Pyrochemical reactions in chemical education – Part I
This file was originally intended for teachers of chemistry as a short
introduction to pyrochemical demonstrations. However, the compositions and
the methods of preparation are indeed general, and the stars prepared here
can be used for
most pyrotechnical applications, including starmines and
bombshells.
Enjoy.
Pyrochemical reactions in chemical education – Part I:
Coloured
smokes, coloured flames and sparklers
A. 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
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.
Binders are organic compounds designed to keep the star in one piece. They
may be ordinary
resins or gums (like shellac), carbohydrates (dextrin),
plastics (polyvinyl chloride, Parlon
rubber) or thermosetting resins
(polyurethane, epoxy resin).
Chlorine donors are
compounds designed to add chlorine or hydrochloric acid
to the flame. In coloured flames, they
serve three main functions:
1) They aid in evaporation of the emitters
(colour-producing substances).
2) Usually, the emitters are themselves chlorides (or
diatomic species
containing chlorine) of the colour-giving elements. Metal chlorides cannot
3) In magnesium-containing
compositions, the chlorine donors reduce the
disturbing background radiation of magnesium
oxide and makes the flame
transparent. The chlorine donors also cool the flame.
The most common chlorine donors are polyvinyl chloride (PVC) and Parlon
rubber. The latter is
richer in chlorine (ca 70%). They can both be used as
binders, too. Ammonium perchlorate is
the only practicable oxidiser which
simultaneously acts as a chlorine donor.
Preparation of dextrin:
Spread some flour or potato starch on a plate and heat at 220
oC in an oven
for 15…30 minutes. Avoid overheating – when the product is brownish and
readily soluble in water, it’s ready. Dextrin can be used either as a mere
fuel or also as a
binder. Water is used as a solvent in this case. Sometimes
dextrin is mixed dry with the other
ingredients and water is added
afterwards. However, see instructions below for a safer
alternative.
Shellac and red gum may also be mixed with the other ingredients (if
they
are powdered) and ethanol used as a solvent. Red gum is also called accroides resin.
Shellac and red gum are also good fuels.
Parlon rubber is soluble in acetone.
Notes on using PVC:
PVC is available in two forms, hard (unplasticised) PVC and
plasticised,
flexible PVC. Unplasticised PVC is available from PVC manufacturers.
Plasticised PVC is the form used for practical purposes. It contains some
high-boiling organic
liquids (plasticisers) to keep it flexible.
It is very useful in pyrotechnics, too.
Plasticised PVC tubing or hose
can be cut into pieces and dissolved in tetrahydrofuran
(THF).
This solution can be used as such. It can contain up to 18% of PVC (w/v),
but
give a week or two for the PVC to dissolve. A solution of hard PVC is
rather messy to handle.
It is advisable to add 2 parts of dibutyl phtalate,
dioctyl phtalate or tricresyl phosphate
to the solution per 100 parts of PVC
as a plasticiser. Instead of cutting the wet star
composition into pellets
with a knife (as described below), you can just let a PVC-bound mass
alone
on a plate until it’s dry, carefully remove it and cut it into stars with
a pair
of scissors. Easy. But only with plasticised PVC.
Unfortunately, THF is the only
practically useful solvent for PVC. It is
really expensive, and if you want to reduce costs,
use some other
binder and powdered PVC.
Tetrahydrofuran, ethanol and acetone are
all narcotic if inhaled. Work in
a fume cupboard (hood). Stains of the binders are sometimes
hard to
remove. Plasticised PVC is the easiest, but shellac requires hot ethanol
and
probably some detergent for successful removal.
The stars are normally 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 pizza cutter, cut the cake into cubes 8 mm each
side. A knife or a ruler may also be used. Best results are achieved by keeping the cutter clean.
Allow the mass to dry (in a safe place; you may use a fridge fan to aid in drying) and remove the
stars by bending and twisting the bowl. This is why we used plastic. However, see note on using PVC
above.
3. Sometimes it is necessary to coat the stars with an igniting composition.
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.
You can mix your own powder for
coating. It isn’t actually true black powder,
not even "meal powder" (mixed and
ball-milled powder), but it serves this
purpose well.
75 parts of fine, sieved
potassium nitrate
10 parts of fine sulfur (preferably flour)
15 parts of fine, sieved
charcoal
This mix can be stored in small plastic containers.
Specific procedures
are described below if they are needed. The above
procedure is usually appropriate.
