Chemistry Demos with Pyrotechnic Compositions (long)
Petri Pihko
Coloured smokes and coloured flames
I. Introduction
As an assistant leader of our chemistry club, I've often been asked to
perform some chemistry magic tricks before the audience. I know from
personal experience that probably the most interesting demonstrations
are those demonstrating the true magic of chemistry - that is, chemical
reactions. Those who have a fume cupboard (hood) for demonstration
purposes can broaden their spectrum of useful demonstrations by including
the most beautiful effects known to mankind to their demonstration sets -
namely, pyrochemical reactions.
Many demonstration manuals (notably Shakhashiri's Chemical Demonstrations)
include several reasonably safe "flash'n'boom" demonstrations. However,
while these demonstrations are relatively easy to perform and do not
require any special equipment, they still fail quite often and unexpectedly.
This is often due to high sensitivity of the compositions, which prevents
the demonstrators from making accurate preparations.
The thermite demonstration looks great, but it is usually performed in a very
large scale. If you haven't tried it before, or if you change anything, try
it first (preferably outside) without any audience before performing it live.
Also, make sure it will go off when you want it. I suggest using a good fuse
and black powder instead of the methods presented in the manuals. Magnesium
ribbon burns with a dazzling flame, but it is neither a fuse nor a match.
The demonstrations presented here are based on tiny pellets of the
composition held together with a binder. They are also called stars.
These are not as rapidly prepared as the usual demonstration compositions,
but contrary to these, the stars can be safely stored in metal cans. They
are at least as safe as matchheads. A 50-gram lot of stars is good for
tens of demonstrations.
The stars are usually prepared as follows (this is probably the safest
method):
1. Dissolve the binder in the solvent used (usually water or ethanol). Pour
the solution into a flat plastic bowl and mix in the other ingredients.
Make sure all the ingredients are thoroughly moistened before adding the
next one. This will almost entirely exclude any accidental friction between
the fuels and the oxidisers. You can use glass or stainless steel tools for
mixing. Grind all the ingredients _separately_ in a mortar before weighing
and mixing them.
2. After you've arrived in a more or less homogenous mix, allow the excess of
the solvent to evaporate (care! do not allow the mix to become too dry, since
you'll have to moisten it again!) and press the mass into a flat cake, about
8 mm thick. With a knife or a ruler, cut the cake into cubes 8 mm each way.
Allow the mass to dry (in a safe place) and remove the stars by bending and
twisting the bowl. This is why we used plastic.
3. Sometimes it is necessary to coat the stars with an igniting composition.
This can be done by moistening the stars with a suitable solvent in a bowl
and sprinkling the igniting composition on the stars. Give the stars a good
shake before adding more solvent or powder. It is advisable to mix
a little binder into the igniting composition before using it for coating.
For example, if you're going to coat the stars with black powder, you should
add some (say, 5%) dextrin to the black powder and moisten the stars with
water. Do not use _too much_ solvent - the stars will stick into each other,
and you'll get only large lumps! Use a dropper for adding the solvent.
II. Coloured smokes
These formulas contain volatile organic pigments, which will partially
evaporate (sublime) from the heat of the main composition (lactose/potassium
chlorate). Dextrin is used as a binder. The stars are usually coated with
black powder to ensure ignition. It is advisable to use these stars in a
cardboard tube to prevent the smoke from catching fire, which will destroy
the effect. The easier alternative is to use a wire gauze (as usual) and to
blow the flame out.
1. Blue smoke
40 parts of copper phtalocyanine (Phtalo Blue)
25 parts of lactose (milk sugar)
33 parts of potassium chlorate
2 parts of dextrin (a water-soluble binder)
Add water and proceed as above. Coat the stars with black powder/dextrin
or a simple ignition composition consisting of
4 parts of potassium chlorate
1 part of sucrose (cane sugar)
1 part of dextrin
This time you'll have to mix the chemicals in a dry state, ie, as plain
powders. Use a small plastic bowl and a glass rod for mixing. Do not grind.
Do not store the composition. Sprinkle it on the moistened stars. Use at
least 20% of the weight of the stars.
