By Duane Thomas
In the first three parts of this five-part series on the components of priming compounds, we discussed the initiator which is stored energy, the sensitizer which places the initiator in a more-excited state thus makes it more likely to detonate when exposed to impact energy, and the frictionator which creates little micro-hotspots in the priming compound when crushed. Now let’s address the fourth component, the oxidizer.
As the name indicates, the oxidizer provides oxygen to sustain the combustion of the initiator, the sensitizer, and the fuel components. Common oxidizers are barium nitrate, potassium nitrate, or manganese dioxide. The priming compound’s burn rate (how quickly it combusts) and its flame duration (how long the flame persists) depend heavily on the oxidizer’s ability to release oxygen efficiently and thus sustain the reaction.
Oxidizers like barium nitrate are commonly used in lead-based primers, and release oxygen rapidly. This accelerates the burn rate, producing a hot, short-duration flame. Potassium nitrate, used in some lead-free primers, releases oxygen more slowly, resulting in a slower burn rate and longer flame duration. Manganese dioxide, which we see occasionally used in specialized primers, splits the difference and does a good job balancing burn rate and duration, but it’s less commonly used since it’s a comparatively expensive material, and less efficient than either barium nitrate or potassium nitrate in many formulations.
While oxidizers primarily affect burn rate and flame duration, their particle size and resulting reactivity to impact can also influence sensitivity. Specifically, smaller particle size in the oxidizer increases a priming compound’s sensitivity. It sounds counterintuitive but smaller particles actually have a larger surface area relative to their size than larger particles, thus burn faster and are more sensitive to ignition.
Easy way to think about this is a sugar cube versus an equivalent amount of granulated sugar. You drop them both into a cup of tea, which is going to dissolve faster? It’s the same thing with an oxidizer. Like the granulated sugar dissolves more easily than the sugar cube, finer oxidizer particles are more apt to detonate than larger particles. Not only will they release their oxygen faster, thus nourish an intense, short-duration flame, they’re more likely to detonate to start. Depending on whether you actually want a high-intensity, short flame, or a lower-intensity but longer flame, tweaking the oxidizer’s particle size, and formulation, can allow the effect we want. And going for smaller or larger oxidizer particles can make the priming compound more or less sensitive.
We discussed in Part 3 how the frictionator increases a priming compound’s sensitivity by creating micro-hotspots between itself, the initiator and the sensitizer when crushed. Because finer oxidizer particles have more surface area in contact with the frictionator, they also do their bit to make the frictionator work better. While a frictionator can reduce the impact energy needed to ignite the initiator/sensitizer mix by 10-30%, a small-particle oxidizer can lower this even more.