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.
Note that the low flame temperatures of these stars aid in
formation of
possibly toxic by-products (comparable to campfire smoke). The dyes
themselves are quite safe, although Rhodamine B should be handled with care
– it stains
everything and contact should be avoided.
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
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.
C. 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 510…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.
If you use the stars for demonstration purposes, 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 the introduction. The stars
take unexpectedly long to dry.
They can be dried in the sun or in a vacuum, but do not 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.
44 parts of potassium perchlorate
31 parts of strontium
nitrate
8 parts of polyvinyl chloride (PVC) _or_ 7 parts of saran (PVDC)
15 parts of red
gum (accroides resin)
5 parts of shellac (binder)
Proceed as above.
30 parts of ammonium perchlorate
35 parts of potassium perchlorate
18 parts of strontium
carbonate
2 parts of hexamine
2 parts of fine charcoal
16 parts of red gum
(accroides resin)
4 parts of dextrin
Add water, proceed as above; _but_ do not
coat these stars with black powder! Ammonium perchlorate and potassium nitrate (from black powder)
react to produce ammonium nitrate and potassium perchlorate. Ammonium nitrate is hygroscopic; the
stars will never be dry in ambient humidity.
The following coating composition can be
used:
80 parts of potassium perchlorate
15 parts of fine charcoal
4 parts
of red gum (accroides resin)
9 parts of manganese dioxide (optional!)
4 parts of fine
aluminium (preferably fine flake or pyro grade; optional!)
2 parts of dextrin
Aluminium and manganese dioxide aid in ignition, but are not necessary.
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)
or 18 parts of PVC (corresponding amount of the
solution in tetrahydrofuran)
Mix Parlon with magnesium. Add 50 volume parts of acetone,
mix well and
mix in the other ingredients.
If PVC is used, add the correct amount
of the solution in THF to the other ingredients and proceed as described above for PVC.
The composition leaves lots of ash. Ammonium perchlorate improves it:
56 parts of
barium nitrate
32 parts of lab grade magnesium powder
17 parts of Parlon rubber (or PVC,
solvent: THF)
25 parts of ammonium perchlorate
Proceed as described for the
previous composition. Use 60 volume parts of
acetone for Parlon. If you use PVC, use the
procedure above for using it.
Greens can also be based on aluminium:
65
parts of barium nitrate
10 parts of very fine aluminium (preferably dark pyro grade (sic!))
4 parts of sulfur
2 parts of boric acid
Add
acetone and proceed as usual. Coat with black powder.
An improved, fierce-burning formula with
ammonium perchlorate:
65 parts of barium nitrate
20 parts of saran (or parlon,
but saran is better in this case)
3 parts of red gum (accroides resin)
7 parts of
sulfur
10 parts of very fine aluminium, preferably dark pyro
15 parts of ammonium
perchlorate
2 parts of boric acid
2 parts of dextrin
Beautiful green,
without magnesium:
50 parts of ammonium perchlorate
35 parts of barium nitrate
Dissolve shellac in boiling ethanol and proceed as usual.
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,
another cycle,
since the flash rapidly consumes the reactants in the flash
zone.
Nitrocellulose
is used as a binder, since other binders tend to interfere
with the twinkling.
3.
Blue stars
60 parts of ammonium perchlorate
17 parts of sulfur
20 parts of
copper(II) oxide CuO
6 parts of red gum (accroides resin) _or_ shellac (powdered)
3
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) or PVC (solution in THF)
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.
65 parts of
potassium perchlorate
16 parts of cuprous chloride (CuCl)
10 parts of sulfur
11
parts of Parlon rubber (or 12 parts of PVC)
7 parts of red gum (accroides resin)
Use either Parlon or PVC as a binder.
60 parts of ammonium perchlorate
20 parts
of copper(II) oxide CuO
10 parts of sulfur
10 parts of dextrin
12 parts of
polyvinyl chloride PVC (use a solution in THF)
Add the PVC solution to the other
ingredients. Allow some THF to evaporate,
form a cake 1 cm thick and allow it to dry on a
plastic plate (check that it
doesn’t dissolve in THF!). Remove the dry cake and cut it into
stars with a
pair of scissors.
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.
D. Sparklers, silver rains
Sparks are produced
whenever hot liquid or solid particles are expelled
from the flame. In pyrotechnics, charcoal
and aluminium are the most
common sources of sparks. Magnesium does not produce good
sparks,
since it evaporates at 1100 oC and usually burns completely in the flame.
