IGNITION COMPOSITIONS AND FIRST FIRES
Compositions with high ignition temperatures (i.e., above 600°C) can be difficult to ignite using solely the "spit" from a black powder fuse or similar mild ignition stimulus. In such situations, an initial charge of a more-readily-ignitible material, called a "first fire," is frequently used. The requirements for such a mixture include:
1. Reliable ignitibility from a small thermal impulse such as a fuse. The ignition temperature of a "first fire" should be 500°C or less.
2. The mixture should attain a high reaction temperature, well above the ignition temperature of the main composition. Metal fuels are usually used when high reaction temperatures are needed.
3. A mixture that forms a hot, liquid slag is preferred. Such slag will provide considerable surface contact with the main composition, facilitating ignition. The production of hot gas will usually produce good ignition behavior on the ground, but reliability will deteriorate at higher altitudes. Liquid and solid products provide better heat retention to aid ignition under these conditions.
4. A slower-burning mixture is preferred over a more rapid one. The slower release of energy allows for better heat transfer to the main composition. Also, most "first fires" are pressed into place or added as moist pastes (that harden on drying), rather than used as faster-burning loose powders.
Potassium nitrate is frequently used in igniters and first fires. Compositions made with this oxidizer tend to have low ignition temperatures (typically below 500 1C), and yet the mixtures are reasonably safe to prepare, use in production, and store. Potassium chlorate formulations also tend to have low ignition temperatures, but they are considerably more sensitive (and hazardous).
Potassium nitrate mixed with charcoal can be used for ignition, as can black powder worked into a paste with water and a little dextrine. Shidlovskiy reports that the composition
works well as an igniter mixture; the solid magnesium oxide (MgO) residue aids in igniting the main composition. Boron mixed with potassium nitrate is a frequently-used, effective igniter mixture, as is the combination of iron oxide with zirconium metal and diatomaceous earth (commonly known as A-lA ignition mixture). Table 1 lists a variety of formulations that have been published.
TABLE 1. Ignition and First Fire Compositions