What characteristic of the flame determines the permissibility of an explosive?

The permissibility of an explosive is largely determined by the characteristics of the flame produced when it detonates. A short flame of short duration indicates that the explosion is more controlled and less likely to cause extensive damage or ignition of surrounding materials. This trait is crucial for ensuring safety, especially in environments where explosive materials are present.

A short flame typically means that the combustion process occurs quickly and does not linger, which is preferred in managing the risks associated with explosives. It often correlates with the explosive's ability to diffuse energy quickly, minimizing the potential for secondary explosions or significant overpressures that can lead to catastrophic failures.

In contrast, a long flame of extended duration can suggest that the explosive is releasing energy over a longer period, which might result in a more significant shockwave and greater potential for secondary detonations. A brightly colored flame or an indigo colored flame does not inherently provide information about the explosiveness or safety of the material in question, as flame color can vary due to different chemical compositions but does not directly correlate with performance or safety characteristics.