Oxidizer Selection
The numerous requirements for a good oxidizer were discussed in detail in Chapter 3. An oxidizer for a colored flame composition must meet all of those requirements, and in addition must either emit the proper wavelength light to yield the desired color or not emit any light that interferes with the color produced by other components.
In addition, the oxidizer must react with the selected fuel to produce a flame temperature that yields the maximum emission of light in the proper wavelength range. If the temperature is too low, not enough "excited" molecules are produced and weak color intensity is observed. A flame temperature that is too hot may decompose the molecular emitter, destroying color quality.
The table gives some data on flame temperatures obtained by Shimizu for oxidizer/shellac mixtures. Sodium oxalate was added to yield a yellow flame color and permit temperature measurement by the "line reversal" method.
Flame Temperatures for Oxidizer/Shellac Mixtures
The data in the table show that potassium nitrate, with its highly endothermic heat of decomposition, produces significantly lower flame temperatures with shellac than the other three oxidizers. The yellow light intensity will be substantially less for the nitrate compositions.
To use potassium nitrate in colored flame mixtures, it is necessary to include magnesium as a fuel to raise the flame temperature. A source of chlorine is also needed for formation of volatile BaCl (green), or SrCl (red) emitters. The presence of chlorine 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 fuel.