SPARKS
The production of brilliant sparks is one of the principal effects available to the fireworks manufacturer and to the "special effects" industry. Sparks occur during the burning of many pyrotechnic compositions, and they may or may not be a desired feature.
Sparks are produced when liquid or solid particles - either original components of a mixture or particles created at the burnning surface - are ejected from the composition by gas pressure produced during the high-energy reaction. These particles -- heated to incandescent temperatures - leave the flame area and proceed to radiate light as they cool off or continue to react with atmospheric oxygen. The particle size of the fuel will largely determine the quantity and size of sparks; the larger the particle size, the larger the sparks are likely to be. A combination of fine fuel particles for heat production with larger particles for the spark effect is often used by manufacturers.
Metal particles - especially aluminum, titanium, and "magnalium" alloy - produce good sparks that are white in appearance. Charcoal of sufficiently large particle size also works well, producing sparks with a characteristic orange color. Sparks from iron particles vary from gold to white, depending on the reaction temperature; they are the brilliant sparks seen in the popular "gold sparkler" ignited by millions of people.
Magnesium metal does not produce a good spark effect. The metal has a low boiling point (1107°C), and therefore tends to vaporize and completely react in the pyrotechnic flame. "Magnalium" can produce good sparks that burn in air with a novel, crackling sound. Several spark-producing formulas are given in Table 1. Remember, the particle size of the fuel is very important in producing sparks - experimentation is needed to find the ideal size.
TABLE 1. Spark-Producing Compositions
For a good spark effect, the fuel must contain particles large enough to escape from the flame prior to complete combustion. Also, the oxidizer must not be too effective, or complete reaction will occur in the flame. Charcoal sparks are difficult to achieve with the hotter oxidizers; potassium nitrate (KNO 3 ) - with its low flame temperatures - works best. Some gas production is required to achieve a good spark effect by assisting in the ejection of particles from the flame. Charcoal, other organic fuels and binders, and the nitrate ion can provide gas for this purpose.