http://rspa.royalsocietypublishing.org/content/213/1114/285.abstract

The Initiation, Burning and Thermal Decomposition of Gunpowder
J. D. Blackwood and F. P. Bowden
Abstract

This paper describes an experimental study of the initiation of burning in gunpowder and of the way in which this develops and grows to explosion. There is evidence that initiation is due essentially to the formation of local hot spots; these may be formed by the adiabatic compression of air pockets, by the friction of grit particles and in other ways. The necessary temperature for these hot spots is remarkably low (ca. 130 degrees C) but it is sufficient to melt the sulphur. It is suggested that the formation of a liquid phase is important and that the process starts with the reaction of molten sulphur with the oxyhydrocarbons present in the charcoal. These oxyhydrocarbons which can be extracted from the charcoal with a suitable solvent, melt at a low temperature and can also provide a liquid phase. A high pressure is necessary before these reactions can proceed at a sufficient rate. A study is made of the way in which the initial reactions develop until the heat evolved is sufficient to start the main reaction, which is the oxidation of carbon by potassium nitrate. There is evidence that the acceleration of the initial reaction which leads up to the rapid burning is controlled by the oxyhydrocarbons present in the charcoal. High-speed camera studies show that the gunpowder grains themselves burn comparatively slowly in a layerwise fashion and suggest that the rapid propagation of the reaction from grain to grain is due to the emission of a hot spray of molten droplets of potassium salts.