ceived as blue), or by the emission of several ranges of light that 2. The emitting species must be sufficiently volatile to exist combine to yield a particular color. For example, the mixing of in the vapor state at the temperature of the pyrotechnic
15 2
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Color and Light Production
153
TABLE 7.6 The Visible Spectruma
A temperature range is therefore required, high enough to achieve the excited electronic state of the vaporized species Observed color - if
but low enough to minimize dissociation.
this wavelength is
5. The presence of incandescent solid or liquid particles in Wavelength
removed from
the flame will adversely affect color quality. The result-
(nanometers)
Emission color
white light
ing "black body" emission of white light will enhance overall emission intensity, but the color quality will be lessened.
<380
None (ultraviolet region)
A "washed out" color will be perceived by viewers. The 380-435
Violet
Yellowish-green
use of magnesium or aluminum metal in color compositions will yield high flame temperatures and high overall inten-435-480
Blue
Yellow
sity, but broad emission from incandescent magnesium ox-480-490
Greenish-blue
Orange
ide or aluminum oxide products may lower color purity.
6. Every effort must be made to minimize the presence of un-490-500
Bluish-green
Red
wanted atomic and molecular emitters in the flame. Sodium 500-560
Green
Purple
compounds can not be used in any color mixtures except yellow. The strong yellow atomic emission from sodium 560-580
Yellowish-green
Violet
(589 nanometers) will overwhelm other colors. Potassium 580-595
Yellow
Blue
emits weak violet light (near 450 nanometers), but good red and green flames can be produced with potassium com-595-650
Orange
Greenish-blue
pounds present in the mixture. Ammonium perchlorate is 650-780
Red
Bluish-green
advantageous for color compositions because it contains no metal ion to interfere with color quality. The best oxidizer
>780
None (infrared region)
to choose, therefore, should contain the metal ion whose emission, in atomic or molecular form, is to be used for a
color production, if such an oxidizer is commercially avail-Source : H. H. B auer , G. D. Christian, and J. E. O'Reilly, Inable, works well, and is safe to use. Using this logic, the strumental Analysis, Allyn & Bacon, Inc., Boston, 1979.
chemist would select barium nitrate or barium chlorate for green flame mixtures. Strontium nitrate, although hygroscopic, is frequently selected for red compositions. The use of a salt other than one with an oxidizing anion (e.g. , strontium carbonate for red) may be required by hygro-reaction. The flame temperature will range from 1000-scopicity and safety considerations. However, these inert 2000°C (or more), depending on the particular composition ingredients will tend to lower the flame temperature and used.
therefore lower the emission intensity. A low percentage 3. Sufficient heat must be generated by the oxidizer/fuel re-of color ingredient must be used in such cases to produce action to produce the excited electronic state of the emitter.
a satisfactory color.
A minimum heat requirement of 0.8 kcal/gram has been men-7. If a binder is required in a colored flame mixture, the mini-tioned by Shidlovskiy [5].
mum possible percentage should be used. Carbon-contain-4. Heat is necessary to volatilize and excite the emitter, but ing compounds may be oxidized to the atomic carbon level you must not exceed the dissociation temperature of mo-in the flame and produce an orange color. The use of a lecular species (or the ionization temperature of atomic binder that is already substantially oxidized (one with a species) or color quality will suffer. For example, the high oxygen content, such as dextrine) can minimize this green emitter BaC1 is unstable above 2000°C and the best problem. Binders such as paraffin that contain little or blue emitter, CuCl, should not be heated above 1200°C [5].
no oxygen should be avoided unless a hot, oxygen-rich composition is being prepared.
Color and
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Light Production
155
TABLE 7.7 Flame Temperatures for Oxidizer/Shellac Mixtures Oxidizer Selection
The numerous requirements for a good oxidizer were discussed in Flame temperatures for various oxidizers (°C)a detail in Chapter 3. An oxidizer for a colored flame composition Potassium
Ammonium
must meet all of those requirements, and in addition must either perchlor-perchlor-
emit the proper wavelength light to yield the desired color or not Potassium
Potassium
ate
ate
chlorate
emit any light that interferes with the color produced by other nitrate
Composition
KClO,,
NH,,C10,,
KCIO
components.
3
KNO 3
In addition, the oxidizer must react with the selected fuel to I.
75% Oxidizer
2250
2200
2180
produce a flame temperature that yields the maximum emission of 1675
15% Shellac
light in the proper wavelength range. If the temperature is too 10% Sodium
low, not enough "excited" molecules are produced and weak color oxalateb
intensity is observed.
A flame temperature that is too hot may
decompose the molecular emitter, destroying color quality.