170
cial use, and the less-reactive - but safer - nitrate compounds
% by weight 41.6
58.4 (for a stoichiometric mixture)
are usually selected. Potassium perchlorate is used with aluminum Formula A in Table 7.2 therefore contains an excess of oxidizer.
and magnesium in some "photoflash" mixtures ; these are extremely It is the slowest burning mixture and produces the least heat.
reactive compositions, with velocities in the explosive range.
Formula B is very close to the stoichiometric point. Formula C
The nitrates are considerably endothermic in their decomposi-contains excess magnesium and is the most reactive of the three; tion and therefore deliver less heat than chlorates or perchlor-the burning of the excess magnesium in air must contribute subates, but they can be used with less fear of accidental ignition.
stantially to the performance of this composition.
Barium nitrate is often selected for white-light mixtures. The A significant altitude effect will be shown by these illuminating barium oxide (BaO) product formed upon reaction is a good, compositions, especially those containing excess metal. The de-broad-range molecular emitter in the vapor phase (the boiling creased atmospheric pressure - and therefore less oxygen - at point of BaO is ca. 2000°C), and condensed particles of BaO
higher altitudes will slow the burning rate as the excess fuel will found in the cooler parts of the flame are also good emitters of not be consumed as efficiently.
incandescent light.
Sodium nitrate is another frequent choice. It is quite hygroscopic however, so precautions must be taken during production
"Photoflash" Mixtures
and storage to exclude moisture. Sodium nitrate produces good To produce a burst of light of short duration, a composition is heat output per gram due to the low atomic weight (i.e. , 23) of required that will react very rapidly. Fine particle sizes are sodium, and the intense flame emission from atomic sodium in the used for the oxidizer and fuel to increase reactivity, but sensi-vapor state contributes substantially to the total light intensity.
tivity is also enhanced at the same time. Therefore, these mix-Potassium nitrate, on the other hand, is not a good source of tures are quite hazardous to prepare, and mixing operations atomic or molecular emission, and it is rarely - if ever - used should always be carried out remotely. Several representative as the sole oxidizer in white-light compositions.
photoflash mixtures are given in Table 7.3.
Magnesium metal is the fuel found in most military illuminating An innovation in military photoflash technology was the decompositions, as well as in many fireworks devices. Aluminum and velopment of devices containing fine metal powders without any titanium metals, the magnesium /aluminum alloy "magnasium," and oxidizer. A high-explosive bursting charge is used instead.
antimony sulfide (Sb2S 3 ) are used for white light effects in many This charge, upon ignition, scatters the metal particles at high
14 6
Chemistry of Pyrotechnics
Color and Light Production
147
TABLE 7.2 The Sodium Nitrate /Magnesium Systema
% Sodium
Linear burning Heat of reaction,
nitrate
% Magnesium
rate, mm/sec
kcal/gram
A.
70
30
4.7
1.3
B.
60
40
11.0
2.0
C .
50
50
14. 3
2.6
aReference 5.
temperature and they are then air-oxidized to produce light emission. No hazardous mixing of oxidizer and fuel is required to prepare these illuminating devices.
SPARKS
The production of brilliant sparks is one of the principal effects available to the fireworks manufacturer and to the "special effects" industry. Sparks occur during the burning of many pyrotechnic compositions, and they may or may not be a desired feature.Sparks are produced when liquid or solid particles - either original components of a mixture or particles created at the burn-ning surface - are ejected from the composition by gas pressure produced during the high-energy reaction. These particles --
heated to incandescent temperatures - leave the flame area and proceed to radiate light as they cool off or continue to react with atmospheric oxygen. The particle size of the fuel will largely determine the quantity and size of sparks; the larger the particle size, the larger the sparks are likely to be. A combination of fine fuel particles for heat production with larger particles for the spark effect is often used by manufacturers.
Metal particles - especially aluminum, titanium, and "magnalium" alloy - produce good sparks that are white in appearance. Charcoal of sufficiently large particle size also works well, producing sparks with a characteristic orange color. Sparks from iron particles vary from gold to white, depending on the
148
Chemistry of Pyrotechnics
Color and Light Production
149
TABLE 7.3 Photoflash Mixtures
TABLE 7.4 Spark-Producing Compositions
Refer-
% by
Oxidizer (% by weight)
Fuel (% by weight)
ence
Composition
weight
Effect
Reference
I.
Potassium per-
40
Magnesium
34
7
I.
Potassium nitrate,
58
Gold sparks
6
chlorate, KC10,,
Aluminum
26
KNO 3
Sulfur
7
II.
Potassium per-
40
Magnesium aluminum
60
7
Pure charcoal
35
chlorate, KC1O,,
alloy, "Magnalium"
(50/50)
II.
Barium nitrate,
50
Gold sparks (gold
6
Ba(N0 3 ) 2
sparkler)
III.
Potassium per-