellowish if seen from a distance)
Table 1. The
perceived colour of heated solid bodies.
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.
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.
D-line atomic emission 589
bands, 591-599 nm, 603-608 nm
being the most intense
molecular bands a:
617-623 nm b: 627-635 nm c: 640-646 nm
molecular bands 600-613
molecular bands a:
511-515 nm b: 524-528 nm d: 530-533 nm
molecular bands 403-456
nm, several intense bands, less intense
bands between 460 nm and 530 nm
Table 2. Sources of coloured light.
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.
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).
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.
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
perchlorate, 67% by weight
Strontium carbonate 13.5%
Pine root pitch (fuel) 13.5%
Rice starch (binder) 6%
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
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.
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.
Highlighted in the Periodic Table below are some of the important
elements that make a firework work.