2. Yellow smoke
43 parts of quinoline yellow (quinophtalone yellow, Chinolingelb)
24 parts of potassium chlorate
16 parts of lactose
6 parts of sodium hydrogen carbonate (sodium bicarbonate)
2 parts of dextrin (as a binder)
Add water, proceed as above. Like the blue smoke stars, these stars should
be coated either with black powder or the igniting composition described
above.
3. Red smoke
40 parts of Rhodamine B (C.I. Basic Violet 10, C.I. 45170)
24 parts of potassium chlorate
16 parts of lactose
4 parts of sodium hydrogen carbonate
2 parts of dextrin
Add water and proceed as usual. Coat the stars as above.
Many other organic pigments and dyes can be used in coloured smokes.
The dyes should volatilise (sublime) readily between about 300...450
degrees oC, which excludes almost all dyes containing -NO2 (nitro) or -SO3H
(sulfonic acid) groups. Unfortunately, the leftovers are usually the
malicious azo or anthraquinone dyes, with known toxic properties. The dyes
suggested here are all safer than them.
If you wish to develop your own formula for a dye you think should work,
just substitute the dye with Rhodamine B in the previous formula. Every
dye would require a formula of its own, but the third formula is a good
starting point. If the dye you're using is evaporates near or slightly
above 300 oC, use a) 45 parts of the dye instead of 40 parts and b) 8
parts of sodium hydrogen carbonate instead of 4 parts.
III. Coloured flames
These are both easier to prepare and use than the coloured smokes. The
colours result mainly from atomic and molecular emissions in the flame.
For the yellow colour, the emitter is atomic Na (two lines near 589 nm).
For the red, both SrO and SrCl (radical) act as emitters, the former
giving a series of bands around 610 nm, and the latter emitting near 660
nm. Green comes from the molecular emission of BaCl radical (a number of
bands in the 500...535 nm range), but the colour is often disturbed by
BaO, which emits a series of bands mainly in the 530...600 nm range
(yellow). Moreover, BaCl is unstable at above 2000 oC. Finally, blue can
be obtained from the emission of CuCl below 1200 oC. The temperature of
a typical Bunsen flame is about 1800 oC, and the primary emitter at that
temperature is CuOH, which gives a green flame. The secret of a vivid blue
is a cool flame.
It is not necessary to coat the stars, since they will readily take fire
anyway. The chlorate compositions are generally more sensitive than the
perchlorate compositions. Avoid sparks and static electricity. Do not grind!
1. Red stars
20 parts of potassium chlorate
60 parts of strontium nitrate
20 parts of shellac (binder)
Dissolve shellac in boiling ethanol. Add the other ingredients and proceed
as described in chapter 1. The stars take unexpectedly long to dry. They
can be dried in the sun or in a vacuum, but do try any heating! The smaller
the stars are, the faster they'll dry.
65 parts of potassium chlorate
15 parts of strontium carbonate
20 parts of shellac
Proceed as above.
31 parts of strontium nitrate
44 parts of potassium perchlorate
10 parts of sulfur (preferably ground sulfur, but the flowers will do)
15 parts of shellac
Proceed as above.
2. Green stars
A simple but nice (somewhat yellowish) green can be made from
7 parts of barium nitrate
7 parts of potassium chlorate
2 parts of shellac
Dissolve shellac in boiling ethanol and proceed as described above for red
stars.
Dazzling green:
50 parts of barium nitrate
32 parts of lab grade magnesium powder
18 parts of Parlon (chlorinated isoprene rubber)
Mix Parlon with magnesium. Add 50 volume parts of acetone, mix well and
mix in the other ingredients.
This formula leaves lots of ash. The following composition is easier to
ignite:
56 parts of barium nitrate
32 parts of lab grade magnesium powder
17 parts of Parlon rubber
25 parts of ammonium perchlorate
Proceed as described for the previous composition. Use 60 volume parts of
acetone if necessary.
Beautiful green, without magnesium:
50 parts of ammonium perchlorate
35 parts of barium nitrate
15 parts of shellac
Proceed as above.
Twinkling green (wow!)
23 parts of magnesium powder (use any lab grade powder)
60 parts of ammonium perchlorate
17 parts of barium sulfate
Binder solution: Dissolve 3 parts of nitrocellulose (smokeless powder or
celluloid film) into 30 parts (w/v) of boiling acetone. If you're going to
prepare these stars more than once, prepare more of the solution,
since nitrocellulose dissolves slowly even in refluxing acetone. Approx.
