Red Flame Compositions
The best flame emission in the red region of the visible spectrum is produced by molecular strontium monochloride, SrCl. This species - unstable at room temperature - is generated in the pyrotechnic flame by a reaction between strontium and chlorine atoms. Strontium dichloride, SrC12 , would appear to be a logical precursor to SrCl, and it is readily available commercially, but it is much too hygroscopic to use in pyrotechnic mixtures.
The SrCl molecule emits a series of bands in the 620-640 manometer region - the "deep red" portion of the visible spectrum. Other peaks are observed. Strontium monohydroxide, SrOH, is another substantial emitter in the red and orange-red regions. The emission spectrum of a red flare is shown in Figure1.
FIG. 1. Emission spectrum of a red flare
Strontium nitrate - Sr(NO3)2 - is often used as a combination oxidizer/color source in red flame mixtures. A "hotter" oxidizer, such as potassium perchlorate, is frequently used to help achieve higher temperatures and faster burning rates. Strontium nitrate is rather hygroscopic, and water can not be used to moisten a binder for mixtures using this oxidizer. Strontium carbonate is much less hygroscopic and can give a beautiful red flame under the proper conditions. However, it contains an inert anion - the carbonate ion, C03 - and low percentages must be used to avoid burning difficulties.
To keep the SrCl from oxidizing in the flame, Shidlovskiy recommends using a composition containing a negative oxygen balance (excess fuel). Such a mixture will minimize the reaction
and enhance color quality. Several red formulas are presented in Table 1.
TABLE 1. Red Flame Compositions