RETARDANTS
Occasionally, a pyrotechnic mixture will function quite well and produce the desired effect, except for the fact that the burning rate is a bit too fast. A material is needed that will slow down the reaction without otherwise affecting performance. This can be accomplished by altering the ratio of ingredients (e.g., reducing the amount of fuel) or by adding an inert component to the composition. Excess metallic fuel is less effective as a "coolant" because of the ability of many fuels - such as magnesium - to react with the oxygen in air and liberate heat. Also, metals tend to be excellent heat conductors, and an increase in the metal percentage can speed up a reaction by facilitating heat transfer through the composition during the burning process.
Materials that decompose at elevated temperatures with the absorption of heat (endothermic decomposition) can work well as rate retardants. Calcium and magnesium carbonate, and sodium bicarbonate, are sometimes added to a mixture for this purpose.
However, gas generation occurs that may or may not affect the performance of the mixture.
Although endothermic, these reactions are thermodynamically spontaneous at high temperature due to the favorable entropy change associated with the formation of random gaseous products from solid starting materials.
Inert diluents such as clay and diatomaceous earth can also be used to retard burning rates. These materials absorb heat and separate the reactive components, thereby slowing the pyrotechnic reaction.