30 parts of the solution (v/w) is used each time.
Mix the ingredients into the binder solution in the order they appear above.
Proceed as usual. Note that acetone evaporates very rapidly and the stars
usually dry within a few hours.
Magnesium reacts slowly with ammonium perchlorate producing ammonia and
magnesium perchlorate, especially in the presence of moisture. Thus, the
twinklers cannot be stored for more than 6 months, and they must be kept
in a closed bag.
During the smoulder phase, magnesium reacts with ammonium perchlorate
in the dark. In the flash phase, magnesium reacts with barium sulfate,
producing hot MgO and creating a green flame. The flash is followed by
another cycle, since the flash rapidly consumes the reactants in the flash
zone.
3. Blue stars
60 parts of ammonium perchlorate
17 parts of sulfur
20 parts of copper(II) oxide CuO
13 parts of dextrin (binder)
Add 25 volume parts of water to dextrin and mix in the other ingredients.
Use more water if necessary. Proceed as described above for stars in
general.
63 parts of potassium perchlorate
13 parts of copper oxide
14 parts of Parlon rubber (binder)
10 parts of red gum or shellac (powdered)
Mix red gum or shellac powder with Parlon. Add 50 volume parts of acetone,
mix well and mix in the other ingredients. Proceed as usual.
4. Yellow stars
6 parts of potassium chlorate
2 parts of sodium hydrogen carbonate
2 parts of dextrin
Mix dextrin with 4 volume parts of water and mix in the other ingredients.
Proceed as described above for stars in general.
IV. Uses
The stars should be used in a fume cupboard (hood), since they emit large
quantities of irritating smoke. There are several alternatives for ignition.
My own method is to place the stars on a wire gauze and ignite them with
a Bunsen burner. Use only a few stars at a time. The twinkling green stars
should be used one at a time for the best effect.
The stars will burn for a few seconds. They usually leave very little
residue. Clean up with a wet cloth and wash the gauze with water.
The effects can be used for demonstrating a variety of principles:
thermochemistry (the heats of reactions), kinetics and activation energies
(why don't the stars go off at once?), electronic spectroscopy (atomic
and molecular emissions), the chemistry of chlorine compounds (chlorates
vs. perchlorates), phase changes (evaporation of dyes) and even complex
kinetics (oscillations), if you can use the magnificent twinkling stars.
V. Hazards
The preparation and use of pyrotechnic compositions is not free from danger.
The smoke compositions are relatively safe, since they contain many inert
materials. Coloured flames, on the other hand, are more hazardous to
prepare. Follow the instructions carefully and avoid all grinding or static
electricity. Do not prepare too large batches (20...50 grams is ideal for
a beginner). Dry and store the stars in a safe place and label them. Do not
store any powdered compositions.
By following the instructions, the preparation and use of ready-made stars
is at least as safe as the pyrotechnic demonstrations described in
Shakhashiri's great demonstration manual (Chemical Demonstrations: A hand-
book for Teachers of Chemistry, part 1.)
Disposal: The best way of disposing old stars is to burn them. They can be
safely burned in small batches (no more than 50 grams at a time) if you use
a safety fuse and some igniting composition. Do this _outside_. If this is
not possible, do it in smaller batches in a fume cupboard. It is advisable
to have a fire extinguisher handy, as always.
VI. Pyrotechnic literature
John A. Conkling: Chemistry of Pyrotechnics (Marcel Dekker, New York, NY
1986)
A great textbook for anyone, especially for those who are interested in the
chemistry beyond the fireworks.
See also Conkling's great articles in Scientific American (July 1990, pp
96-102) and Chemical&Engineering News (June 29, 1981, pp 24-32).
T.Shimizu: Fireworks - The Art, Science and Technique, 2nd ed. (Pyrotechnica
Publications, 1988)
This book is a must for any professional, either for a pyrotechnist or a
teacher. A comprehensive treatise of commercial fireworks and the underlying
science.
Send comments to:
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. .* . * . . Petri Pihko ke...@tolsun.oulu.fi
. .*. .* *. Pihatie 15 C kem...@rainbow.oulu.fi
.*.* .. . SF-90650 OULU kem...@finou.oulu.fi
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