SENSITIVITY
Sensitivity of a high-energy mixture to an ignition stimulus is influenced by a number of factors. The heat output of the fuel is quite important, with sensitivity generally increasing as the fuel's heat of combustion increases. Mixes containing magnesium or aluminum metal, or charcoal, can be quite sensitive to static spark or a fire flash, while mixes containing sulfur as the lone fuel are usually less sensitive, due to the low heat output of sulfur. Ignition of a small quantity of material by static energy does not liberate sufficient energy, in a sulfur mix, to generate a selfpropagating process. A greater quantity of material must react at once to produce ignition.
Another important factor is the thermal stability and heat of decomposition of the oxidizer. Potassium chlorate mixtures tend to be much more sensitive to ignition than potassium nitrate compositions, due to the exothermic nature of the decomposition of KC1O 3 . Mixtures containing very stable oxidizers - such as ferric oxide (Fe 2O 3 ) and lead chromate (PbCr0 4 ) - can be quite difficult to ignite, and a more-sensitive composition frequently has to be used in conjunction with these materials to effect ignition.
A mixture of a good fuel (e.g., Mg) with an easily-decomposed oxidizer (e.g., KC1O 3 ) should be quite sensitive to a variety of ignition stimuli. A composition with a poor fuel and a stable oxidizer should be much less sensitive, if it can be ignited at all! Ignition temperature, as determined by DTA or a Henkin-McGill study, is but one measure of sensitivity, and there is not any simple correlation between ignition temperature and static spark or friction sensitivity. Some mixtures with reasonably high ignition temperatures (KC1O4 and Al is a good example) can be quite spark sensitive, because the reaction is highly exothermic and becomes self-propagating once a small portion is ignited. Sensitivity and output are not necessarily related and are determined by different sets of factors. A given mixture can have high sensitivity and low output, low sensitivity and high output, etc. Those mixtures that have both high sensitivity and substantial output are the ones that must be treated with the greatest care. Potassium chlorate/sulfur/aluminum "flash and sound" mixture is an example of this type of dangerous composition.