COLOR
Certain elements and compounds, when heated to high temperature, have the unique property of emitting lines or narrow bands of light in the visible region (380-780 nanometers) of the electro- magnetic spectrum. This emission is perceived as color by an observer, and the production of colored light is one of the most important goals sought by the pyrotechnic chemist. Table 1 lists the colors associated with the various regions of the visible spectrum. The complementary colors - perceived if white light minus a particular portion of the visible spectrum is viewed -- are also given in Table 1.
TABLE 1. The Visible Spectrum
To produce color, heat (from the reaction between an oxidizer and a fuel) and a color-emitting species are required. Sodium compounds added to a heat mixture will impart a yellow color to the flame. Strontium salts will yield red, barium and copper compounds can give green, and certain copper-ontaining mixtures will produce blue. Color can be produced by emission of a narrow band of light (e.g. , light in the range 435-480 nanometers is perceived as blue), or by the emission of several ranges of light that combine to yield a particular color. For example, the mixing of blue and red light in the proper proportions will produce a purple effect. Color theory is a complex topic, but it is one that should be studied by anyone desiring to produce colored flames.
The production of a vividly-colored flame is a much more challenging problem than creating white light. A delicate balance of factors is required to obtain a satisfactory effect
1. An atomic or molecular species that will emit the desired wavelength, or blend of wavelengths, must be present in the pyrotechnic flame.
2. The emitting species must be sufficiently volatile to exist in the vapor state at the temperature of the pyrotechnic reaction. The flame temperature will range from 1000- 2000°C (or more), depending on the particular composition used.
3. Sufficient heat must be generated by the oxidizer/fuel reaction to produce the excited electronic state of the emitter. A minimum heat requirement of 0.8 kcal/gram has been mentioned by Shidlovskiy.
4. Heat is necessary to volatilize and excite the emitter, but you must not exceed the dissociation temperature of molecular species (or the ionization temperature of atomic species) or color quality will suffer. For example, the green emitter BaC1 is unstable above 2000°C and the best blue emitter, CuCl, should not be heated above 1200°C. A temperature range is therefore required, high enough to achieve the excited electronic state of the vaporized species but low enough to minimize dissociation.
5. The presence of incandescent solid or liquid particles in the flame will adversely affect color quality. The resulting "black body" emission of white light will enhance over all emission intensity, but the color quality will be lessened. A "washed out" color will be perceived by viewers. The use of magnesium or aluminum metal in color compositions will yield high flame temperatures and high overall intensity, but broad emission from incandescent magnesium oxide or aluminum oxide products may lower color purity.
6. Every effort must be made to minimize the presence of unwanted atomic and molecular emitters in the flame. Sodium compounds can not be used in any color mixtures except yellow. The strong yellow atomic emission from sodium (589 nanometers) will overwhelm other colors. Potassium emits weak violet light (near 450 nanometers), but good red and green flames can be produced with potassium compounds present in the mixture. Ammonium perchlorate is advantageous for color compositions because it contains no metal ion to interfere with color quality. The best oxidizer to choose, therefore, should contain the metal ion whose emission, in atomic or molecular form, is to be used for color production, if such an oxidizer is commercially available, works well, and is safe to use. Using this logic, the 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 hygroscopicity and safety considerations. However, these inert ingredients will tend to lower the flame temperature and therefore lower the emission intensity. A low percentage of color ingredient must be used in such cases to produce a satisfactory color.
7. If a binder is required in a colored flame mixture, the minimum possible percentage should be used. Carbon-containing compounds may be oxidized to the atomic carbon level in the flame and produce an orange color. The use of a binder that is already substantially oxidized (one with a high oxygen content, such as dextrine) can minimize this problem. Binders such as paraffin that contain little or no oxygen should be avoided unless a hot, oxygenich composition is being prepared.