II.
70% Oxidizer
2125
2075
2000
1700
Table 7.7 gives some data on flame temperatures obtained by 20% Shellac
Shimizu for oxidizer/shellac mixtures. Sodium oxalate was added 10% Sodium
to yield a yellow flame color and permit temperature measurement oxalate
by the "line reversal" method [11].
III.
65% Oxidizer
1850
1875
1825
The data in Table 7. 7 show that potassium nitrate, with its 1725
25% Shellac
highly endothermic heat of decomposition, produces significantly 10% Sodium
lower flame temperatures with shellac than the other three oxi-oxalate
dizers.
The yellow light intensity will be substantially less for the nitrate compositions.
To use potassium nitrate in colored flame mixtures, it is nec-a Reference 11.
essary to include magnesium as a fuel to raise the flame tempera-bThe sodium oxalate (Na
ture.
A source of chlorine is also needed for formation of volatile 2 C 20,,) produces a yellow flame. The intensity of the yellow light emission can be used to determine the BaCl (green), or SrCl (red) emitters. The presence of chlorine flame temperature.
in the flame also aids by hindering the formation of magnesium oxide and strontium or barium oxide, all of which will hurt the color quality.
Shidlovskiy suggests a minimum of 15% chlorine donor in a color composition when magnesium metal is used as a often contain sawdust as a coarse, slow-burning retardant to help fuel [5].
achieve lengthy burning times.
To achieve rapid burning - such as in the brightly-colored Fuels and Burning Rates
"stars" used in aerial fireworks and Very pistol cartridges -
compositions will contain charcoal or a metallic fuel (usually mag-Applications involving colored flame compositions will require nesium). Fine particle sizes will be used, and all ingredients will either a long-burning composition or a mixture that burns rap-be well-mixed to achieve a very homogeneous - and fast burning -
idly to give a burst of color.
mixture.
Highway flares ("fusees") and the "lances" used to create fireworks set pieces require long burning times ranging from 1-30 minutes. "Fast" fuels such as metal powders and charcoal are Color Intensifiers
usually not included in these slow mixtures. Partially-oxidized Chlorine is the key to the production of good red, green, and blue organic fuels such as dextrine can be used. Coarse oxidizer and flames, and its presence is required in a pyrotechnic mixture to fuel particles can also retard the burning rate. Highway flares
156
Chemistry of Pyrotechnics
Color and Light Production
157
TABLE 7.8 Chlorine Donors for Pyrotechnic Mixtures MgO particles is thereby reduced, and color quality improves significantly.
Melting point,
% Chlorine
MgO + HCl + MgCl + OH
Material
Formula
°C
by weight
Polyvinyl chloride
(-CH2CHC1-)n Softens ca. 80
56
Red Flame Compositions
decomposes
The best flame emission in the red region of the visible spectrum ca. 160
is produced by molecular strontium monochloride, SrCl. This
"Parlon" (chlorinated
Softens 140
ca. 66
species - unstable at room temperature - is generated in the polyisopropylene )
pyrotechnic flame by a reaction between strontium and chlorine atoms. Strontium dichloride, SrC1 2 , would appear to be a logi-Hexachlorobenzene C 6C16
229
74.7
cal precursor to SrCl, and it is readily available commercially,
"Dechlorane"
C10C112
160
78.3
but it is much too hygroscopic to use in pyrotechnic mixtures.
(hexachloropenta-
The SrCl molecule emits a series of bands in the 620-640 mano-diene dimer)
meter region - the "deep red" portion of the visible spectrum.
Other peaks are observed. Strontium monohydroxide, SrOH, is Hexachloroethane
C 2C16
185
89.9
another substantial emitter in the red and orange-red regions
[1, 11]. The emission spectrum of a red flare is shown in Figure 7.1.
Strontium nitrate - Sr(NO 3) 2 - is often used as a combination achieve a good output of these colors. Chlorine serves two impor-oxidizer/color source in red flame mixtures. A "hotter" oxidizer, tant functions in a pyrotechnic flame. It forms volatile chlorine-such as potassium perchlorate, is frequently used to help achieve containing molecular species with the color-forming metals, en-higher temperatures and faster burning rates. Strontium nitrate suring a sufficient concentration of emitters in the vapor phase.
is rather hygroscopic, and water can not be used to moisten a Also, these chlorine-containing species are good emitters of nar-binder for mixtures using this oxidizer. Strontium carbonate is row bands of visible light, producing the observed flame color.
much less hygroscopic and can give a beautiful red flame under Without both of these properties - volatility and light emission -
the proper conditions. However, it contains an inert anion - the good colors would be difficult to achieve.
carbonate ion, C032 - and low percentages must be used to avoid The use of chlorate or perchlorate oxidizers (KC1O 3 , KC1O,,, burning difficulties.
etc.) is one way to introduce chlorine atoms into the pyrotechnic To keep the SrCl from oxidizing in the flame, Shidlovskiy rec-flame. Another method is to incorporate a chlorine-rich organic ommends using a composition containing a negative oxygen balance compound into the mixture. Table 7.8 lists some of the chlorine (excess fuel). Such a mixture will minimize the reaction donors commonly used in pyrotechnic mixtures. A dramatic in-2 SrCl + 0
crease in color quality can be achieved by the addition of a small 2 -> 2 SrO + C1 2
percentage of one of these materials into a mixture. Shimizu rec-and enhance color quality [ 51. Several red formulas are presented ommends the addition of 2-3% organic chlorine donor into compo-in Table 7.9
sitions that don't contain a metallic fuel, and the addition of 10-15% chlorine donor into the high temperature mixtures containing Green Flame Compositions
metallic fuels [11].
Shimizu attributes much of the value of these chlorine donors Pyrotechnic compositions containing a barium compound and a good in magnesium-containing compositions to the production in the chlorine source can generate barium monochloride, BaCl, in the flame of hydrogen chloride, which reacts with magnesium oxide flame and the emission of green light will be observed. BaCl - an to form volatile MgCl molecules. The incandescent emission from unstable species at room temperature - is an excellent emitter in