Charcoal and iron sparks are often orange or golden yellow, whereas Al
sparks are whiter. The
colour of a glowing solid or liquid particle is almost
completely determined by its surface
temperature. The intensity distribution often obeys the Planck’s black-body radiation curve,
although the actual intensities are usually lower than the theory predicts. This has sometimes been
called grey-body radiation. The intensity maximum shifts towards shorter wavelengths (blue endof the
visible spectrum) as the temperature increases. At the same time, the overall intensity is markedly
increased.
The sparkler composition must generate enough gases in order to expel
the hot particles. Moreover, if the burning proceeds mainly through liquid
phases, it reduces
the amount of sparks remarkably. Thus, ammonium
perchlorate is ideal for sparklers and silver
rains. Potassium perchlorate
and potassium chlorate are generally not used, except with
potassium
nitrate. The particle size of the materials may also have a profound effect
on
the quality and quantity of sparks, especially with aluminium. Fine
flakes are best.
Dextrin and shellac are usually used as binders. Epoxy resin and polyurethane
can also
be used.
The spark compositions are safer to prepare than coloured stars, since
they
don’t usually contain chlorates. Still, we are using fine metal powders here,
and
the unpelletised compositions may be powerful explosives, though not
especially sensitive.
Powdered aluminium is most sensitive to static
electricity. When in doubt, use metal cups and
wear cotton clothing. Or,
even better, add all the ingredients to a solution of a binder one
at a time.
This will exclude any danger in the preparation. The dried stars are not
as
dangerous as the plain powders.
Furthermore, the stars may be hazardous to use, due to
their very nature.
Have a fire extinguisher handy and wear cotton clothing. I also advise
you
to protect your face and eyes. As always, remember not to test the stars
the first
time before the audience. The shelf life of the stars is quite
long.
The stars
are prepared as usual. Since fine powders are used, the stars are
easy to cut.
1.
Gold flitter
16 parts of fine potassium nitrate
3 parts of sulfur
2 parts
of powdered charcoal
4 parts of sodium oxalate (or 2 parts of ultramarine)
11 parts of
fine, grey aluminium powder (preferably pyro aluminium)
5 parts of flake aluminium or medium
aluminium powder (Al bronze works well)
4 parts of dextrin (binder)
Add water and
proceed as usual. The particle sizes of aluminium powders
will markedly affect the result. If
Al bronze is available, you can use all
16 parts of it instead of the two different Al
powders.
2. Silver shower I
35 parts of potassium nitrate
8 parts of
fine charcoal
2 parts of boric acid
7 parts of sulfur (flowers of sulfur)
60
parts of potassium perchlorate
20 parts of fine pyro aluminium (atomised aluminium, 0.1 um)
15 parts of coarse flake aluminium
10
parts of dextrin
Add water and proceed as above. As before, the particle size and
surface
area of the reactants has a profound effect on the results.
3. Silver
shower II
65 parts of ammonium perchlorate
22 parts of fine aluminium powder or
flake aluminium (not too coarse)
18 parts of shellac
Dissolve shellac in boiling
ethanol, mix in the other ingredients and
proceed as usual. Shellac stars take a long time to
dry; try drying in the
sun. The particle size of aluminium is not as critical as in the
above
formulas.
4. Simple silver shower
15 parts of flitter (or any
grade except the finest pyro grades) aluminium
55 parts of potassium nitrate
2 parts of
boric acid
10 parts of fine charcoal
5 parts of dextrin (binder)
Add water
and proceed as usual.
If you don’t want to obtain N+1 grades of unspecified Al powders,
feel free
to experiment with the grades you have. Both stabilised and unstabilised
Al
powders will work. Just substitute the powder you have with the Al
powder suggested in the
formula. (Use small batches.) If the composition
burns too fast and emits only a few sparks,
you have to add coarser
grades. If you have enormous difficulties with ignition as well as a
poor
result with regard to sparks, your Al is too coarse.
E. 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:
(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.
F. 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.
The stars won’t explode, but
the loose powders may have a good chance of
doing that, especially when confined. The stars
merely burn (and are a fire
hazard if they are accidentally ignited) and generate noxious
smoke.
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.
G.
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.
More references can be found in Books in Print (in most
libraries).
–> End ‘o File