Gunpowder may be ignited by the percussion of copper against copper, copper against iron, lead against lead, and even with lead against wood, when the shock is very great. It is more difficult to ignite gunpowder between copper and bronze,[1] or bronze and wood than between the other substances. Again, out of ten samples which were wrapt in paper and struck upon an anvil with a heavy hammer, seven of grained powder exploded and nine of mealed.
[1] Bronze consists of 78 parts copper to 20 of tin. Bell metal—78 copper and 22 tin. Gun metal—100 copper to 8 to 10 tin. Brass—2 copper, 1 zinc and calamine stone, to harden and colour.
Influence of shape of grain on ignition.
If the part to which the heat is applied be of an angular shape, the inflammation will take place quicker than if it be of a round or flat form, on account of the greater surface that is exposed to the increased temperature.
The form of the grain influences the velocity of the transmission of flame.
If the grains are of a rounded form, there would be larger interstices, and a greater facility will be afforded to the passage of the heated gas, and therefore this shape is most favourable to the rapid and complete inflammation of each grain in the whole charge. On the other hand, particles of an angular or flat form, fitting into each other as it were, offer greater obstruction to this motion, and the velocity of the transmission of inflammation is thereby diminished.
Effect of size on the velocity of transmission of inflammation.
If the grains be small, the interstices will be small also, and the facility to the expansion of the gas thereby diminished. In the experiments with trains of powder, the increased surface exposed to the heated gas was found to more than compensate for the diminished facility to its expansion, and generally a train of small-grained powder laid upon a surface without being enclosed, will be consumed more quickly than a train of large-grained powder.
Large grain best suited for heavy ordnance.
But this is not the case in a piece of ordnance, a circumstance which amongst others will account for the diminished initial velocity given to the shot by a charge of small-grained musket powder, below that produced by the large-grained usually adopted for this service.